Occurrence of Oospores of Phytophthora Infestans in the Field and under Controlled Conditions

Both A1 and A2 mating types of Phytophthora infestans were detected in El-Behera governorate and the surrounding area where potato and tomato were intensively cultivated during the 2005-2006 growing season. The A2 mating type constituted 14.8% of the total isolates recovered (162) while A1 mating type constituted 83.9%. A 7.4% of the total potato and tomato fields (94) surveyed yielded A2 mating type while 89.3% of the fields yielded A1 mating type and three fields only yielded both A1 and A2 mating types. Investigations were focused on fields where both mating types were occurred as opportunity for oospore formation presumably high. No oospores were detected in any of the blighted potato tubers and tomato fruits as well as the blighted potato and tomato leaves collected at harvest from fields showed both mating types. This was despite that two of 63 tubers sampled yielded both the A1 and A2 mating types and abundance of oospores were produced in vitro on a variety of culture media testes particularly the Rye A and V8 Juice. The artificial inoculation of both mating types into whole freshly harvested (dormant) and old (sprouting) potato tubers as well as the tomato fruits resulted in a successful blight but no oospores were produced in the dormant tubers while the old sprouting ones showed little number of oospores. The tomato fruits, however, supported more oospores with the same isolate pairings and also the inoculated leaf discs of both potato and tomato yielded more oospores. Temperature ranging from 10 to 23 o C supported oospore formation with maximum oospore production was at 10 o C. Wetness exhibited a crucial role for oospore J.Agric.&Env.Sci.Alex.Univ.,Egypt Vol.7 (1)2008 13 formation as free moisture (floating leaf discs) supported oospore production in inoculated leaf discs while no oospores were formed in leaf discs on moist filter paper. Meantime, aeration supported oospore formation in inoculated leaf discs in nonsealed Petri dishes while sealed Petri dishes suppressed oospore formation in most pairings (five out of the six pairings conducted). The ratio of coinoculated A1 and A2 mating types was not an important factor in production of oospores. Even a ratio of 5A1 : 95A2 or 95A1 : 5A2 was resulted in oospore production which indicated that a low proportion of A2 mating type in the field may suffice for sexual reproduction of the pathogen and oospore formation. The apparently absence of oospores of P. infestans under field conditions in Egypt could be attributed to the absence of prolonged rainy season and the lack for A2 mating type in most fields. keywords: Phytophthora infestans, mating type, oospores, potato, tomato.