![]() ![]() Late blight resistant varieties are available. Do not leave rejected tubers in cull piles in the field, otherwise they will provide a source of spores for the next crop.Destroy the leaves before harvest if late blight is present to avoid the infection of tubers when they are lifted.Frequently, inspect the crop for spots on the leaves, especially if fungicides are not being used routinely to prevent infection (see under Chemical Control).Do not apply too much nitrogen fertilizer as this will increase the growth of leaves, and also delay the time to crop maturity.Remove self-grown potatoes and Solanum weeds (i.e., volunteer plants) as they may have late blight infections.Avoid overhead irrigation otherwise, conditions will be created for the production of spores and their infection of both leaves and tubers.Plant the seed potatoes on ridges so that the spores have further to travel to reach the tubers.Choose a short duration, 'early' variety that set fruits and matures quickly to produce a crop in the shortest time possible, and potentially avoid serious build up of disease.Do not plant crops where they were grown in the previous season, or year. Select sites where there is good drainage, and where there is good air movement, so that leaves dry quickly after rain and dews.Check for volunteer plants from the previous crop and, if found, remove and burn them.If infected by late blight they will rot. If unsure about freedom from late blight, test a few seed tubers by lettings them sprout at 15-20☌ for 10-15 days. It is important to use certified seed, i.e., that which is free from late blight infection, and from viruses.This means getting rid of the cull piles, and also any other unharvested tubers left in the fields. Note it is important for all growers in a region to cooperate in getting rid of the sources of the disease. Until recently the movement of tubers from countries where the A2 strain existed was regulated, but with the wide distribution of that strain in recent year, the policy has been reassessed. Spores are produced on the surface of tubers in the cull piles (reject tubers), and on volunteer plants from tubers left in the ground. Survival of late blight between crops occurs as oospores, where A1 and A2 strains occur together, but this is not as important as survival in tubers. Sporangia can travel as far as 15-20 km. Spread over longer distances, across countries and continents, occurs in tubers used as 'seed' for planting. Spread of late blight over short to moderate distances between plants and fields occurs as sporangia and zoospores in wind or wind-driven rain. Under those conditions, sporangia and zoospores spread the disease rapidly and fields of potato are destroyed in a less than 2 weeks. Oospores germinate and produce sporangia.Įpidemics of late blight occur when night temperatures are cool, followed by warm days with mists and rains. There are two ways of doing this depending on temperature: (i) in warmer conditions (21-26☌), the spores germinate like seeds and infect the plant directly, or (ii) in cooler conditions (18-22☌) each spore produces 6-8 smaller spores (called 'zoospores') which burst out of the larger spore and swim short distances over the plant surface before germinating and infecting. Sporangia and zoospores survive only for a few days in plant remains or in the soil.īy contrast, oospores are resting spores they have thick walls for survival either in plant remains, in the soil or in the tubers. If the sporangia land on a potato plant, and the surface is wet, they germinate. The sporangia on the leaves, stems and tubers within the cottony growth are spread by wind, rain and water splash. Late blight has two ways of reproducing (i) spores called 'sporangia' are produced without going through a sexual process or (ii) spores called 'oospores' are produced when two strains (A1 and A2) come together and mate. Plants are destroyed a few days after the first spots appear. Rots can also form later in apparently healthy tubers in storage. Spores, formed in the white cottony growth at the edge of the spots on leaves (Photo 2) and stems, wash into the soil and infect the tubers through cracks, 'eyes' (buds) and lenticels, and cause brownish-reddish rots (Photo 3) the tubers become soft and smelly as bacteria invade and destroy them. Within a few days the leaves turn yellow, shrivel and die (Photo 1). The spots are not limited by the leaf veins. ![]() Brown irregular-shaped spots occur on the leaves spreading from the tips and margins and expanding rapidly. Symptoms on potato and tomato are similar. The life cycle, however, has many fungal-like characteristics. Phytophthora infestans is not a fungus, but an oomycete or a water mould, related to algae.
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