Abstract
A description is provided for Plasmopara viticola. Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Ampelopsis brevipedunculata, A. hederacea, A. heterophylla, A. veitchii, Ampelocissus acetosa, A. latifolia, A. salmonea, Cissus caesia, C. gracilis, C. hypoglauca, C. polyantha, Cordifolia sempervirens, Cinerea sp., Parthenocissus quinquefolia, P. tricuspidata, Solonis robusta, Vitis aestivalis, V. amurensis, V. arizonica, V. berlandieri, V. californica, V. cinerea, V. coignetiae, V. cordifolia, V. girdiana, V. labrusca, V. lanata, V. monticola, V. pagnuccii, V. riparia, V. romaneti, V. rupestris, V. silvestris, V. treleasei, V. vinifera. DISEASE: Grape vine downy mildew; the fungus is an obligately biotrophic plant pathogen. All tissues bearing stomata are infected, becoming discolored, malformed and necrotic. Lesions on affected organs develop a white felt of sporangiophores. Leaves are most susceptible to attack during active growth in early summer, and when very mature in the autumn. Sporangiophores may appear directly on healthy green leaf tissues with no overlying lesion, or as a dense felt under yellow oily lesions on the upper surface (if humidity is high, 5-15 days after infection), or may be absent, leaves presenting only a mosaic of small, angular yellow or dark-red blemishes, limited by the secondary veins (mainly on old leaves in the autumn). During early growth, whole branches are attacked, but later only the extremities of branches are invaded. Infected shoots turn brown, curl up or become hooked at their tips. Nodes are more susceptible to attack than internodes. Tendrils, petioles, inflorescences and bunches also develop similar brown spots and lesions. Bunches are susceptible until the grapes are 5-6 mm diam., after which infection is rare (grey rot followed later by brown rot). Subsequent browning and desiccation of the bunch is caused by penetration of the bunch stalk by mycelium from earlier infections elsewhere. GEOGRAPHICAL DISTRIBUTION: See CMI Distribution Maps of Plant Diseases 221. TRANSMISSION: Oospores present in infected leaf tissues from the previous season's crop germinate in the spring, when air temperatures exceed 12°C and at least 10 mm rain falls in 24 hours, releasing zoospores into water or onto very moist soil from sporangia (64, 2458). Zoospores are projected onto vine leaves near the soil by rain splash, germinate to give hyphae and penetrate tissues via their stomata. Sporangia are liberated in moist air only, are disseminated by air currents, and remain viable for five days in dry air, producing secondary infection sites. Production of sporangia occurs at a relative humidity of 95-100%, and an air temperature of 13-27°C (optimum 18-22°C). Mycelium may overwinter between the bud scales and in diseased leaves, but it has not been established whether this contributes substantially to re-infection of healthy leaf tissues the following spring. There is no evidence for systemic transmission.
