Three matings of Phytophthora infestans yielded 190, I 70 and I 85 single oospore progeny (SOP), for cross # I, 2 and 3, respectively. Each cross had a common UK A2 metalaxyl-sens itive (RF 040) parent. Cross # 1 A 1 parent was a UK (RF 032) metalaxylsensitive isolate. Cross #2 A 1 parent was a UK (RF 039) metalaxyl resistant isolate.
Cross #3 A I parent was both metalaxyl and propamocarb resistant and of Californian origin. Ratios of A 1 to A2 mating type was 132:40, 88:72, and 12 I :42, for cross # 1, 2 and 3, respectively. Self-fertile SOPs totalled eight, four and 16, for cross # 1, 2 and 3, respectively, and 22 SOP (10, 6 and 6, cross # 1, 2 and 3, respectively) were of undetermined mating type. The morphology of A2 SOPs were observed to be ' lumpy' and could be divided into three categories of lumpiness. Molecular markers, namely RAPDs were used to prove hybridity of SOPs and confirmed with AFLPs. Only 12% of SOPs were non-pathogenic on the most susceptible potato cv. Bintje. No correlation was observed between mating type and any of the aggressiveness components, but there was correlation between latent period and infection frequency, latent period and average lesion expansion and also between sporulation and lesion expansion . On the most susceptible of seven potato cultivars, cv. Home Guard, in cross # 1, 1 1 % of the progeny had a latent period shorter than that of the A I parent and on ly one SOP had a latent period shorter than the A2 parent. In cross #2, only 8% SOPs had a shorter latent period than either parent. In cross #3, 8% had a shorter latent period than both parents. On cv. Stirling, the most resistant UK potato cultivar used, between 5.5% and 23% SOPs were non-pathogenic and in cross # I, 2 and 3, 6%, 13% and 28% of the SOPs had shorter
latent periods than e ither parent. There were significant ranges in sens itivity of single oospore progeny to four of the five fungicides tested. Cross # I (A I MS x A2 MS) produced SOPs that were mainly intermediate. A resistant x sensitive mating (cross #2 & #3) produced all three phenotypes of metalaxyl sensitivity with the majority being intermed iate > resistance > sensitive. UK A 1 parental isolates (crosses #2) that were meta laxyl-resistant were a lso resistant (or intermediate) to the other fungicides tested except on azoxystrobin where a ll isolates were sensitive. Likewise, the metalaxyl -sensitive A 1 parent (cross# 1) was a lso sensitive to a ll the other fungic ides. Although the A2 UK parental isolate was sensitive to metalaxyl, it was res istant to cymoxanil and intermediate to propamocarb hydrochloride and fluazinam. Except for azoxystrobin, all crosses produced sens it ive, intermediate and resistant single oospore progeny.
Sporangial asexual progeny from the parental isolates and from some SOPs were not d ifferent from the parental isolates for fu ngic ide sensitivity. However, some zoosporangial progeny di ffered from the ir sporangial parents. One cymoxanil sensitive sporangial progeny of the A2 parent gave rise to five very resistant zoosporangial progeny. These aspects of variation are discussed in the context of dynamcs of potato late blight.