Venturia inaequalis. [Descriptions of Fungi and Bacteria].
Abstract A description is provided for Venturia inaequalis. Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Principally on apple (Malus pumila), and other species of Malus. Also recorded on Pyrus spp., Sorbus spp. Pyracantha, Cotoneaster integerrima, Crataegus oxyacantha, Viburnum, Sarcocephalus esculentus (36, 278) (Herb. IMI). DISEASE: Causes scab or black spot of apple, a common disease which can produce serious losses in both quantity and quality of fruit. The fungus can infect shoots, buds, blossoms, leaves and fruit. Symptoms appear initially as small, dull, pale spots particularly on the underside of leaves in spring. These enlarge as a mass of radiating subcuticular hyphae develops, forming dark circular lesions visible on both leaf surfaces; a slight puckering or blistering effect may also occur. Older lesions turn grey as the infected host tissue is killed. Scab on fruit is usually more conspicuous than on leaves, but varies according to resistance, pathogen virulence etc. Severe lesions may involve most of the fruit and become suberized and cracked, thus allowing entrance of secondam rotting organisms. Early infection of young fruit may cause shedding or distorted growth. On older fruit smaller secondary, lesions (pepper spot) often develop around a large primary scab. Infection of mature fruit may become visible during storage as sunken, black lesions. Lesions on young shoots appear as light brown blister-like swellings. GEOGRAPHICAL DISTRIBUTION: Worldwide wherever apples are grown (CMI Map 120, ed. 3, 1966). TRANSMISSION: The fungus overwinters primarily on leaf litter where the saprophytic sexual (perithecial) stage occurs. In wet spring weather the resulting perithecia release masses of airborne ascospores which infect the susceptible young foliage (42, 204). Dormant overwintering lesions on shoots and bud scales may also occur; these produce conidia which can infect young spring growth. Primary (spring) lesions produce conidia which cause secondary infection of foliage, fruit and shoots during wet summer weather. Conidia are dispersed chiefly by rain splash but may be present in the air near heavily infected trees during dry conditions (40, 757).