Late blight caused by Phytophthora infestans is the major disease of potato. Scientists place this fungus in a group called ‘Oomycetes’, which are water molds. The pathogen is highly aggressive and can potentially infect all plant parts, causing rapid die back and death. In Nepal, more than 2l potato diseases have been reported among which late blight is economically important. In high hills above 2000 m above mean sea level yield loss in potato due to late blight has been recorded up to 90 percent.
Host crop for late blight: The primary hosts of Late blight are potatoes and tomatoes; however, other related plants can also be affected. These include eggplants, peppers, petunias, and Solanaceous weeds, such as nightshade species and wild tomato.
How does the disease spread?
In season, the disease spreads by spores produced on infected plant material, such as transplants, volunteer potatoes, weeds, and diseased crop debris. This pathogen produces two types of spore called sporangia and zoospore. Sporangia can move between plants within fields by rain or water splashing or short distances in soil, for example, zoospores wash off foliage and move through the soil in water films to infect tubers. Sporangia can also move long distances, possibly up to 100 km, on the wind or in storm fronts.
General plant symptoms: Brown irregular-shaped spots occur on the leaves spreading from the tips and margins and expanding rapidly. The spots are not limited by the leaf veins. Within a few days, the leaves turn yellow, shrivel, and die. Black or brown spots occur on the stems. Spores, formed in the white cottony growth at the edge of the spots on leaves and stems, wash into the soil and infect the tubers through cracks, “eyes” (buds) and lenticels, and cause brownish-reddish rots; the tubers become soft and smelly as bacteria invade and destroy them. Rots can also form later in apparently healthy tubers in storage. Plants are destroyed a few days after the first spots appear.
Disease epidemiology: The cool (12–15°C) and high humidity (>90%) weather with heavy dews or rains alternating with warm (18–20°C) moist period favor for rapid development of the disease. Infection and disease development is observed in a range of 7.2–26.6°C
Prevention and control measures: There are different approaches and methods for the prevention and control of the diseases, among them, some of the important methods are given below.
- Use tolerant varieties, e.g. Jankdev, Khumal, laxmi, IPY8
- Use planting material from reliable sources e.g. registered agro-dealers, government farm
- Select healthy seeds for planting (a good seed size is 25-40 gm)
- Adjust planting date according to the weather to prevent appropriate weather conditions for disease development.
- Check for volunteer plants from the previous crop and, if found, remove and burn them.
- Select sites where there is good drainage, and where there is good air movement, so that leaves dry quickly after rain and dews. Avoid fields surrounded by trees.
- Practice crop rotation. Do not plant crops where they were grown in the previous season or year. Use 2-3-year crop rotation.
- Choose a short duration, “early” variety that sets fruits and matures quickly to produce a crop in the shortest time possible, and potentially avoid a serious build-up of disease.
- Use 7 kg Urea, 11kg Di-Ammonium Phosphate and 5 kg Muret of Potash/ropani, splitting it 2.5 kg urea at field preparation and 4.5kg during earthing-up stage
- Apply well-decomposed compost during field preparation (mix with Trichoderma- if available – at 2.5kg /1 mt compost /ropani
- Plant the seed potatoes on ridges so that the spores have further to travel to reach the tubers.
- Avoid overhead irrigation; otherwise, conditions will be created for the production of spores and their infection of both leaves and tubers.
- Remove self-grown potatoes and Solanum weeds (i.e., volunteer plants) as they may have late blight infections.
- Do not apply too much nitrogen fertilizer as this will increase the growth of leaves, and also delay the time to crop maturity.
- Frequently, inspect the crop for spots on the leaves, especially if fungicides are not being used routinely to prevent infection.
- When it is wet, foggy, or frosty monitor your crop every day, look for downy growth, and watery soaked lesions on the underside of leaves and brown lesions on the upper side. If symptoms exist to go for immediate intervention
- If it is not wet, foggy or frosty check your crop once a week
- If there is frost, irrigate the field – this will increase temperatures and help reduce disease incidents.
- Destroy the leaves before harvest if late blight is present to avoid the infection of tubers when they are lifted. Use a herbicide to kill the leaves.
- After harvest:
- Do not leave rejected tubers in cull piles in the field, otherwise, they will provide a source of spores for the next crop
- Earth-up one week after emergence
- The leaf extracts from onions, garlic, Malustoringo, Reynoutria japonica, and Rheum coreanumrevealed positive inhibition of mycelial growth of infestans.
- The antagonist Bacillus subtilis B5 was found effective in inhibiting the growth of infestans
- Various naturally occurring microorganisms, i.e., Trichoderma viride, Penicillium virdicatum, P.aurantiogiseum, and Penicillium aurantiogriseum showed antagonistic effect against infestans
- The most widely used fungicide is Dithane M-45 (mancozeb) at 3 gm/Lit to both side of the leave
- The systemic fungicide ridomil (metalaxyl) at 2gm\lit can be used.
- Apply copper oxychloride at a rate of 1.5g/lit ensure spraying on both sides of the leaves at 7-10 days interval for 3-4 times