In this article we will discuss about the early blight of potato caused by deuteromycetes.
Host: Solanum Tuberosum
Pathogen: Alternaria Solani
Introduction to the Early Blight of Potato:
Early blight is a common foliage disease of potato and tomato. It is of common occurrence both in cold as well as in warm regions in India and abroad where, potatoes and tomatoes are grown.
According to Mathur, Singh and Nagarkote (1971), there have been outbreaks of early blight of potato during the past-5 or 6 years in the plains of Uttar Pradesh. The pathogen causes injury to the leaves as a leaf spot disease and instigates permature defoliation. Finally it incites tuber rot of potato and fruit rot of tomato.
Symptoms of Early Blight Disease:
The disease appears on the leaflets, 3-4 weeks after the crop is sown as small, isolated, scattered pale brown to dark spots, oval or angular in shape mostly up to 3 or 4 mm in diameter. Each spot is usually delimited by a narrow chlorotic marginal zone which fades into the normal green.
The chlorotic zone increases with the increase in size of the spot. As a rule the oldest (lowest) leaves are affected first and the disease progresses upwards.
The necrotic tissue of the spot often shows a series of concentric ridges which produce a target-board effect, a symptom characteristic of this disease (Fig. 22.23 D). The number of spots on the leaflets may be a few but if the conditions are favourable the spots increase in number and size involving the entire leaf surface.
In the advanced stage when the number of spots is numerous, the leaf shows signs of old age and droops. According to R.S. Singh (1968), the spots become hard in dry weather and the leaflets curl.
Under humid conditions, the diseased areas coalesce and big rotting patches appear on the leaf surface. In severe cases of infection the leaves dry up, shrivel and drop off. Falling of leaves starts with the older (lower) ones until a few remain at the top.
Sometimes there is complete defoliation. Stems and petioles may also develop brown to dark lesions which may finally lead to either worthless plants or collapse of the entire over-ground portion of the plant.
Folsom and Bonds (1925) reported infection and rotting of potato tubers. The surface lesions on the potato tubers are a little darker than the healthy skin and slightly sunken.
They are irregular or circular in shape up to 2 cm in diameter. The tissue beneath the lesion shows a brown corky dry rot up to 6 mm in depth. The older lesions develop fissures.
Effects of Early Blight Disease:
The effect of disease on the potato crop may sometimes be considerable. The peak period of attack of foliage blight on potato usually coincides with the time when the plants have begun formation of tubers.
The injury to the foliage which in extreme cases may lead to premature defoliation reduces photosynthetic activity of the plant. Consequently the tubers formed remain small and a few in number. This reduces crop yield. The disease as well instigates rotting of tubers.
The pathogen causing early blight of potato and tomato is a form-species Altemana solani (Ell. and Martin) Jones and Grout of form class Deuteromycetes.
The mycelium consists of light brown, slender, septate sparsely branched hyphae which become dark-coloured with age. The hyphae ramify in the intercellular spaces but later penetrate the cells of the invaded tissues (Fig. 22.23 C).
The conidiophores which are relatively short (50- 90 µ long and 9 µ broad) and dark-coloured arise from the older diseased tissue of the host and emerge through the stomata. The conidia which measure 120 to 296 µ in length and 12-20 µ in breadth are dark-coloured, beaked, muriform and multiseptate (Fig 22 23 E).
There are 5-10 transverse septa and a few longitudinal ones. The beak is long, septate and rarely branched. The conidia are borne singly but in pure cultures, in short chains of two. Each conidium develops from a bud formed on the terminal cell of the conidiophore.
The mature conidia are detached readily and dispersed chiefly by air currents, water and insects. On a suitable host they germinate readily in moist weather each by putting out 5-10 germ tubes (Fig. 22.23 E1 and B).
Disease Cycle (Fig. 22.23):
The mycelium and conidia of the pathogen remain viable for a considerable time, the former for about a year or more in the infected dry leaves and the latter for 17 months at room temperature.
The source of primary inoculum is the infected plant debris such as the dried leaves, stems, potato tubers and contaminated tomato seeds. Primary infection may be brought about by conidia (A) or mycelium from the infected debris in the soil.
According to Walker (1969), the conidia germinate (B) at the optimum temperature of 28° to 30°C within 35 to 45 minutes. The germ tubes gain entry into the lower leaves of the host plant through stomata.
Walker (1969) reported that the fungus penetrates the host leaf and stem directly through the epidermis (C). The incubation period varies from 48 to 72 hours. The primary infection usually appears on the foliage as leaf spots within 2 or 3 days under favourable conditions of temperature and moisture (D).
Secondary spread immediately follows through conidia produced on the primary spots within 5-7 days after infection. Conidia start forming when the spots are about 3 mm in diameter. Rand (1917) reported that heavy dew with rains now and then promote abundant sporulation.
The mature conidia are readily detached and dispersed through the agency of air currents, water and insects. Flees, beetles and Colorado bettles are reported to assist infection by carrying conidia on their bodies and facilitate penetration of germ tubes by inflicting wounds on the host surface.
It is held that some of the pathogenic effects of the disease such as chlorosis are due to the secretion of a toxic material by the fungus. Brain et al. (1952) isolated the toxin from culture filtrates and named it alternaric acid.
Epidemiology of of Early Blight Disease:
Climatic or soil conditions which are unfavourable to the host and thus reduce its physiological vigour tend to promote the disease.
The disease spreads and becomes serious when the season begins with abundant moisture followed by high temperatures because these conditions are unfavourable to the host. They reduce its physiological vigour. Weaker plants are more susceptible to disease than the healthier ones. On the other hand, high soil fertility tends to reduce the severity of disease. Periods of continued drought also check its spread.
Control Measures of Early Blight Disease:
As in the case of other soil borne diseases, crop rotation and field sanitation are the two rational measures which provide effective check against primary infection from spores in the infected plant debris. Infected dry leaves and dead haulms should be raked together and burned immediately after harvest.
Application of regular spray schedule is another effective measure. When the plants are 15- 20 cm high, weekly sprays (with Bordeaux mixture or other suitable fungicides) throughout the period of plant growth effectively control the early blight. Mathur, Singh and Nagarkote (1971) found that spraying potato crop with Dithane M-45 was most effective against early blight disease.
It also resulted in significant increase in the yield. Vorster (1962), Bebrchen and Brien (1965) and Harison et al. (1965) reported similar results with manganese carbamate from other parts of the world. Compacci and Santo (1959), Paharia (1961) and Kadyroa (1964) recommended repeated application of Zineb. The use of Brestan 60 has also proved profitable.