Characterization of Reactions to Powdery Mildew (Podosphaera pannosa) in Resistant and Susceptible Rose Genotypes

Fungal development of powdery mildew Podosphaera pannosa (Wallr.: Fr.) de Bary on rose leaves depends on constitutive or induced resistance mechanisms present in attacked rose genotypes. The relationship between fungal development and plant resistance was investigated microscopically on young greenhouse leaves of four rose genotypes with different levels of resistance: Rosa wichuraiana, R. laevigata anemoides and R. hybrida cultivars ‘Excelsa’ and ‘Gomery’. Induced plant reactions, hydrogen peroxide production and cross sections through infected leaves were examined. The variation in development of the fungus on these rose genotypes depended on the relative presence of normal haustoria, abnormal haustoria, induced cell reactions, papilla formation or physical barriers. Formation of papillae could arrest up to one third of the successful penetrations. Papillae formation was often succeeded by total cell reaction. Abnormal haustoria were detected as rudimentary haustoria, haustoria with abnormal shape or haustoria without extra haustorial matrix. Post‐haustorial cell reactions, with and without cell collapse, were detected. In non‐collapsed cells, appositions were directed to both cell wall and haustorium. This was followed by accumulation of non‐identified, probably antifungal compounds. Both single and multicell reactions occurred. Hydrogen peroxide was detected during papilla formation and induced cell reactions.