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Abstract
Powdery mildew caused by Erysiphe polygoni is one of the major foliar diseases on bigleaf hydrangeas (Hydrangea macrophylla). To determine resistance components of bigleaf hydrangea to powdery mildew, fungal development and host necrotic cells were compared on six bigleaf hydrangea cultivars using a detached leaf disk bioassay. Germination percentages of conidia were not significantly different among cultivars. However, percent geminated conidia with secondary germ tubes (GCSGT), percent necrotic host cells, infection efficiency, latent period, and sporulation were significantly different among cultivars. In general, ‘Veitchii’ was resistant, ‘Nikko Blue’ was susceptible, and ‘Madame Emile Mouillere’, ‘Forever Pink’, ‘Lilacina’, and ‘Holstein’ were intermediate. Necrotic cells were macroscopically visible in all cultivars regardless of assigned resistance levels. Significantly more necrotic cells were detected in resistant cultivar ‘Veitchii’ compared with other cultivars. These results suggest that hypersensitive reaction is not a qualitative trait of resistance, but the frequency of necrotic cells could be one of several resistance components that contribute to restrain fungal growth and colony development. Additionally, percent GCSGT, infection efficiency, latent period, and sporulation could be used to evaluate partial resistance in bigleaf hydrangea to powdery mildew using a detached leaf disk assay.