In this article we will discuss about:- 1. Occurrence of Phytophthora 2. Symptoms of Phytophthora 3. Predisposing Causes 4. Control of the Disease 5. Salient Features 6. Economic Importance.
- Occurrence of Phytophthora
- Symptoms of Phytophthora
- Predisposing Causes of Phytophthora
- Control of the Disease Caused by Phytophthora
- Salient Features of Phytophthora
- Economic Importance of Phytophthora
1. Occurrence of Phytophthora:
The genus is closely allied to Pythium. Waterhouse (1956) reported about 75 species of Phytophthora. They are more terrestrial than Pythium and thus less dependent on free water for reproduction. Some of the species live as saprophytes but the majority live on flowering plants attacking the aerial parts and causing leaf blight, canker fruit rot diseases.
One of the species phytophthora infestans (Mont). Debary is of great economic importance and widespread interest. It is called the potato blight organism. Phytophthora literally means a plant destroyer. P. infestans a serious potato disease known as Potato resulting in foliage blight and rot of tubers.
The potato blight disease is either absent or occurs in a mild form in the plains because high temperature and drought which immediately follow the potato growing season kill the fungus in the soil and seed tubers stored at ordinary temperature. On the hills above 6000 ft. it occurs on an epidemic scale because the potato crop is sown in the rainy season. The temperature during the day is never above 22-23°C .
Since 1943 the disease has been making periodic appearance in the plains of Northern India and is considered a serious disease of potatoes. The disease was first introduced in India in the Nilgiris between 1870-1880. Excess of water coupled with a range of temperature between 22-23°C promotes optimum fungal growth. Above 26°C the mycelium perishes.
2. Symptoms of Phytophthora:
The disease manifests itself after the flowering period, usually in the month of January The first signs are small, black patches which appear at the margins and tips of the leaflets. The patches gradually enlarge and spread over the entire surface of the leaflets. Soon the disease invades the petioles and stems finally killing the entire tops of plants which fall over in a rotten pulp. Subsequently the tubers are affected (Fig. 6.27).
3. Predisposing Causes of Phytophthora:
The factors which favour the spread of the disease are:
1. Warm, muggy and moist weather.
2. Continuous rain, heavy dews and ground fog.
Day temperature over above 22-23°C.
It is suggested that in the temperature and cold regions of the world and at higher altitudes in the Tropics the fungal parasite overwinters in the form of mycelium in the infected potato tubers. With the return of favourable weather in following spring, the dormant mycelium reactivates and grows in the new tissues of the sprouts from the infected tubers forming the new mycelium.
The optimum temperature for its growth is 26°C. Above this temperature the hyphae perish. The new fungus mycelium sporulates on the aerial parts of the potato plants producing sporangiophores and sporangia. The sporangia carried by water or dispersed by wind bring about secondary infections.
4. Control of the Disease Caused by Phytophthora:
The late blight disease of potato can be effectively controlled provided suitable control measures are used at the right time and in a large scale.
The methods suggested to control the disease can be divided under the following two categories:
A. Chemical methods:
The following methods using chemicals can effectively control the disease:
Spraying the affected plants with fungicide mixtures have been used for quite some time. Bordeaux mixture is the most popular fungicide used to effectively control the disease in the field.
The early sprayings with 4: 4: 50 strength and late sprayings with 6: 6: 50 strength are commonly used. Various formulations of Fytolon Blue copper and Blitox 50 are gradually replacing Bordeaux mixture. In recent years,’ Dithane Z-78 or Dithane M-45 and Maneb have been used for spraying with effective results.
(b) Treatment of tubers before storage:
This saves the tubers from secondary infection by rot-causing organisms. A-90 minute dip of tubers in Mercuric chloride solution (1: 1000) prevents the secondary infection.
B. Agricultural methods:
The following agricultural methods are commonly used to control the disease:
(a) High ridging:
A 4-6 inches high ridging at the time of earthing helps to reduce the infection of tubers.
(b) Selection of seeds:
Infected tubers should be discarded and only disease-free stock be used for sowing. Although apparently disease-free tubers are no guarantee to loss but it certainly reduces the infection and makes spraying more effective.
The crop should be clear and all sanitary precautions must be taken to reduce the infection. Weeds should not be allowed to grow. At the time of harvesting, contact between tubers and infected leaves should be avoided.
(d) Sorting of tubers from a blighted field:
Harvesting should be done in clear dry weather and only healthy infection-free tubers should be collected. Before putting these in bins, the potato-tubers should be inspected again and only healthy potatoes should be stored.
(e) Storage of Potatoes:
The tubers should be stored in a cool, dry and well aerated chambers with temperatures ranging between 38°-40°F.
(f) Resistant varieties:
Use of resistant varieties developed throughout the world have proved to be the best method of controlling late blight disease of potato.
5. Salient Features of Phytophthora:
1. Phytophthora infestans parasitizes potato and causes the potato blight or late blight of potato.
2. The mycelium is well developed and profusely branched. It consists of aseptate, hyaline, coenocytic hyphae.
3. The hyphae are both intercellular and intracellular. The intercellular hyphae produce usually globular occasionally short, straight or curved peg-like haustoria which penetrate the cell walls of the host tissues to obtain nutrition. The haustoria are more commonly found in the tubers.
4. Asexual reproduction is by means of sporangia produced on special aerial hyphae, the sporangiophores which arise in tufts from the internal mucelium and emerge to the outside by way of the stomata of the host plant.
5. The sporangiophores are sympodially branched and are characterised by the bodular swelling at each node below the sporangium.
6. The papillate sporangia are lemon-shaped or oval and multinucleate. They are borne terminally and single but are pushed to the side by proliferation and continued growth of the hyphal tips bearing them.
7. The main factors governing germination of sporangia are moisture and temperature. The moisture is supplied by rain or dew.
8. At higher temperature in the presence of water the sporangium germinates directly by a germ tube which produces a mycelium.
9. At low temperature (cooler conditions) in the water the sporangium produces about 5-8 biflagellate zoospores.
10. The zoospores are differentiated in the sporangium itself and are liberated by the bursting of the apical papilla. No vesicle is formed.
11. The liberated zoospore germinates in the usual manner to produce a germ tube which forms an appressorium and causes infection by means of a fine, infection peg.
12. Sexual reproduction is oogamous and the fungus is heterothallic. The antheridia and oogonia are produced on two distinct, opposite mating types of hyphae.
13. The antheridial and oogonial hyphae grow towards each other. The tip of the oogonial branch punctures and grows through the antheridium and swells on the other side to form the globose oogonium above the antheridium. The latter forms a funnel-shaped collar around the oogonial stalk at the base of the oogonium.
14. The sex organs at maturity are uninucleate.
15. At the time of germination the oospore produces a germ tube which terminates in a sporangium. The protoplast of the sporangium produces the zoospores but direct germination of the sporangium by a germ tube has been reported by Smoot et al (1958).
16. At times the directly germinates by a germ tube.
6. Economic Importance of Phytophthora:
Phytophthora infestans or potato blight fungus is of considerable economic and historical interest. It causes a serious disease of potatoes known as Potato blight or late blight of potatoes.
In an epidemic form the disease may cause complete crop failure or serious losses in potato crops. This disease caused famines in the past in countries where potato was the mainstay in the diet of the peasantry. An example is the catastrophic potato famine that swept over Ireland in 1845-46.
It resulted in the death of over a hundred thousand people from Spread hunger starvation and was the cause of emigration of many to America in the Eastern united is in 1846. The disease caused loss of about 50 million dollars. The potato blight fungus also causes the late blight of tomatoes. The tomato fruits are infected and undergo rot at all stages.