Tomato late blight fungicides12/29/2023 ![]() Late blight can be a devastating disease of tomato. Check tomato plants regularly for infections, especially when the days are cool and wet, or overcast with heavy dews or mists. Look for the white growth after putting the leaves in a plastic bag overnight. Look for spots and patches on the leaves which grow rapidly and produce a furry white growth on the underside. The A2 strain has spread to many countries, with the result that there are new strains that are more aggressive, produce more spores and have increased tolerance to fungicides. ![]() There is now the potential for new strains of the oomycete through sexual reproduction, and these may be more aggressive, and/or tolerant to fungicides than those present previously. Previously only one strain (A1) was known in Europe. Since the 1990s, a second strain (A2) has appeared in Europe, presumably and introduction from Mexico. It causes large losses in yields of tomatoes (and potatoes), and requires the application of expensive fungicides to maintain control. Late blight is one of the world's worst plant diseases, despite efforts to control it for 150 years. Under those conditions, zoospores spread the disease rapidly and entire planting are destroyed in a few days (Photos 5&6). Epidemics of late blight occur when night temperatures are cool, followed by warm days with mists and rains. It can be present in the gel around them, and also in the seed coat. Long distance spread is possible on seed. Spread by wind or wind-driven rain can occur as far as 15-20 km. There are two ways of germinating: (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 about 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. If the spores land on a tomato, and the surface is wet, they germinate. Spores of the oomycete on the leaves, stems and fruit are spread by wind or water splash. It is also called a 'resting spore' as it can survive for some time in plant remains or in the soil. In the second method, the mating strains produce a spore called an 'oospore', which is thick walled. In the first method, spores called 'sporangia' are produced on leaves, stems and fruit, and these germinate and produce more spores of the same kind. Late blight has two ways of reproducing (i) it produces spores without going through a sexual process or (ii) two strains come together and mate. Spores from the leaves of potato wash into the soil and infect the tubers and cause rots. On the young leaves, black or brown irregular-shaped spots occur they expand rapidly, forming a white cottony growth on the underside, and within a few days the leaves turn yellow, shrivel and die (Photos 1-4). Dark brown, firm, rots occur on the fruit of tomatoes (Photo 2). Symptoms on tomato and potato leaves and stems 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. There is evidence of strains attacking different hosts. ![]() 265) and wild species in the potato ( Solanum) family, e.g., nightshade ( Solanum nigrum). It is recorded on "Solanaceae" from American Samoa. It is recorded on tomato from American Samoa, Australia, Cook Islands, New Zealand, and Papua New Guinea. Wherever tomatoes are grown. Asia, North, South and Central America, Caribbean, Europe, Oceania. Tomato late blight, potato late blight Scientific Name Phytophthora infestans Distribution Close-up of Photo 1 to show the destruction caused by late blight, Phytophthora infestans.
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