Infected seed tubers, tomato transplants, potato cull piles and/or other volunteer potatoes are extremely important sources of inoculum. Temperatures ranging from approximately 50 to mid 90s oF will enable disease progression in the field (Franc et al., 1996).ĭisease is initiated by fungus spores or "inoculum" produced on the surface of living, infected plant tissue. The late blight fungus is especially adapted for growth under conditions where water is present and cool temperatures persist. Late Blight is caused by the fungus Phytophthora infestans. Refer to the "Compendium of Potato Diseases." Infected tubers are commonly invaded by secondary decay organisms such as soft-rot bacteria and, therefore, are quite likely to decay during storage. Older lesions may become firm and sunken due to water loss and tubers will appear shrivelled. Lesions may start as a superficial decay that continues to develop after tubers are harvested and placed into storage. When cut open, affected tissue is water-soaked, reddish brown and extends with an irregular margin into the tuber flesh. Tuber lesions first appear as irregular, dark blotches. See pictures of late blight on stem and leaves. As conditions become moist and cool, disease development resumes. Under continuously wet conditions, the disease progresses rapidly and warm, dry weather will slow or stop disease development. On the leaf underside, a spore-producing zone of white moldy growth approximately 0.1 - 0.2 inches wide may appear at the border of the lesion. Under moist, cool conditions, water-soaked spots rapidly enlarge and a broad yellow halo may be seen surrounding the lesion (Mohan et al., 1996). Late blight will first appear as water-soaked spots, usually at the tips or edges of lower leaves where water or dew tends to collect. A2 may multiply faster than the A1 mating type. Soil survival from season to season may be possible. The A2 mating type can result in oospores which do not need a living host. This would result in a much greater genetic diversity. The concerns about the A2 mating type are that sexual reproduction can now occur between the two types when they are in the same field. But, there have been reports in North America of the appearance of strains that are A2 type (Goodwin and Drenth, 1997). The late blight pathogen (US-1) that has appeared in the past is an A1 type. There is also concern over the appearance of A2 mating types of the pathogen. Some of these new pathogenic strains are also more aggressive and virulent than the old (US-1) strain that affected the world in the 19th century. These mutations, most notably strain US-8, have made the pathogen resistant to control by metalaxyl, the stand-by fungicide for many years. In the late twentieth century, there have been major re-occurrences and concern around the world over this pathogen and its disease due to recent mutations (Fry and Goodwin, 1997). Late blight was responsible for the Irish potato famine in the mid-nineteenth century (Daly, 1996). Late blight (Phytophthora infestans) fungus is in the same genus as the fungus causing pink rot (P.
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