Author
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Deahl, Kenneth |
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Submitted to: American Journal of Potato Research
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 3/15/1998 Publication Date: N/A Citation: N/A Interpretive Summary: During recent years, the potato and tomato industries have seen major disease losses due to the resurgence of the late blight fungus. More than 150 years after it devastated potato crops in the United States and Europe and led to the Irish potato famine, the plant pathogenic fungus, Phytophthora infestans, is again creating a major plant health problem. In this paper, we describe the renaissance of late blight, document how contemporary strains of pathogens are increasingly more fungicide-resistant and monitor the occurrence in the U.S. and Canada of a second worldwide migration and population displacement of this organism. These "new" exotic strains of the pathogen are more virulent and aggressive and we show that the new strains belong to a different mating type from resident populations. Unfortunately, concurrent infection of both strains means sexual reproduction is very possible, as well as even greater genetic diversity. Since these samples were sent for identification from regions reporting late blight by colleagues throughout the U.S. and Canada, this information will be used by growers and scientists in designing new disease management strategies. Technical Abstract: In a collaborative survey of the U.S. & Canada from 1992-95, 865 isolates of the fungus that causes late blight, Phytophthora infestans, were collected in 25 states and 6 provinces. Isolates were analyzed for mating types and response to the phenylamide fungicide, metalaxyl. Most isolates were obtained from infected potato and tomato tissues by plating tissue onto selective media containing antibiotics. Other isolates were received as axenic cultures. Data on the occurrence of the two mating types isolated from diseased samples indicated an overall increase in the frequency of A2 mating type over this period. In each of the 4 years, the percentage of samples from which A2 isolates were obtained was 5.4, 36, 74, and 88%, respectively, in each consecutive year from 1992. In contrast the percentage of samples which A1 isolates were obtained was 94, 65, 25 and 12% respectively, in the same consecutive time period. The incidence of resistance to metalaxyl among isolates increased from 40 to more than 90% between 1992-95. Some of the A2 isolates were sensitive to metalaxyl but most were insensitive. Although sexual reproduction is possible where A1 and A2 isolates occur, genetic recombination could not be established from the data. |
