The parasitic relationship between Phyllosticta ampelicida and Vitis vinifera

Phyllosticta ampelicida, the causal agent of black rot of grape, is one of the most damaging fungal pathogens in viticulture in North America. The interactions of the pathogen with host leaf tissues were studied to better understand the behavior of conidia, appressoria, and subcuticular hyphae, particularly in older leaves that remain symptomless. Pycnidiospores germinated on leaf surfaces within an hour and completed appressorium development within six hours, regardless of leaf age. Younger, still-expanding leaves exhibited disease symptoms as circular necrotic lesions 10-14 d after inoculation. Older leaves that were expanded fully at the time of inoculation did not develop disease symptoms. Fungal growth following penetration from appressoria was limited to the subcuticular region above anticlinal walls of the epidermis and was extensive in younger expanding leaves. Subcuticular growth in older, symptomless leaves was limited to less than 30 mu m from the site of penetration from the appressorium. However, these hyphae were viable, since desiccation of symptomless leaves with paraquat 60 d after inoculation allowed the fungus to grow saprobically and develop pycnidia on the leaf surface, especially over major veins.