Oospore Germination and Formation by the Late Blight Pathogen Phytophthora infestans in vitro and under Field Conditions

The ability of the late blight pathogen Phytophthora infestans to form oospores in leaves of seven potato cultivars was examined at di€erent incubation temper- atures under controlled environmental conditions and under ®eld conditions. At 10°C, the oospore formation in three intermediate-resistant cultivars all di€ered signi®cantly from each other (P < 0.05), with the lowest amount formed in cv. Asterix. This latter cultivar did not form oospores at any other tempera- ture. Under ®eld conditions oospores were formed abundantly in a naturally infected ®eld. A signi®cant date by cultivar interaction showed that P. infestans increased the oospore formation in foliage by time in cvs Columbo, Hertha and Matilda, whereas no signi- ®cant di€erences between dates were found for other cultivars. The genetic structure of P. infestans in the naturally infected ®eld plot, where oospores formed abundantly, was studied by using ampli®ed fragment length polymorphism and a high genetic diversity was revealed. Oospore germination from two Scandinavian (A1 and A2) P. infestans isolates was stimulated in visible light and in 1 : 2 and 1 : 10 soil extract. The e€ect of light and nutrients on oosporogenesis is discussed