Here is a list of fourteen major diseases found in fruits. Also learn about their symptoms and control.
Black rot is one of the most damaging grape diseases. All cultivated varieties of grapes are susceptible to infection by the black rot fungus. If not controlled, some or all of the grapes within a cluster will be rotted. The disease is favoured by warm, humid weather as is found during the summer. Before good control measures were devised, vineyards along the River often were hard hit. Grape growers commonly lost most of their crop, and the grape industry was literally driven out of the area.
Symptoms of black rot first appear as small yellowish spots on leaves. As the spots (lesions) enlarge, a dark border forms around the margins. The centres of the lesions become reddish brown. By the time the lesions reach 1/8 to 1/4 inch in diameter (approximately two weeks after infection), minute black dots appear.
These are fungal fruiting bodies (pycnidia) and contain thousands of summer spores (conidia). Pycnidia are often arranged in a ring pattern, just inside the margin of the lesions. Lesions may also appear on young shoots, cluster stems, and tendrils.
The lesions are purple to black, oval in outline, and sunken. Pycnidia also form in these lesions. Fruit symptoms often do not appear until the berries are about half grown. Small, round, light-brownish spots form on the fruit. The rotted tissue in the spot softens, and becomes sunken.
The spot enlarges quickly, rotting the entire berry in a few days. The diseased fruit shrivels, becoming small, hard, black and wrinkled (mummies). Tiny black pycnidia are also formed on the fruit mummies. The mummies usually remain attached to the cluster.
Grape black rot is caused by the fungus, Guignardia bidwellii. Black rot survives the winter in cane and tendril lesions and fruit mummies. In the spring during wet weather, the pycnidia on infected tissues absorb water and conidia are squeezed out. Conidia are splashed about randomly by rain and can infect any young tissue in less than 12 hours at temperatures between 60-90 degrees F. A film of water on the vine surface is necessary for infection.
A second type of spore, an ascospore, may also be produced in overwintered fruit mummies. Ascospores are forcibly discharged into the air and can travel considerable distances. Research has shown that ascospores are an important source of primary infections in the spring.
(i) Sanitation is Important:
Destroy mummies, remove diseased tendrils from the wires, and select fruiting canes without lesions. It is very important not to leave mummies attached to the vine. Research has shown that mummies on the ground release most or all of their ascospores before the end of bloom.
Mummies left up in the trellis can produce ascospores and conidia throughout the growing season, thus making control of this disease much more difficult. If only a few leaf lesions appear in the spring, remove these infected leaves.
(ii) Plant grapes in sunny open areas that allow good air movement. Proper row orientation to prevailing winds and good weed control beneath the vines also enable plants to dry more quickly during wet weather.
Peach canker is a fungus disease common on apricot, prune, plum, and sweet cherry trees as well as on peach trees. The disease is common in peach orchards and is a frequent cause of limb dying and death of peach trees.
Other common names for peach canker are perennial canker and Valsa canker. The fungi that cause this disease enter the plant through wounds. Infection results in dead and weakened twigs and branches, and in reduced yields.
The first symptoms appear in early spring as gummy drops of sap around wounded bark. The diseased inner bark begins to break down, causing the cankered surface to appear depressed. Black specks, which are fungal spore producing bodies, appear on the bark surface or under the bark tissue. During wet periods spores ooze out of these fungal bodies in tiny orange or amber coloured, curled strands.
During the summer, healthy bark (callus tissue) grows over the edges of the narrow, oval shaped cankers. In the fall, the fungus resumes growth, attacking and killing the new callus tissue. Over a period of years, a series of dead callus ridges form as the canker gets larger. Eventually, the canker may completely surround a branch.
The portion of the branch beyond the canker then dies. Large amounts of gum are usually produced around cankered areas. Peach canker is often confused with other problems which cause cankering and gumming. Among these are bacterial canker, insect borer injury and mechanical injury. When insects are involved, chewed-up wood dust is usually visible under the gum. Mechanical injury can often be verified by carefully reviewing recent operations in the area.
Peach canker is caused by the fungi, Cytospora leucostoma and Cytospora cincta. These fungi are weak pathogens and generally do not attack healthy, vigourous peaches bark. Winter injury, insect damage, and mechanical injury are common types of wounds serving as entry points.
The fungi survive the winter in cankers or in dead wood. During spring and summer, spores produced in the cankers are spread by wind and rain to wounds on the same or nearby trees. The spores are not blown over long distances in the wind.
Infection and canker development depend on temperature and the species of fungus involved. Cytospora cincta is favoured by lower temperatures than Cytospora leucostoma. Because of the manner of infection and development of this disease, no single control measure is adequate. Most known control methods act indirectly by reducing points of entry or by reducing the level of inoculum. Fungicides are generally ineffective for controlling this disease.
(i) Prune young trees carefully to avoid weak, narrow-angled crotches. Narrow- angled crotches are frequent sites of breakage and winter injury.
(ii) Delay pruning until early spring. This promotes quick healing. Remove cankered branches and dead wood while pruning. Do not leave protruding pruning stubs. Cut flush to the next larger branch.
(iii) Eradicate cankers and remove badly cankered limbs, branches or trees. Burn or remove all cankered limbs soon after pruning. These limbs or branches serve as a reservoir for the disease causing fungi. Sanitation is critical, especially during the early life of the orchard.
(iv) Do not plant new peach trees near established trees with canker.
(v) Avoid mechanical and insect injury.
(vi) Promote vigorous, healthy peach trees with proper fertilization, pruning, and water.
(vii) Do not over-fertilize late in the season. Winter injury is more common on these trees because winter hardening is delayed.
(viii) White latex paint applied to the southwest side of trunks and lower scaffold branches may help avoid cold injury.
(ix) Maintain a good control programme for other diseases and insect pests, especially borers.
Leaf curl is a springtime disease that occurs on peach, nectarine and related ornamental plants. The disease, though not a problem every spring, can be severe during cool, wet springs that follow mild winters. The leaf curl fungus damages peach trees by causing an early leaf drop.
This weakens the trees, making them more susceptible to other diseases and to winter injury. Weakened trees also will produce fewer fruit the following season. Yield may be further reduced when blossoms and young fruit become diseased and drop.
Symptoms of leaf curl appear in the spring. Developing leaves become severely distorted, and have a reddish or purple cast. Later, as spores form on the leaf surface, the leaves become powdery gray in colour. Shortly after this, the leaves turn yellow or brown and drop.
