Photo 2. Underside of potato leaf with spot caused by late blight, Phytophthora infestans. Note spores form on the margin of the spot.
Potato late blight
Worldwide. Wherever potatoes are grown. Asia, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded on potato from American Samoa, Australia, Fiji, New Caledonia, New Zealand, Papua New Guinea. It is recorded on "Solanaceae" from American Samoa.
Potato, tomato (see Fact Sheet no. 261), and wild species in the potato (Solanum) family, e.g., nightshade (Solanum nigrum). There is evidence of strains attacking either potato or tomato, and there may be species that are specific to other hosts.
Phytophthora infestans is not a fungus, but an oomycete or a water mould, related to algae. The life cycle, however, has many fungal-like characteristics.
Symptoms on potato and tomato are similar. 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 (Photo 1). Black or brown spots occur on the stems. 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. Rots can also form later in apparently healthy tubers in storage. Plants are destroyed a few days after the first spots appear.
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.
The sporangia on the leaves, stems and tubers within the cottony growth are spread by wind, rain and water splash. If the sporangia land on a potato plant, and the surface is wet, they germinate. There are two ways of doing this depending on temperature: (i) in warmer conditions (21-26oC), the spores germinate like seeds and infect the plant directly, or (ii) in cooler conditions (18-22oC) 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.
By contrast, oospores are resting spores; they have thick walls for survival either in plant remains, in the soil or in the tubers. Oospores germinate and produce sporangia.
Epidemics of late blight occur when night temperatures are cool, followed by warm days with mists and rains. Under those conditions, sporangia and zoospores spread the disease rapidly and fields of potato are destroyed in a less than 2 weeks.
Spread of late blight over short to moderate distances between plants and fields occurs as sporangia and zoospores in wind or wind-driven rain. 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.
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. Spores are produced on the surface of tubers in the cull piles (reject tubers), and on volunteer plants from tubers left in the ground. See life cycle (Diagram).
Late blight is one of the world's worst plant diseases, despite efforts to control it for more than 150 years. It causes large losses in yields of potatoes and tomatoes, and requires the application of expensive fungicides to maintain control. Until the 1980s, the A2 strain was known only in Mexico. Since then it has spread to many countries, and where mating with the A1 strain occurs the offspring are more aggressive, produce more spores and have increased tolerance to fungicides.
CIP (the International Potato Center, Peru) estimates that about 15% of the annual production of developing countries is lost to potato late blight each year. The cost is estimated at about US$3 billion.
Look for spots and patches on the leaves which grow rapidly and produce a furry white growth on the underside. Look for the white growth after putting the leaves in a plastic bag overnight. Check the plants regularly for infections, especially when the days are cool and wet, or overcast with heavy dews, fogs or mists.
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.
Note it is important for all growers in a region to cooperate in getting rid of the sources of the disease. This means getting rid of the cull piles, and also any other unharvested tubers left in the fields.
Late blight resistant varieties are available. They are being bred continually by CIP, so check if they are available in your country.
The following points are important:
AUTHOR Grahame Jackson
Information from CABI (2015) Phytophthora infestans (Phytophthora blight) Crop Protection Compendium (www.cabi.org/cpc); and information (and Diagram) from Schumann G, D’Arcy CJ (2000) Late blight of potato and tomato. The Plant Health Instructor. DOI: 10.1094/PHI-I-2000-0724-01. Updated 2005. Photo 2 Howard F. Schwartz, Colorado State University, Bugwood.org. Photo 3 Agriculture Research Service, United States Department of Agriculture.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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