Symptoms and Management of Late Blight of Potato
Causal organism: Phytopthora infestans.
Late blight is the major limiting factor to increase potato production. In Bangladesh, the infection of late blight was first reported in 1922.
Symptoms of Late Blight of Potato
Symptoms on Leaves
1) Late Blight appears on potato or tomato leaves as pale green, water soaked spots often beginning at leaf tips or edges.
2) In moist weather, lesion enlarge rapidly and turn dark brown to purplish black. It forms brown blighted areas which are often surrounded by a pale yellowish green border.
3) A white downy growth of Sporangiospores and sporangia forms at the edges of the spot, most commonly on the lower leaf surface.
Sysmptoms on Stem
In severe cases on stem, the disease may extends on the stalk quickly and the entire crown may fall over in a rotten pulp in a day or two.
Symptoms on Tubers
If allowed to spray unchecked, the disease will reach the tubers. Affected tubers have a reddish brown decay below the skin. Early attacks of blight may not be visible on tubers but any infected tubers will rot in stored condition.
Disease Cycle of Late Blight of Potato
The Pathogen, over winter infected tubers. When that infected tubers are planted, the disease develop on the emerging plants. The fungus produces abundant spores and help in secondary spread of the disease. The spores are spread by rain water, wind and equipment. In presence of high humidity, the fungus forms sporangiospores/ Conidiophores through the stomata on the lower side of the leaves and produces zoospores (In cool weather, 12C) or Conidia (In warm weather at about 25C) on germination.
Fig: Disease Cycle of late blight of Potato (Phytopthora infestans)
Zoosporangia release many biflagellate zoospores and Conidia (asexually) and Oospores (Sexually) which may produce germ tube and infect the plant.
Favourable Condition of late blight of Potato
• Excessive humidity (90 % and above).
• Suitable temperature (10120C) for germination of Sporangia.
Disease Forecasting of late blight of Potato
The following condition are considered for forecasting the disease
▲ Night temperature below 10C for at least 4 hours.
▲ Minimum day temperature 10C.
▲ Clouds on the next day.
▲ Rainfall during the next 24 hours of at least 0.1 mm.
Control Measure/Management of late blight of Potato
1. Cultural Control of late blight of Potato
♦ Seed tubers should be taken from healthy plant.
♦ Crop residues and alternate host should be destroyed.
♦ Using disease resistant varieties. e.g. Cardinal, Diamant, Koofri sinduri etc.
♦ Crop rotation should be followed.
♦ Tubers should be collected after 7-10 days of cutting and removing top of the plant.
♦ Do not lift tubers when soil is wet.
2. Physical Control of late blight of Potato
♦ Seed (tuber) treatment with hot water, 40C at 30 minutes.
3. Chemical Control of late blight of Potato
♦ Spraying with Ridomil (0.2 %) or Dithane M45 or Copper oxichloride @ 34 g/L for 2-3 times at 10-15 days interval.
♦ Tuber should be treated by dipping in Bordeaux mixture (1%) or other fungicides e.g. Dithane M45 (0.2 %) for 35 minutes. If weather remains wet and cool, spraying should be repeated every 10 days.
♦ Dip the tubers in formalin solution for 35 minutes and cover them under the moist gunny bags for 2 hours and finally dry them before sowing.