There is no secondary spread of this disease from leaves infected in the spring to new leaves produced later in the growing season. Once infected leaves drop, no further symptoms will appear during that growing season. Diseased twigs become swollen and stunted, and may have a slight golden cast. They usually produce curled leaves at their tips.
Though rarely seen, flowers and fruit may also become diseased. They drop shortly after they are infected. Diseased fruit has shiny, reddish, raised, warty spots.
Peach leaf curl is caused by the fungus, Taphrina deformans. The fungus survives the winter as spores (conidia) on bark and buds. Infection occurs very early in the growing season. During cool, wet spring weather the conidia infect new leaves as they emerge from the buds. Host plant tissues are susceptible for only a short period. As the tissues mature they become resistant. The fungus produces another type of spore (ascospore) on the upper surface of the diseased leaves.
During wet weather, ascospores produce additional conidia by budding. These conidia are carried to other parts of the tree by rain and wind, where they will overwinter until the next spring. Environment can limit leaf curl infection. This partially explains why the disease does not occur every year. Leaf curl is worse when the weather is cool and wet.
Low temperatures are thought to retard maturation of leaf tissue, thus prolonging the time infection may occur. The fungus can penetrate young peach leaves readily at temperatures between 50 and 70 degrees F, but only weakly below 45 degrees F. Rain is necessary for infection.
Leaf curl is not difficult to control. Since the fungus survives the winter on the surface of twigs and buds, a single fungicide spray, thoroughly covering the entire tree, will provide control. If leaf curl does result in significant defoliation in the spring, the fruit on affected trees should be thinned to compensate for the loss of leaves. Over-cropping the tree will weaken it and make it more susceptible to winter injury.
Damage from powdery mildew attack results in stunted growth. The foliage becomes distorted and twig growth is reduced. Also, the fruit surface may become russetted or discoloured, and dwarfed. Heavily mildewed trees are weakened, and are more susceptible to other pests and winter injury.
Powdery mildew may be found on buds, blossoms, leaves, twigs, and fruit. Symptoms first appear in the spring on the lower surface of leaves, usually at the ends of branches. Small, whitish felt-like patches of fungal growth appear and quickly cover the entire leaf.
Diseased leaves become narrow, crinkled, stunted and brittle. By mid-summer, tiny, black round specks show up on the lower leaf surface, but more commonly on the twigs. These are fungal fruiting bodies, but their importance in the disease cycle is probably minimal.
The fungus spreads rapidly to twigs, which stop growing and become stunted. In some cases the twigs may be killed back. Leaves and blossoms from infected buds will be diseased when they open the next spring.
Infected blossoms shrivel and produce no fruit. Fruit symptoms are not usually seen unless the disease has built up to high levels on susceptible cultivars. Diseased fruit has a fine network type surface blemish called russetting.
Powdery mildew is caused by the fungus, Podosphaera leucotricha. Powdery mildew overwinters as fungal strands (mycelium) in vegetative or fruit buds which were infected the previous season.
Infected terminals may have a silvery gray colour, stunted growth, and a misshapen appearance and are more susceptible to winter kill than are non-infected terminals. Temperatures near -18 degrees F kill a majority of mildewed buds and the fungus within them. Even at lower temperatures, however, some powdery mildew survives.
As buds break dormancy, the powdery mildew fungus resumes growth and colonizes developing shoots causing primary infections. The powdery white appearance on infected shoots consists of many thousands of spores which are responsible for spreading the fungus and causing secondary infections later in the growing season. Secondary infections are important because they produce the overwintering infected buds.
Secondary infections usually develop on leaves and buds before they harden off and may reduce the vigour of the tree.
(i) Apple varieties vary greatly in their susceptibility to powdery mildew. Jonathan, Granny Smith, Mutsu (Crispin), Rome, Cortland, Baldwin, Monroe and Idared are very susceptible and should be avoided if powdery mildew is a problem. Most other varieties may also be infected if inoculum is present and conditions are favourable for infection.
(ii) Plant trees in sunny locations with good air drainage. This reduces the humidity around trees and reduces the chances of disease.
(iii) Where powdery mildew is a problem, a good fungicide spray programme is generally required for control. This is especially true in commercial plantings.
Fire blight is a common and very destructive bacterial disease of apples and pears. The disease is so named because infected leaves on very susceptible trees will suddenly turn brown, appearing as though they had been scorched by fire. As a result of this disease, blight susceptible pear cultivars are no longer grown in many parts in the Midwest.
Damage and losses from fire blight on apple result from: death or severe damage to trees in the nursery; death of young trees in the orchard; delay of bearing in young trees due to frequent blighting of shoots and limbs; loss of limbs or entire trees in older plantings as the result of girdling by fire blight cankers; and direct loss of fruit due to blighting of blossoms and young fruit.
Fire blight may cause severe damage to many other members of the Rosaceae family. Quince, crabapple, mountain ash, spirea, hawthorn, pyracantha, and cotoneaster are all susceptible. Cultivars within some of these species are resistant.
Blossom and twig blight symptoms appear in the spring. Diseased blossoms become water-soaked and turn brown. The bacteria may then grow down into the blossom bearing twigs (spurs).
Leaves on the spur become blighted, turning brown on apple and black on pear. Droplets of milky tan-coloured bacterial ooze may be visible on the surface of diseased tissue. These droplets contain millions of bacteria which can cause new infections.
Twig blight starts at the growing tips of shoots and moves down into older portions of the twig. Blighted twigs first appear water-soaked and then turn dark brown or black. Blighted leaves remain attached to the dead branches through the summer.
The end of the branch may bend over, resembling a shepherd’s crook or an upside down “J”. As the fire blight bacteria move through blighted twigs into main branches, the bark sometimes cracks along the margin of the infected area on the main branch causing a distinct canker.
Both apple and pear fruit may be blighted. Rotted areas turn brown to black and become covered with droplets of ooze. The fruit remains firm but later dries out and shrivels into mummies.
Fire blight is caused by the bacterium, Erwinia amylovora. The fire blight bacteria overwinter in living tissue at the margins of cankers on the trunk and main branches. The bacteria become active in the spring when temperatures get above 65 degrees F.
Their growth is favoured by rain, heavy dews, and high humidity. By the time trees are blossoming, droplets of ooze containing the bacteria are present on the surface of cankers. Relatively few overwintering cankers become active and produce bacteria in the spring, but a single active canker may produce millions of bacteria, enough to infect an entire orchard.
