DEVELOPMENT OF POTATO LATE BLIGHT EPIDEMICS - DISEASE FOCI, DISEASE GRADIENTS, AND INFECTION SOURCES

Natural potato late blight epidemics were studied to assess the relati ve impact of various inoculum sources of Phytophthora infestans in Sou thern Flevoland (the Netherlands) from 1994 through 1996. Disease surv eys were combined with characterization of isolates for mating type an d DNA fingerprint pattern using probe RG57. Seventy-four percent of th e commercial potato fields with early foci were clearly associated wit h nearby infested refuse piles. Characterization of isolates from refu se piles and fields confirmed the association. Infected seed tubers, v olunteer plants, and infested allotment gardens appeared to be of mino r importance for late blight development in potato fields, Several foc i in refuse piles, potato fields, and allotment gardens contained more than one genotype. Due to favorable weather in August 1994, infested organic potato fields became major inoculum sources, resulting in the spread of P. infestans to adjacent conventional potato fields. Analyse s of disease gradients, both at the field and regional levels, confirm ed the role of the organic fields as mid-season infection sources. The mean slope of field gradients downwind of refuse piles (point sources ) was significantly steeper (100-fold difference) than the mean slope of field gradients downwind of organic fields (area sources). The geno typic composition of the P. infestans populations along the gradient a nd of the source populations in the organic potato crops did not diffe r significantly. Analysis of the region gradient revealed genotype-spe cific disease gradients. Control measures are recommended.