Late blight of potato and its management (control)
What is late blight of potato?
Late blight of potato is one of the most destructive among all the potato diseases. If the conditions are favorable, then no other disease can be as destructive and dangerous as late blight. Timely and proper control, and management of the disease is very necessary and important to avoid heavy loss.
Host: Potato and tomato are the two main host of this disease.
|(1). Appearance (chronology)|
|(2). Characteristic symptoms or of late blight of potato|
|(3). Presence of purplish lession|
|(4). Symptoms of tube infection|
|(5). Cause of the disease (causal organism)|
|(6). Forecasting of the disease|
|(7). Disease cycle|
|(8). Mode of infection|
|(9). Management of the disease|
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(1). Appearance (chronology)
- 1870 to 1880: First appearance, Nilgiri hills of South India.
- After 1880: Second appearance, Darjeeling district of Himalayas.
- 1889 to 1900: Third appearance, Hoogly district of West Bengal.
- 1912 to 1913: Severe attack in Jorhat district of Assam.
- 1928: Pusa district of Bihar.
- 1933: Patna district of Bihar.
- 1943: Meerut district of Uttar Pradesh, and Dehradun district of Uttarakhand.
- After 1943: Plains of India.
(2). Characteristic symptoms or of late blight of potato
Symptom first appears on foliage. Lower leaves show first symptom. Presence of brown dead spots on leaves, and presence of the extended dead areas on leaves. At first, this will look like chlorotic, then it turns brownish black. The size of the spots in not fix. Dead areas appears at the tip of leaf or the margin, from where it spread downward and inward. Moist condition deteriorates the leaves very soon. Wet rot is common in these situations. Dry condition may cause the curling of leaves with hard leaf surface.
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(3). Presence of purplish lession
It is one of the characteristic symptom. A zone outside the purplish lession shows a paler than normal green merging into the latter. On the lower side of the leaf apposite to this zone a mildew growth appears which is whitish or grayish. This whitish area consists of the aerial fructification of the pathogen. Mildew appears in dry season. In cool moist weather, the pathogen becomes active and corresponding symptoms, and progress reappear.
(4). Symptoms of tuber infection
- The first sign of tube infection is a brown to purple discoloration of the skin.
- Latter, it turns into a brownish dry rot, which is about a half an inch below the surface.
- Heavy infestation causes rotting of tubers.
- In moist atmosphere, white tufts of mycelium and sporangiophores of the fungus appear on the surface of the tuber in the stores.
(5). Cause of the disease (causal organism)
|Fungus||Phytophthora infestans (Mont.) de Bary|
5.1: Time of appearance
At any time of during growth of plant.
Most common season is January.
The sporangium first develops at the tip of a branch. Sporangia are multinucleate. The method of germination of sporangia is governed by temperature. Low temperature favours the formation of zoospores. High temperature (24°C or above) favours the formation of sporangium. After few minutes, the zoospores germinate by a tube which penetrates the epidermis of the host directly or through stomata. The ideal temperature for the formation of oospores is 23°C. P. infestans has a large number of races differing in pathogenicity to host genotypes. Maximum outbreak of the disease happens when conditions are favorable for germination of sporangia by zoospores.
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(6). Forecasting of the disease
(1). Night temperature below dew point for at least 4 hour.
(2). Minimum temperature of 10°C.
(3). Clouds on the following day.
(4). Rainfall during the next 24 hours of at least 0.1 mm.
(7). Disease cycle
The viability of sporangia in soil depends on moisture and temperature. Optimum soil moisture is 15 to 20% saturation stage. High temperature and drought kill the fungus in soil. Infected tubers left in the field carry over the living mycelium to the next crop season. The mycelium in these infected seed tubers serves as the source of primary inoculum. It can tolerate exposure to 0°C.
The mycelium from the infected parent tuber grows upward in the stem and sporulates on the shoots. These are infector plants. Fungus may also develop from the tuber in soil. These fungus produce spores which infect lower leaves in contact with the soil.
(8). Mode of infection
The infection by sporangia or zoospores take place at any part of the epidermis of leaves and stem, especially young stems.
Germination of zoospores occurs frequently in susceptible varieties. The main places of tuber infection are, eyes, lenticels, and wounds. Susceptibility of eyes depends on the length of storage and maturity of tuber. Spores present at the top of the plant washed down by the rain. These spores infect the healthy tuber over time. Loose and light soil support the inoculum.
Infection also depends on the depth of the soil. Its infection is common in shallow rooted tuber, e.g., 5 c.m. Tubers preset at the soil depth of 15 c.m. are safe from this disease. Frequent rain cause severe loss if 50% of potato plants are already affected. Contact of healthy tubers with unhealthy leaves at the time of harvesting is an another source of infection. Infection is also common in storage among healthy and unhealthy tubers.
(9). Management of the disease
- Selection of seed tubers.
- Time of harvesting.
- Sorting of potato.
- Treatment before storage.
9.1: Selection of seed tubers
- Ovoid the selection of infected tubers.
- Closely inspect the seed tuber during selection.
- Reject the infected tubers.
- Seed treatment doesn’t check late blight.
- Use healthy tubers to delay onset of the disease and make fungicidal sprays economical and more effective.
- Keep the field free from weed.
- Check field humidity.
- Don’t grow solanaceous crop on the ridge of field.
- Perfom dehaulming before harvesting.
- Destroy the plant debris.
9.3: Time of harvesting
- Alter the time of harvesting.
- Delay the time of harvesting.
- Let the plant be fully mature.
- Kill the potato haulms 2 to 3 weeks prior to harvesting of tubers.
- This practice will eliminate the foliage as well as inoculum.
9.4: Sorting of tubers
- Dig the tubers in a good dry weather.
- Collect sound tubers.
- Directly store it.
9.5: Treatment before storage
- This prevents secondary rot infection.
- Use mercuric chloride.
- The ratio of solution should be 1:1000.
- Use potassium metabisulfite to prevent post harvest decay.
- In vitro treatment: Use hydrogen peroxide (1:50) to prevent tuber rot in storage.
- Store the tubers in cool, dry, and well aerated room.
- Probability of determination is less.
- Avoid moist environment.
- Best temperature for storage is 4°C to 5°C.
- High ridge: Make 15 cm high ridge during earthing up. It will check the infection.
- Mulching: Use polythene mulch to prevent the spreading of inoculum in healthy tubers.
- Phosphonic acid: Give a single spray of phosphonic acid during the mid or late season.
- Weekly foliar spray of phosphonate, phosphate, phosphate potassium, sodium or ammonium suppress pre harvest infection.
9.8: Chemical treatment
- Do it when the seed tubers are the primary source of inoculation.
- It will increase plant emergence and improved crop uniformity.
- You shoul use thiphanate methyl + mancozeb.
- Use dimethomorph + mancozeb or cymoxanil + mancozeb for foliar spray.
(1). Dying of top portion of a potato plant.
(2). Drying of top portion of a potato plant.
(3). Time of appearance of late blight in potato.