Location: Southern Horticultural ResearchTitle: Assesment of resistance components of bigleaf hydrangeas (Hydrangea macrophylla) to Erysiphe polygoni in vitro) Author
Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2009
Publication Date: 4/1/2010
Citation: Li, Y., Windham, M., Trigiano, M., Rinehart, T.A., Reed, S.M., Spiers, J.M. 2010. Assesment of resistance components of bigleaf hydrangeas (Hydrangea macrophylla) to Erysiphe polygoni in vitro. Canadian Journal of Plant Pathology. 31:348-355. Interpretive Summary: Bigleaf hydrangea (Hydrangea macrophylla) is one of the largest species in hydrangeas and is a popular ornamental plant. Powdery mildew caused by Erysiphe polygoni is a foliar disease of bigleaf hydrangea affecting inflorescences and causing yellow blotches on leaves. Plants grown in greenhouses or in shade are particularly susceptible to the disease. Variation of resistance to E. polygoni has been reported among bigleaf hydrangea cultivars. In both susceptible and resistant cultivars, the fungus formed haustoria under primary appressoria and initiated secondary germ tubes. Although cell death were observed in both susceptible and resistant cultivars, the percentage of dead infected cells was greater in the resistant cultivar than in the susceptible cultivar. Additionally, development of hyphae and sporulation were different between susceptible and resistant cultivars. The objectives of this study were to compare six bigleaf hydrangea cultivars with different levels of resistance to powdery mildew in the fungal development including spore germination, secondary germ tube initiation, infected cell death, infection efficiency, latent period and sporulation and to characterize of resistance mechanisms in resistant cultivars that will be used as sources to develop new resistant cultivars.
Technical Abstract: Resistance components of bigleaf hydrangea (Hydrangea macrophylla) to powdery mildew caused by Erysiphe polygoni were investigated using a detached leaf disk bioassay. Percentages of spore germination of the fungus were similar on all six bigleaf hydrangea cultivars that have different levels of resistance to powdery mildew. However, percent geminated conidia with secondary germ tubes (GCSGT), percent dead infected 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 although there were variations in reaction for individual components among cultivars. Necrotic dead cells were macroscopically visible in all cultivars regardless of resistance levels. Nonetheless, more dead infected cells were detected in resistant cultivar ‘Veitchii’. These results suggested that hypersensitive reaction is not a qualitative trait of resistance, but the frequency of dead cells could be one of resistance components that contribute to restrain fungal growth and colony development. Additionally, percentage of GCSGT, infection efficiency, latent period and sporulation could be used to evaluate resistance in bigleaf hydrangea to powdery mildew using a detached leaf disk assay.