Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/22/2008
Publication Date: 2/1/2009
Citation: Li, Y., Windham, M.T., Trigiano, R.N., Reed, S.M., Spiers, J.M., Rinehart, T.A. 2009. Bright-field and fluorescence microscopic study of development of Erysiphe polygoni in susceptible and resistant bigleaf Hydrangea. Plant Disease. 93:130-134. Interpretive Summary: Powdery mildew, caused by Erysiphe polygoni, is a foliar disease of bigleaf hydrangea. Plants grown in greenhouses or in shade are particularly susceptible to the disease. Powdery mildew colonies develop on the upper and lower leaf surfaces and affect inflorescences. Fungal colony growth can cause yellow or purple blotches on infected leaves. Variation of resistance to E. polygoni among bigleaf hydrangea cultivars has been noted in greenhouses and gardens. However, the infection process of the fungus and the nature of defense reactions in host plants are unknown. For developing efficient integrated disease management strategies of powdery mildew for hydrangea, it is essential to understand how the fungus infects plants and how plants respond to fungal attacks. The objectives of this study were to describe the infection process, examine host responses to the fungal attacks and compare fungal development in susceptible and resistant cultivars of bigleaf hydrangea.
Technical Abstract: Temporal development of Erysiphe polygoni and responses of bigleaf hydrangeas (Hydrangea macrophylla) to the fungal attack were investigated using bright-field and fluorescence microscopy. Conidia began to germinate at 2 h after inoculation (HAI) and formed primary appressoria at the tip of the primary germ tubes within 4 HAI. Secondary germ tubes were initiated from primary appressoria or other parts of conidia 12 HAI. Hyphae developed through elongation of secondary germ tubes and paired lateral appressoria were formed along hyphae within 48 HAI. Conidiophores and conidia were formed 5 days after inoculation. In the susceptible cultivar Nikko Blue and the resistant cultivar Veitchii, the fungus established a parasitic relationship, which was indicated by the formation of haustoria under primary appressoria and development of secondary germ tubes at 24 HAI. A hypersensitive response (HR) and accumulation of callose were detected in both resistance and susceptible cultivars at 3 DAI. Resistance to powdery mildew in ‘Veitchii’ was evident by manifestation of higher percentage of necrotic infected cells and restricted colony development compared to the susceptible ‘Nikko Blue’.