The bacteria in droplets of ooze are spread by splashing rain or insects (mostly bees, flies, and ants) to open blossoms. The bacteria multiply rapidly in the blossom nectar, and invade the blossom tissue through natural openings called nectaries.
The optimum temperature range for blossom blight infection is 65 to 86 degrees F. However, shoot infection more commonly occurs through wounds created by sucking insects, such as aphids, leafhoppers, and tarnished plant bugs; by wind whipping; or by hail.
Fire blight bacteria multiply rapidly within an infected shoot. Droplets of ooze can form on the shoots within 3 days. Shoots remain highly susceptible to infection until vegetative growth ceases and the terminal bud is formed.
Fire blight is one of the most difficult diseases of apple to control, and there is no one procedure that will give complete control. Though control is not an easy task, the use of several practices in an integrated manner should result in minimal damage from fire blight.
(i) Plant apple, crabapple, and pear varieties that are less susceptible to fire blight. A listing of the relative susceptibility of some of the more common apple and pear varieties. Fire blight is not as severe a disease problem on most crabapple varieties. A few crabapple varieties which can develop severe fire blight include: Silver Moon, Snowdrift, Red Jade, and Van Esseltine.
(ii) Prune out fire blight cankers and blighted twigs. To decrease the inoculum level for the following season, prune out blighted twigs and cankers during the dormant season. During the dormant season (winter) there is much less chance of spreading bacteria. Branches that are more than half-girdled by cankers should be removed.
Cut off blighted twigs by making cuts at least 4 inches below the visible dead wood. Cankers can be cut out of trunks or large branches by removing dead tissue down to wood that appears healthy. If blighted twigs are pruned out during summer, cuts should be made 12 to 15 inches below diseased wood and pruning tools should be disinfested by dipping in a 2:10 solution of household bleach in water after each cut. We recommend that commercial growers do a thorough job of pruning out blighted wood in the dormant season and not in summer.
(iii) Follow proper pruning and fertilization practices. Excessive nitrogen fertilizer and heavy pruning will promote vigourous growth of succulent tissue which is more susceptible to fire blight. Adjust management practices on susceptible varieties to promote moderate growth.
Make fertilizer applications in early spring or late fall after growth has ceased.
(iv) Sucking insects create wounds through which fire blight bacteria can enter. These pests should be controlled throughout the growing season. To protect bees, do not apply insecticides during bloom.
Apple scab is one of the most serious diseases of apple and ornamental crabapple. Disease development is favoured by wet, cool weather that generally occurs in spring and early summer. Both leaves and fruit can be affected.
Infected leaves may drop resulting in unsightly trees, with poor fruit production. This early defoliation may weaken trees and make them more susceptible to winter injury or other pests. Infected fruits are blemished and often severely deformed. Infected fruits may also drop early.
Symptoms first appear in the spring as spots (lesions) on the lower leaf surface, the side first exposed to fungal spores as buds open. At first, the lesions are usually small, velvety, olive green in colour, and have unclear margins.
On some crabapples, infections may be reddish in colour. As they age, the infections become darker and more distinct in outline. Lesions may appear more numerous closer to the mid-vein of the leaf. If heavily infected, the leaf becomes distorted and drops early in the summer.
Trees of highly susceptible varieties may be severely defoliated by mid to late summer. Fruit symptoms are similar to those found on leaves. The margins of the spots, however, are more distinct on the fruit. The lesions darken with age and become black and “scabby.” Scabs are unsightly, but are only skin deep. Badly scabbed fruit becomes deformed and may fall before reaching good size.
In the spring, the fungus in old diseased leaves produces millions of spores. These spores are released into the air during rain periods in April, May and June. They are then carried by the wind to young leaves, flower parts and fruits. Once in contact with susceptible tissue, the spore germinates in a film of water and the fungus penetrates into the plant. Depending upon weather conditions, symptoms (lesions) will show up in 9 to 17 days.
The fungus produces a different kind of spore in these newly developed lesions. These spores are carried and spread by splashing rain to other leaves and fruits where new infections occur. The disease may continue to develop and spread throughout the summer. Because a film of water on leaves and fruit is required for infection to occur, apple scab is most severe during years with frequent spring rains.
i. The use of resistant or scab immune varieties is the ideal method for controlling scab. Currently there are several apple varieties that are totally resistant to scab. Backyard growers are strongly encouraged to consider using these resistant varieties in order to reduce or eliminate the need for fungicide applications around the home.
Scab resistant apple varieties include:
ii. Prima, Priscilla, Sir Prize, Freedom, Liberty, Jonafree, Enterprise, Goldrush, Redfree, Pristine, Williams Pride, Novamac and Nova Easygro. All other varieties, including most commercially grown varieties are susceptible to scab; however, they differ in their degree of susceptibility. McIntosh, Cortland, Red Delicious and Rome Beauty are all very susceptible to scab. Golden Delicious and Jonathan are less susceptible. Lists of scab resistant ornamental crabapples are available from many nurseries and garden centres.
iii. Rake and destroy fallen leaves. This will reduce the number of spores that can start the disease cycle the next year.
iv. Where resistance to scab is not present, fungicide application is the primary method of control.
Anthracnose is a disease common to raspberries, blackberries, and other brambles or cane fruits. It causes severe damage to black and purple raspberries and susceptible varieties of red raspberries throughout the United States.
The disease reduces the size and quality of fruit on infected canes. In addition, it may kill canes or weaken them so that they do not survive the winter. Other common names for this disease are “cane spot” and “gray bark.”
Anthracnose first appears in the spring on the young shoots as small, purplish, slightly raised or sunken spots. Later, they enlarge and become ash gray in the centre with slightly raised purple margins.
The spots are often so close together on black and purple raspberries that they form large irregular areas (cankers). The cankers may encircle the cane, sometimes causing the death of the cane beyond the canker. The bark in badly cankered areas often splits. Late season infections result in superficial gray, oval spots.
The spots have definite margins, but are not sunken. They may become so numerous that the spots blend together, covering large portions of the cane. This is the characteristic “gray bark” symptom which is common on red raspberry.
Dark coloured specks (fungal fruiting bodies) develop in circles on the gray bark. Anthracnose sometimes attacks the leaves and can cause some leaf drop. Small spots, about 1/16 inch in diameter, with light gray centres and purple margins appear on the leaves. Lesion centres later fall out, leaving a shot hole effect.
Anthracnose is caused by the fungus Elsinoe veneta. The fungus survives the winter in lesions on diseased canes. The following spring and summer, during wet and rainy periods, spores are released.
