Effect Of Fungicide On Sexual Reproduction And Variation Among Asexual Progeny Of Phytophthora Infestans

Phytophthora infestans (Mont.) de Bary is the causal agent of late blight on potato. So far, the most effective means for late blights control are planting Tesistant cultivars and the application of fungiside. The aim of this study was testing the effects of fungicides on oospore formation on six isolates of Phytophthora infestans. The two self-fertile isolates formed oospores in single sulture in most case. Two of the four non-self-fertile isolates produced oospore in single sulture only in medium with metalaxyl but not in media with other chemicals that tested, or in medium with high levels of NaCl or extreme pH. The rest two non-self-fertile isolates did not produce any oospore in single culture under all the conditions tested. The results indicated that metalaxyl can stimulate the oospore production in single culture of certain isolates. The mechanism of this phenomenon was likely different from the reaction of microorganism to adversity.The asexual progenies of isolate 2511-1 and isolate SF3-lof P. infestans were derived respectivly from metalaxyl treatmented and metalaxyl untreated mycelium by single zoospore isolation. The extent of phenotypic varibility in the asexual progenies was investigated for colony types, mycelial growth rate and the change of mating type. The mating types of all the asexual progenies are same as the respective parent isolate. However, both asexual progenies derived from metalaxyl treated and non-treated mycelia showed a considerable level of phenotypic variations on colony morphology and mycelial growth rate and a higher level of variation were detected in progeny derived from metalaxyl treated mycelium.To determine if phenotypic variation could be detected at DNA level. Nineteen AFLP primer pairs were screen out by using AFLP and 5 of them were used. Polymorphism was detected in 60 SF3-1 single-zoospore isolates and 30 2511-1 single-zoospore isolates. Metalaxyl could enhance the genetic variation in progenies of P. infestans more considerable variation. Cluster analysis using the unweighted pair-group method with arithmetic averages (UPGMA) separated the asexual progenies from metalaxyl treated mycelium into significantly more AFLP groups than the asexual progenies from metalaxyl un-treatment mycelium. Results of this study suggested that there is a higher level of genetic variation among single-zoospore isolates derived asexually from metalaxyl treated mycelium than that derived from metalaxyl untreated mycelium. The phenomenon of variation in asexual generation may play an important role in evolution, survival and development of pathogenicity and fungicide resistance of an organism.