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Summary The infection of potato (Solanum tuberosum) leaves with the late blight pathogen Phytophthora infestans, or treatment with fungal elicitor, leads to the massive accumulation of pathogenesis-related (PR) proteins in the extracellular leaf space. The most abundant of these proteins was purified to apparent homogeneity and identified as a new, basic member of the PR-1 family of defence proteins, designated PR-1b. Antibodies raised against the protein and a cDNA isolated by differential screening were used to study the temporal and spatial patterns of PR-1b protein and mRNA distribution in healthy and infected potato tissues. PR-1b was present in old leaves and at low levels also in the carpels of flowers. In leaves, strong accumulation of PR-1b mRNA and protein occurred in response to infection by the oomycete pathogen Phytophthora infestans or the bacterial pathogen Pseudomonas syringae pv. maculicola. PR-1b mRNA and protein accumulation was clearly initiated at the infection site, but a delayed and sustained accumulation was also observed in neighbouring, uninfected leaves of potato plants. Tissue- and cell type-specific expression of PR-1b was analysed by immunohistochemical and in situ RNA hybridization techniques. Appreciable amounts of PR-1b protein and mRNA were localized in epidermal cells, guard cells of the stomata, glandular trichomes, crystal idioblasts, and cells of the vascular system of infected leaves. However, no significant differences in the amounts and distribution patterns of PR-1b could be observed between compatible and incompatible interactions of potato and Phytophthora infestans, indicating that PR-1b expression is not involved in determining cultivar/race-specific resistance in potato.

The ability of the late blight pathogen Phytophthora infestans to form oospores in leaves of seven potato cultivars was examined at different incubation temperatures under controlled environmental conditions and under field conditions. At 10°C, the oospore formation in three intermediate‐resistant cultivars all differed significantly from each other (P < 0.05), with the lowest amount formed in cv. Asterix. This latter cultivar did not form oospores at any other temperature. Under field conditions oospores were formed abundantly in a naturally infected field. A significant date by cultivar interaction showed that P. infestans increased the oospore formation in foliage by time in cvs Columbo, Hertha and Matilda, whereas no significant differences between dates were found for other cultivars. The genetic structure of P. infestans in the naturally infected field plot, where oospores formed abundantly, was studied by using amplified 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 effect of light and nutrients on oosporogenesis is discussed.

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