Spores are carried by splashing rain to healthy first-year primocanes. These spores may then germinate and infect young tissues on developing primocanes. Disease development is favoured by extended periods of wet weather.
(i) All steps possible should be taken to improve air circulation within a planting, to allow faster drying of foliage and canes. Reducing the number and duration of wet periods should reduce the potential for infection. Excessive applications of fertilizer (especially nitrogen) should be avoided, since it promotes excessive growth of very susceptible succulent plant tissue.
Plants should be maintained in narrow rows and thinned to improve air circulation and allow better light penetration. Weeds are very effective in reducing air movement; therefore, good weed control within and between rows is important for improving air circulation within the planting.
(ii) After harvest, remove and destroy all old fruited canes (floricanes) and any new primocanes that are infected. It is best to remove old canes shortly after harvest, and it is critical to have them removed before new growth starts in the spring. Remember, the fungus overwinters on old-infected canes.
(iii) Remove all wild brambles growing in the area because they can serve as a reservoir for the disease.
(iv) Where the disease is established in the planting, fungicide applications are generally required to achieve adequate control.
8. Spur Blight of Red Raspberries:
Spur blight is caused by the fungus Didymella applanata. Spur blight occurs only on red and purple raspberries. Spur blight has been considered to be a serious disease of red raspberry; however, recent studies in Scotland suggest that spur blight actually does little damage to the cane. The extent of damage caused by spur blight in the United States is not clearly understood.
The spur blight fungus has been reported to reduce yields in several ways. It can blight the fruit bearing spurs that are produced on the side branches, cause premature leaf drop, and kill buds on the canes that later develop into fruit bearing side branches. In addition, berries produced on diseased canes may be dry, small and seedy.
The symptoms first appear on young first-year primocanes in late spring or early summer. Purple to brown areas (lesions) appear just below the leaf or bud, usually on the lower portion of the stem. These lesions expand, sometimes covering all the area between two leaves.
In late summer or early fall, bark in the affected area splits lengthwise and small black specks, which are fungal fruiting bodies (pycnidia) appear in the lesions. They are followed shortly by many slightly larger, black, erupting spots; another form of fungal fruiting body (perithecia).
Leaflets sometimes become infected and show brown, wedge-shaped diseased areas, with the widest portion of the wedge towards the tip of the leaf. Infected leaflets may fall off, leaving only petioles without leaf blades attached to the cane. When diseased canes become fruiting floricanes during the next season, the side branches growing from diseased buds are often weak and withered.
Spur blight is caused by the fungus, Didymella applanata. It survives the winter in lesions on diseased canes. The following spring and summer, during wet and rainy periods, spores are released and carried by splashing rain and wind to nearby primo canes. There they germinate in the presence of water and produce new infections, where the fungus will again over winter.
All steps possible should be taken to improve air circulation within a planting, to allow faster drying of foliage and canes. Reducing the number and duration of wet periods should reduce the potential for infection.
Excessive applications of fertilizer (especially nitrogen) should be avoided, since it promotes excessive growth of very susceptible succulent plant tissue. Plants should be maintained in narrow rows and thinned to improve air circulation and allow better light penetration.
Weeds are very effective in reducing air movement; therefore, good weed control within and between rows is important for improving air circulation within the planting. Wild brambles, especially wild red raspberries, growing in the area should be removed. They can serve as a reservoir for the disease.
After harvest, remove and destroy all old fruited floricanes and any first-year primocanes that are infected. It is important that old canes be removed before new canes emerge in the spring. If spur blight becomes an important problem in the planting, growers may want to consider the use of fungicide. Special fungicide sprays specifically for control of spur blight are generally not warranted.
Brown rot is a common and destructive disease of peach and other stone fruits (plum, nectarine, apricot, and cherry).
The brown rot fungus may attack blossoms, fruit, spurs (flower and fruit bearing twigs), and small branches. The disease is most important on fruits just before ripening, during and after harvest. Under favourable conditions for disease development, the entire crop can be completely rotted on the tree. Peaches not kept in cool storage may be rotted in two to three days by the fungus.
The symptoms of brown rot are very similar on all stone fruit. Symptoms first appear in the spring as the blossoms open. Diseased flowers wilt, turn brown, and may become covered with masses of brownish-gray spores. The diseased flowers usually remain attached into the summer.
Young fruits are normally resistant, but may become infected through wounds. As fruits mature they become more susceptible to attack, even in the absence of wounds. Fruit infections appear as soft brown spots which rapidly expand and produce a tan powdery mass of conidia.
The entire fruit rots rapidly, then dries and shrinks into a wrinkled “mummy.” Rotted fruit and mummies may remain on the tree or fall to the ground. Fruit infection may spread rapidly, especially if environmental conditions are favourable and fruits are touching one another.
The fungus may move from diseased blossoms or fruit into the spurs. The fungus may then invade and cause diseased areas (cankers) on the twigs below. Succulent shoots are sometimes infected by direct penetration near their tip. A canker may form encircling the twig, causing death of the twig beyond the canker (twig blight).
Brown rot is caused by the fungus, Monilinia fructicola. The brown rot fungus survives the winter in mummified fruits (either on the ground or still on the tree) and in twig and branch cankers produced the preceding year. Both sources may produce spores that can infect blossoms and young shoots.
The inner surface of each bowl is lined with thousands of spore-containing sacs (asci). At this stage, the slightest disturbance of air movement will cause an apothecium to forcibly discharge millions of spores.
These spores (ascospores) are carried by wind to the open or unopened blossoms and young shoots. If a film of water (either from dew or rain) is present for 5 hours or longer, the spores can germinate and penetrate the plant.
Infected blossoms soon wilt and tan-gray tufts, composed of masses of another type of spore (conidia), develop on the outside of the flower shuck. If the infected blossom does not drop off, the fungus soon grows through the pedicel to the twig and forms a canker.
Masses of conidia are soon produced on the newly cankered twig surface during moist periods throughout May and June. These summer spores are easily detached, and, like the ascospores, are mainly wind-borne.
They are also splashed by rain or carried by insects to the growing fruit. Brown rot conidia can germinate and infect at temperatures of 32 to 90 degrees F. Wet weather and temperatures ranging from 60 to 70 degrees F are most favourable for disease development.
Following spring and summer rainy periods, mummified fruit still hanging in the tree become covered with masses of conidia that may result in blossom blight or fruit rot. Mummies hanging in the tree do not produce ascospores.
