Inoculum sources and genotypic diversity of Phytophthora infestans in Southern Flevoland, the Netherlands

Genotypic changes in populations of Phytophthora infestans in Southern Flevoland (150 km(2)) were analysed by characterising isolates from potato refuse piles, conventional and organic potato fields, and potatoes and tomatoes in allotment gardens for mating type (1712 isolates) and DNA fingerprint pattern using probe RG57 (1048 isolates). The overall percentages of genotypes (and of isolates) that were A2 varied from 32 (4) in 1994 to 45 (56) in 1996. Among the 1048 isolates 170 different genotypes were identified, of which 138 (81%) were 'rare' (i.e., detected in only one sampling site in the research area during 1993-1996). Many rare genotypes were encountered in organic potato fields and in allotment gardens. In 1994 and 1995, four genotypes were abundant. The highest percentages of isolates with these 'common' genotypes were encountered in refuse piles and conventional potato fields. The common genotypes were nearly absent in 1996, suggesting that the population may have passed through a bottleneck at the transition from 1995 to 1996. The Shannon index of genotypic diversity was high in allotment gardens and in organic potato fields. For the total populations the normalised Shannon index of genotypic diversity increased from 0.34 in 1994, with weather favourable to late blight, to 0.61 in 1996, with unfavourable weather. The high numbers of rare genotypes detected every year indicate that oospores may act as an infection source in commercial potato fields. However, refuse piles were identified as the most important infection sources for commercial fields in 1994 and 1995. In 1996 disease in commercial organic fields was probably initiated by a few genotypes originating from seed tubers. In allotment gardens oospores were probably the most important infection source.