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.
0 kommentar(er)