Although the flesh of young fruit is very susceptible to brown rot infection, the fruit has such a tough skin that the germ tubes of the summer spores do not normally penetrate. For this reason, young uninjured fruits are fairly safe from infection.
However, any type of injury to the fruit will provide entry points for brown rot spores. Insect and hail wounds, fruit cracking, limb rubs, twig punctures, and a variety of picking and packing injuries greatly increase the losses due to brown rot. Growers must realise that brown rot spores are practically everywhere during the fruit-ripening period. Infection is almost certain to occur if the weather is moist and if the fruit skin is broken in some way.
(i) Sanitation is very important in controlling brown rot. All dropped and rotted fruit should be picked up and destroyed promptly. At the same time, remove all mummies from the trees. Prune out all cankers during the dormant season. Overripe or rotting fruit in the packing shed should be removed and destroyed at once.
(ii) Control of insects that feed on fruit is essential. Remember that anything that causes wounding of the fruit will increase the incidence of brown rot. Special care should be taken during harvesting and packing to prevent puncturing or bruising of ripe fruit.
(iii) Remove wild or neglected stone fruit trees that serve as reservoirs for the disease.
(iv) Fruit should be cooled and refrigerated (as close to 32 degrees F as possible) immediately after harvest.
(v) The use of fungicide is an important part of the disease management programme for brown rot.
Orange rust is the most important of several rust diseases that attack brambles. All varieties of black and purple rasp berries and most varieties of erect blackberries and trailing blackberries are very susceptible. Orange rust does not infect red raspberries.
Unlike all other fungi that infect brambles, the orange rust fungus grows “systemically” throughout the roots, crown and shoots of an infected plant, and is perennial inside the below ground plant parts. Once a plant is infected by orange rust, it is infected for life. Orange rust does not normally kill plants, but causes them to be so stunted and weakened that they produce little or no fruit.
Orange rust-infected plants can be easily identified shortly after new growth appears in the spring.
Newly formed shoots are weak and spindly. The new leaves on such canes are stunted or misshapen and pale green to yellowish. This is important to remember when one considers control, because infected plants can be easily identified and removed at this time. Within a few weeks, the lower surfaces of infected leaves are covered with blister-like pustules that are waxy at first but soon turn powdery and bright orange.
This bright orange, rusty appearance is what gives the disease its name. Rusted leaves wither and drop in late spring or early summer. Later in the season, the tips or infected young canes appear to have outgrown the fungus and may appear normal. At this point, infected plants are often difficult to identify. In reality, the plants are systemically infected, and in the following years, infected canes will be bushy and spindly, and will bear little or no fruit.
Causal Organism and Disease Development:
Orange rust is caused by two fungi that are almost identical, except for a few differences in their life cycles. Arthuriomyces peckianus occurs primarily in the northeastern quarter of the United States and is the causal agent for the disease. Gymnoconia nitens is a microcyclic (lacks certain spores).
(i) Whenever possible, start with disease-free, certified nursery stock.
(ii) When diseased plants first appear in early spring, dig them out (including roots) and destroy them before pustules form, break open, and discharge the orange masses of spores. If plants are not removed, these spores will spread the disease to healthy plants.
(iii) Remove all wild brambles from within and around the planting site. Wild brambles serve as a reservoir for the disease.
(iv) Maintain good air circulation in the planting by pruning out and destroying old fruited canes immediately after harvest, thinning out healthy canes within the row, and keeping the planting free of weeds.
(v) Fungicide sprays are generally not considered an effective control method for orange rust.
Black knot of plums and cherries is a widespread and serious disease throughout the United States. Black knot is a common disease on wild plums and cherries and in home orchards where pruning and spraying are not regularly practiced.
The disease becomes progressively worse during each growing season and unless effective control measures are taken, it can stunt or kill the tree. The black knot fungus can infect American, European, and Japanese varieties of cultivated plums and prunes. Sweet, tart, and Mahaleb cherries are also affected by the fungus, but are generally less susceptible than plum or prune. Occasionally, it may also infect apricots, peaches and other Prunus species.
The black knot fungus mainly affects twigs, branches, and fruit spurs. Occasionally, trunks may also become diseased. Usually, infections originate on the youngest growth. On infected plant parts, abnormal growth of bark and wood tissues produce small, light- brown swellings that eventually rupture as they enlarge.
In late spring, the rapidly growing young knots have a soft (pulpy) texture and become covered with a velvety, olive-green growth of the fungus. In summer, the young knots turn darker and elongate. By fall, they become hard, brittle, rough and black. During the following growing season, the knots enlarge and gradually encircle the twig or branch.
The cylindrical or spindle-shaped knots may vary from one-half inch to a foot or more in length and up to 2 inches in diameter. Small knots may emerge from larger knots forming extensive galls. After the second year, the black knot fungus usually dies and the gall is invaded by secondary fungi that give old knots a white or pinkish colour during the summer.
Smaller twigs usually die within a year after being infected. Larger branches may live for several years before being girdled and killed by the fungus. The entire tree may gradually weaken and die if the severity of the disease increases and effective control measures are not taken.
Causal Organism and Disease Development:
Black knot is caused by the fungus, Dibotryon morbosum. The fungus overwinters in knots on twigs and branches or in the infected wood immediately surrounding them. In the spring, the fungus produces spores (ascospores) in sacs contained within tiny fruiting bodies on the surface of the knots. These ascospores are ejected into the air during rainy periods and are blown for moderate distances by wind currents.
Only succulent green twigs of the current season’s growth are susceptible to infection. Ascospores that land on them may germinate and cause infection if the twigs remain wet for a sufficient length of time. Normal growth is disrupted in the infected regions, and a knot is formed as the fungus causes the plant to produce tumor like growths.
Knots may become visible by the late summer of the year of infection but often are not noticed until the following spring, when they begin to enlarge rapidly. New ascospores capable of spreading the disease may be formed in the young knots the year following infection but often are not formed until the second spring.
Although the precise environmental conditions required for infection are uncertain, only a few hours of rain apparently are required at temperatures above 55 degrees F (13 degrees C), whereas much longer rainy periods are required to produce infection at temperatures below this threshold.
Most plum varieties grown, including Stanley and Damson, are susceptible to this disease. It has been reported that Early Italian, Brodshaw, Fallenburg, Methley and Milton are somewhat less susceptible than Stanley; Shiro, Santa Rose, and Formosa are much less susceptible; and President is apparently resistant to black knot.
