Overwintering of Sphaerotheca mors-uvae on black currant and gooseberry

The ability of Sphaerotheca mors-uvae to perennate as cleistocarps, and as mycelium in buds was examined during the winters of 1965-6, 1966-7 and 1967-8 in relation to its two principal hosts, gooseberry and black currant. Cleistocarps on black currant leaves were examined from August 1965 to April 1966 and from July 1966 to March 1967. In 1965 cleistocarps were first observed on the leaves on 5 August; in 1966 on 11 July. These continued to develop through August and September and by October approximately 70% contained well-defined ascospores. The ascospore content remained generally at this level until February 1966 and November 1966; then the numbers of cleistocarps with ascospores fell and by April 1966 and March 1967 few such cleistocarps remained. From 21 March 1966 and 15 February 1967, but not otherwise, discharge of ascospores from the overwintered cleistocarps was readily obtained in laboratory tests. The viability and infectivity of the ascospores was demonstrated by allowing them to discharge on to leaf discs of black currant in the laboratory and also on to leaf discs and plants in the field. Sporulating colonies of S. mors-uvae developed within 8 days. Cleistocarps from shoots of black currant were examined from 4 August 1966 to 9 March 1967, and from 27 July 1967 to 1 January 1968. They developed in a similar manner to those on black currant leaves and by September in both 1966 and 1967 over 60% contained ascospores. This level was not maintained; the number of cleistocarps with ascospores fell gradually and by 8 December 1966 and 1 January 1968 few remained. Only in one laboratory test (21 November 1967) were ascospores discharged from a sample of these cleistocarps. Cleistocarps from shoots of gooseberry were examined from July 1966 to March 1967, and from August 1967 to January 1968. The pattern of ascospore development and subsequent decline in number of cleistocarps with ascospores was similar to that observed for black currant shoots. No discharge of ascospores could be demonstrated in laboratory tests. Evidence that S. mors-uvae perennates in buds of gooseberry was obtained by dissecting buds and by inducing buds on surface-sterilized shoots to burst under conditions which precluded chance infection. Field observations also suggested that bud infection occurred on gooseberry. Similar experiments failed to demonstrate the fungus in buds of black currant, and there was no indication of bud infection of this host in the field.