GENETIC CHANGE WITHIN POPULATIONS OF PHYTOPHTHORA-INFESTANS IN THE UNITED-STATES AND CANADA DURING 1994 TO 1996 - ROLE OF MIGRATION AND RECOMBINATION

Dramatic changes occurred within populations of Phytophthora infestans in the United States and Canada from 1994 through 1996. Occurrence of the US-8 genotype, detected rarely during 1992 and 1993, increased ra pidly and predominated in most regions during 1994 through 1996. US-7, which infected both potato and tomato and made up almost 50% of the s ample during 1993, was detected only rarely among 330 isolates from th e United States analyzed during 1994. It was not detected at all in mo re limited samples from 1996. Thus, ability to infect both potato and tomato apparently did not increase the fitness of this genotype relati ve to US-8, as predicted previously. US-I, the previously dominant gen otype throughout the United States and Canada, made up 8% or less of t he samples analyzed during 1994 through 1996. A few additional genotyp es were detected, which could indicate the beginnings of sexual reprod uction of FI infestans within the United States and Canada. However, c lonal reproduction still predominated in all locations sampled; opport unities for sexual reproduction probably were limited, because the Al and A2 mating types usually were separated geographically. The high se nsitivity of the US-I genotype to the fungicide metalaxyl also could h ave reduced opportunities for contact between the mating types in fiel ds where this compound was applied. The previous correlation between m etalaxyl sensitivity and genotype was confirmed and extended to a new genotype, US-17: all US-1 isolates tested were sensitive; all isolates of the US-7, US-8, and US-17 genotypes tested to date have been resis tant. Isolates of P. capsici and P. erythroseptica, two other species often found on tomato and potato, could be easily distinguished from e ach other and from P. infestans using a simple allozyme assay for the enzyme glucose-6-phosphate isomerase. This technique could be useful f or rapid identification of species, in addition to genotype of P. infe stans. It generally was not possible to predict which genotypes would be present in a location from 1 year to the next. Long-distance moveme nt of US-8 in seed tubers was documented, and this was probably the pr imary means for the rapid spread of this genotype from 1993 through 19 96.