Japanese varieties of plums are generally less susceptible than most American varieties. When planting new plum or prune trees, avoid planting trees next to or downwind from an old or abandoned orchard with a significant black knot problem. Similarly, remove all wild plum and cherry trees (potential disease reservoirs) from fence rows or woodlands next to the orchard site.
Established orchards or backyard trees should be scouted or examined each year for the presence of black knot, and infected twigs should be pruned out and destroyed or removed before bud break. Chopping prunings with a flail mower (to strip infected bark and knots from wood) is an alternative method of destroying infected prunings if burning or burying is impractical.
It is important to prune at least 2-4 inches (5-10 cm) below each knot because the fungus grows beyond the edge of the knot itself. If pruning is not possible because knots are present on major scaffold limbs or the trunk, they can be removed by cutting away the diseased tissue down to healthy wood and out at least ½ inch (1 cm) beyond the edge of the knot.
Fungicides can offer significant protection against black knot, but are unlikely to be effective if pruning and sanitation are ignored. The timing of fungicide sprays should be adjusted to account for inoculum levels and weather conditions.
Where inoculum is high because of an established black knot problem or a neighbouring abandoned orchard, protection may be needed from bud break until early summer. Where inoculum has been maintained at low to moderate levels, sprays are most likely to be useful from white bud through shuck split (the period of maximum availability of ascospores).
Fungicides are most necessary and will provide the greatest benefit if applied before rainy periods, particularly when temperatures are greater than 55 degrees F (13 degrees C). In evaluating control programmes, remember that knots often do not become apparent until the year following infection.
Verticillium wilt of strawberry can be a major factor limiting production. When a plant is severely infected by the Verticillium wilt fungus, the probability of it surviving to produce a crop is greatly reduced.
The Verticillium fungus can infect about 300 different host plants, including many fruits, vegetables, trees, shrubs and flowers, as well as numerous weeds and some field crops. The fungus can survive in soil, and, once it becomes established in a field or garden, it may remain alive for 25 years or longer.
Cool, overcast weather interspersed with warm, bright days is most favourable for development of Verticillium wilt. Infection and disease development may occur when soil temperature is from 70 to 75 degrees F (21 to 24 degrees C). Many soils contain the Verticillium wilt fungus. The fungus can be introduced into un-infested soil on seed, tools and farm machinery, and in the soil and roots of transplants.
The first symptoms of Verticillium wilt in new strawberry plantings often appear about the time runners begin to form. In older plantings, symptoms usually appear just before picking time. Symptoms on above-ground plant parts may differ with the susceptibility of the cultivar affected.
In addition, above-ground symptoms are difficult to differentiate from those caused by other root infecting fungi. Isolation from diseased tissue and culturing the fungus in the laboratory are necessary for positive disease identification.
On infected strawberry plants, the outer and older leaves droop, wilt, turn dry and become reddish-yellow or dark brown at the margins and between veins. Few new leaves develop, and those that do tend to be stunted and may wilt and curl up along the mid vein.
Severely infected plants may appear stunted and flattened, with small yellowish leaves. Brownish to blueish-black streaks or blotches may appear on the runners or petioles. New roots that grow from the crown are often dwarfed with blackened tips. Brownish streaks may occur within the decaying crown and roots.
Verticillium wilt is caused by the soil borne fungus fumigant that is most effective is a mixture of chloropicrin (tear gas) 33 per cent and methyl bromide 66 per cent. Downy mildew is a major disease of grapes throughout the eastern United States.
The fungus causes direct yield losses by rotting inflorescences, clusters and shoots. Indirect losses can result from premature defoliation of vines due to foliar infections. This premature defoliation is a serious problem because it predisposes the vine to winter injury. It may take a vineyard several years to fully recover after severe winter injury.
On leaves, young infections are very small, greenish-yellow, translucent spots that are difficult to see. With time the lesions enlarge, appearing on the upper leaf surface as irregular pale-yellow to greenish-yellow spots up to ¼ inch or more in diameter.
On the underside of the leaf, the fungus mycelium (the “downy mildew”) can be seen within the border of the lesion as a delicate, dense, white to grayish, cotton-like growth. Infected tissue gradually becomes dark brown, irregular, and brittle.
Severely infected leaves eventually turn brown, wither, curl, and drop. The disease attacks older leaves in late summer and autumn, producing a mosaic of small, angular, yellow to red-brown spots on the upper surface. Lesions commonly form along veins, and the fungus sporulates in these areas on the lower leaf surface during periods of wet weather and high humidity.
On fruit, most infection occurs during 2 distinct periods in the growing season. The first is when berries are about the size of small peas. When infected at this stage, young berries turn light brown and soft, shatter easily, and under humid conditions are often covered with the downy-like growth of the fungus. Generally, little infection occurs during hot summer months. As nights become cooler in late summer or early fall, the second infection period may develop.
Berries infected at this time generally do not turn soft or become covered with the downy growth. Instead, they turn dull green, then dark brown to brownish-purple. They may wrinkle and shatter easily and, in severe cases, the entire fruit cluster may rot.
These infected fruit will never mature normally. On shoots and tendrils, early symptoms appear as water-soaked, shiny depressions on which the dense downy mildew growth appears. Young shoots usually are stunted and become thickened and distorted. Severely infected shoots and tendrils usually die.
Downy mildew is caused by the fungus Plasmopara viticola. The fungus overwinters in infected leaves on the ground and possibly in diseased shoots. The overwintering spore (oospore) germinates in the spring and produces a different type of spore (sporangium). These sporangia are spread by wind and splashing rain. When plant parts are covered with a film of moisture, the sporangia release small swimming spores, called zoospores.
Zoospores, which also are spread by splashing rain, germinate by producing a germ tube that enters the leaf through stomatas (tiny pores) on the lower leaf surface. The optimum temperature for disease development is 64 to 76 degrees F (18 to 25 degrees C).
The disease can tolerate a minimum temperature of 54 to 58 degrees F (12 C to 13 degrees C), and a maximum temperature of about 86 degrees F (30 degrees C). Once inside the plant, the fungus grows and spreads through tissues.
At night during periods of high humidity and temperatures above 55 degrees F (13 degrees C), the fungus grows out through the stomatas of infected tissue and produces microscopic, branched, tree-like structures (sporangiophores) on the lower leaf surface.
More spores (sporangia) are produced on the tips of these tree-like structures. The small sporangiophores and sporangia make up the cottony, downy mildew growth. Sporangia cause secondary infections and are spread by rain.
