Production, survival and infectivity of oospores of Phytophthora infestans

The formation of oospores of Phytophthora infestans was studied in tomato a nd potato crops and volunteer plants under field conditions, and in laborat ory tests with leaf discs of potato cultivars differing in their level of r ace-nonspecific resistance. Oospores were readily detected in blight-affect ed tomato leaflets and fruits, and in leaflets of field crops and volunteer potato plants. Oospores extracted from blighted potato leaflets yielded 13 oospore-derived progeny. Oospores were also produced following inoculation of leaf discs of eight potato cultivars expressing different levels of rac e-nonspecific resistance with a mixture of sporangia of A1 and A2 isolates. The highest numbers of oospores were produced in cvs Bintje (susceptible) and Pimpernel (resistant), and the lowest in Nicola (intermediate resistanc e). The relationship between lesions per leaflet and oospore incidence, aff ected by varying A1 : A2 ratios, was explored using a simple mathematical m odel, and validated by comparing actual oospore production in leaflets with multiple lesions of the race-nonspecific-resistant potato clone Lan 22-21 with the predictions generated by the model. Survival of oospores was inves tigated after their incorporation in either a sandy or a light clay soil in buried clay pots exposed to the local weather conditions. Over 6 years the se soils were regularly assessed for their infection potential using floati ng leaflets in a spore-baiting bioassay. Sandy and clay soils contaminated with oospores remained infectious for 48 and 34 months, respectively, when flooded. Infections of floating potato leaflets occurred within 84-92 h and ceased after 11 days. Soil samples remained infective if dried and re-floo ded on two, but not more, occasions.