Any practice that speeds the drying time of leaves and fruit will reduce the potential for infection. Select a planting site where vines are exposed to all-day sun, with good air circulation and soil drainage. Space vines properly in the row, and, if possible, orients the rows to maximize air movement down the row.
Sanitation is important. Remove dead leaves and berries from vines and the ground after leaf drop. It may be beneficial to cultivate the vineyard before bud break to cover old berries and other debris with soil. Cultivation also prevents overwintering spores from reaching developing vines in the spring. To improve air circulation, control weeds and tall grasses in the vineyard and surrounding areas. When pruning, select only strong, healthy, well-coloured canes of the previous year’s growth.
Practices such as shoot positioning and leaf removal that help to open the canopy for improved air circulation and spray coverage are also very important. Grape varieties vary greatly in their susceptibility to downy mildew. In general, vinifera (Vitis vinifera) varieties are much more susceptible than American types, and the French hybrids are somewhat intermediate in susceptibility.
Cabernet Franc, Cabernet Souvignon, Catawba, Chancellor, Chardonnay, Delaware, Fredonia, Gewurytraminer, Ives, Merlot, Niagra, Pinot Blanc, Pinot Noir, Riesling, Rougeon and Sauvignon Blanc are reported to be highly susceptible to downy mildew. A good fungicide spray programme is extremely important. Downy mildew can be effectively controlled by properly timed and effective fungicides.
Many commercial strawberry cultivars are susceptible to the red stele fungus. This root rot disease has become a serious problem facing strawberry production in the northern two-thirds of the United States. The disease is most destructive in heavy clay soils that are saturated with water during cool weather when the fungus is most active. The red stele fungus can survive in soil for up to 13 years or longer once it becomes established in the field or garden.
Normally, the disease is prevalent only in the lower or poorly drained areas of the planting; however, it may become fairly well distributed over the entire patch, especially during a cool, wet spring.
When plants start wilting and dying in the lower portions of the strawberry planting, the cause is very likely to be red stele. Infected plants are stunted, lose their shiny-green luster, and produce few runners. Younger leaves often have a metallic, blueish-green cast. Older leaves turn prematurely yellow or red. With the first hot, dry weather of early summer, diseased plants wilt rapidly and die.
Diseased plants have very few new roots, when compared with the roots of healthy plants that have thick and bushy roots with many secondary feeding roots. Infected strawberry roots usually appear gray, while the new roots of a healthy plant are yellowish- white.
The most reliable symptom of red stele is found within the roots and may be observed by gently digging up a few plants that are just beginning to wilt, taking care to preserve the root system. Plants with red stele usually have few fine lateral roots so the main fleshy roots have a “rat-tail” appearance. During intermediate stages of disease development, these fleshy roots will be white near the crown of the plant but will show a dark rot progressing upward from the tips.
When the white outer portion of the root just above this rotten zone is peeled off or sliced through, the root core (or stele) will appear to be dark red. It may be necessary to examine several rotting roots before finding a red stele, but this symptom is very distinctive and is diagnostic for the disease. Reddened steles are relatively difficult to find after harvest because most infected roots have died and begun to decay by then.
Causal Organism and Disease Cycle:
Red stele is caused by the soil borne fungus Phytophthora fragariae. This fungus is not a natural inhabitant of most agricultural soils but probably is introduced on nursery stock or by the movement of infested soil and run-off water from fields in which the disease occurred previously.
P. fragariae is very persistent and can survive in a field for many years once it has become established, even if no strawberries are grown during that time. The organism that causes red stele of strawberry is not known to cause disease on any other crop, with the possible exception of loganberry.
P. fragariae persists in the soil as thick-walled resting spores (oospores). When the soil is moist or wet, some of the oospores germinate and form structures called sporangia, which are filled with the infectious spores of the fungus (zoospores).
These microscopic zoospores are released into the soil when it becomes completely saturated with water (flooded or puddled) and use tail-like structures to swim short distances through water-filled soil pores to the tips of strawberry roots, to which they are chemically attracted.
Zoospores also may swim to the soil surface, where surface run-off water may carry them relatively long distances. Zoospore activity may occur at soil temperatures ranging from about 38 to 77 degrees F (4 to 25 degrees C), but is most significant from 44 to 59 degrees F (7 to 15 degrees C). Thus infection is most likely in the spring and fall. Once zoospores have infected the root tip, the fungus begins to grow up into other parts of the root, causing the characteristic dark rot and red stele symptoms.
New sporangia are formed along the outside of infected root issue and release additional zoospores whenever the soil is saturated, thereby continuing to spread the disease. The fungus produces oospores within infected roots as they begin to rot and die, and these oospores are released into the soil when the roots decay, thus completing the disease cycle.
Since significant production and movement of infective zoospores occurs only during periods when the soil is completely saturated, the key to control is drainage. Strawberries should not be planted in low-lying or heavy soils where water accumulates or is slow to drain.
Only resistant varieties should be planted in a field where red stele is known to have caused losses within the last 5 to 10 years. The following June bearing varieties are reported to be resistant to Red Stele- Allstar; Delite; Earliglow; Guardian; Lester; Midway; Redchief; Scott; Sparkle; Sunrise and Surecrop.
The ever bearing varieties are also reported to be resistant. All “resistant” varieties, however, are resistant only to certain common races of the red stele fungus and can become diseased if exposed to other races of the pathogen.
It is important to minimize the chance of introducing the red stele fungus into a field where it does not already exist. Buy nursery stock only from a reputable supplier, and take care not to transfer soil on farm implements from an infested field into a clean one. New fungicides active against red stele also help in controlling this disease but are most effective when used in combination with good soil water management practices.
Fungal diseases of the leaf may occur as soon as the first leaves unfold in early spring and continue until dormancy in the late fall. Generally, these diseases do not cause significant economic damage. The primary damage from leaf diseases is a loss of vigour through reduced leaf area. If outbreaks of these leaf diseases become significant, the plants will become weakened resulting in increased susceptibility to root diseases and winter injury.
The three major leaf diseases that are caused by fungi have a similar disease cycle and are controlled in a similar manner. Leaf spot, leaf scorch, and leaf blight are the most common leaf diseases and they all overwinter in infected dead or living leaves. They all produce spores that spread the disease by causing new infections during moist, warm conditions.
Leaf spot is caused by the fungus, Mycosphaerella fragariae. Symptoms of leaf spot first appear as circular, deep purple spots on the upper leaf surface. These spots enlarge and the centres turn grayish to white on older leaves and light brown on young leaves. A definite reddish purple to rusty brown border surrounds the spots.
On fruit, superficial black spots may form under moist weather conditions. The spots form on ripe berries around groups of seeds. The spots are about ¼ inch in diameter, and there are usually only one or two spots per fruit.
However, some fruits may be more severely infected. The fungus overwinters as spores in lesions on leaves. The fungus produces more spores in spots on the upper and lower leaf surface that spread the disease during early summer. These spores are spread by splashing rain. Middle-aged leaves are most susceptible. Lesions also develop on stems, petioles and runners.
Leaf scorch is caused by the fungus Diplocarpon earliana. Symptoms of leaf scorch consist of numerous small, irregular, purplish spots or “blotches” that develop on the upper surface of leaves. The centres of the blotches become brownish. Blotches may coalesce until they nearly cover the leaflet, which then appears purplish to reddish to brown. The fungus overwinters on infected leaves.
The fungus produces spore forming structures in the spring on both surfaces of dead leaves. These structures produce spores abundantly in midsummer. In the presence of free water these spores can germinate and infect the plant within 24 hrs. Older and middle-aged leaves are infected more easily than young ones.
Leaf blight is caused by the fungus, Phomopsis obscurans. Symptoms of leaf blight infections begin as one to several circular reddish-purple spots on a leaflet. Spots enlarge to V-shaped lesions with a light brown inner zone and dark brown outer zone. Lesions follow major veins progressing inward. The whole leaflet may turn brown. In severe cases, stolons, fruit trusses and petioles may become infected which may girdle and kill the stem.
The fungus overwinters as mycelium or fruiting structures on the old leaves that remain attached to the plant. Spores are spread by rain splash early in the spring. Leaf blight is most destructive to older leaves in the late summer. Petioles, calyxes and fruit may also be infected earlier in the season.
Leaf spot and leaf scorch are controlled most effectively by the use of resistant varieties. The following June bearing varieties are reported to be resistant to both leaf spot and leaf scorch- Allstar; Canoga; Cardinal; Delite; Earliglow; Honeoye; Jewell; Lester; Midway and Redchief. The ever bearing varieties, Tribute and Tristar, are reported to be tolerant to leaf spot and leaf scorch. There are no varieties with reported resistance to leaf blight.
These cultural practices should help reduce infection:
(i) Remove the older and infected leaves from runner plants before setting.
(ii) Take care in spacing runner plants in matted-row culture.
(iii) Plant in light, well-drained soil in a location exposed to all-day sun and good air circulation.
(iv) Control weeds in the planting. Weeds reduce air circulation and increase drying time for leaves. (Leaves stay wet longer in weedy plantings.)
(v) Removing infected leaves after harvest (during renovation) is helpful in reducing inoculum and controlling all the leaf diseases.
If leaf diseases are a problem in the planting, fungicides will aid in control.
14. Crown gall:
Crown gall is caused by the bacterium Agrobacterium tumefaciens. This bacterium has the widest host range of any plant pathogen. It is capable of causing tumors, or “galls,” on virtually all plant species, except the monocots (grasses). A similar bacterium, Agrobacterium rubi, causes galls on the canes of brambles. All fruit crops grown are susceptible.
The disease is particularly destructive on brambles (raspberries and blackberries) and grapes. It can also cause severe problems on apple, pear, blueberry, all stone fruits and on ornamentals.
The bacteria induce galls or tumors on the roots, crowns, trunks and canes of infected plants. These galls interfere with water and nutrient flow in the plants. Seriously infected plants may become weakened, stunted and unproductive.
The disease first appears as small overgrowths or galls on the roots, crown, trunk or canes. Galls usually develop on the crown or trunk of the plant near the soil line or underground on the roots. Above ground or aerial galls may form on canes of brambles and highly susceptible cultivars of grape.
Although they can occur, aerial galls are not common on fruit trees. In early stages of development the galls appear as tumor-like swellings that are more or less spherical, white or flesh-coloured, rough, spongy (soft) and wart-like. They usually form in late spring or early summer and can be formed each season.
As galls age they become dark brown to black, hard, rough, and woody. Some disintegrate with time and others may remain for the life of the plant. The tops of infected plants may appear normal. If infection is severe, plants may be stunted, produce dry, poorly- developed fruit, or show various deficiency symptoms due to impaired uptake and transport of nutrients and water.
The crown gall bacterium is soil-borne and persists for long periods of time in the soil in plant debris. It requires a fresh wound in order to infect and initiate gall formation. Wounds that commonly serve as infection sites are those made during pruning, machinery operations, freezing injury, growth cracks, soil insects and any other factor that causes injury to plant tissues. Bacteria are abundant in the outer portions of primary galls, which is often sloughed off into the soil.
In addition to primary galls, secondary galls may also form around other wounds and on other portions of the plant in the absence of the bacterium. The bacteria overwinter inside the plant (systemically) in galls, or in the soil.
When they come in contact with wounded tissue of a susceptible host, they enter the plant and induce gall formation, thus completing the disease cycle. The bacteria are most commonly introduced into a planting site on or in planting material.
(i) Obtain clean (disease free) nursery stock from a reputable nursery and inspect the roots and crowns yourself to make sure they are free from galls. Avoid planting clean material in sites previously infested with the bacteria.
(ii) Avoid all unnecessary root, crown and trunk wounding by careless cultivation and other machinery operation, and control soil insects. Any practice that reduces wounding is highly beneficial. Preventing winter injury (especially on grapes) is also beneficial.
(iii) On grapes, the double trunk system of training may be a useful system for minimizing losses due to crown gall. If one trunk is infected, it can be removed. The remaining trunk can be pruned leaving a full number of buds until the second trunk can be renewed. Galls on the upper parts of the trunk or on canes can be removed by pruning.
(iv) A relatively new biological control agent for crown gall is available for apple, pear, stone fruit, blueberry, brambles and many ornamentals. It is not effective on grape. The agent is a non-pathogenic strain of bacterium (Agrobacterium radiobacter strain 84) that protects the plants against infection by the naturally occurring strains of pathogenic bacteria in the soil.
Nursery stock is dipped in a suspension of commercially prepared Agrobacterium radiobacter strain 84 at planting time. The antagonistic bacteria act only to protect disease free plants from future infection by the crown gall bacterium; they cannot cure infected plants.