Overwintering of Sphaerotheca humuli, the cause of hop powdery mildew

The ability of Sphaerotheca humuli to overwinter as cleistocarps in infected hop cones and leaves and in aerial buds on rootstocks was examined during the winters of 1970-1, 1971-2 and 1972-3. Periodical examination of cleistocarps, collected in October and overwintered in Terylene bags on the soil of a hop garden, consistently revealed two periods of maturation ending in November and in March, when over 50% contained eight, well-defined ascospores. In laboratory tests cleistocarps, kept either in the hop garden or dry at 4, 8 or 18^oC during the winter, could not be encouraged to dehisce earlier than April when naturally dehisced cleistocarps were first detected in the field. More ascospores were discharged from cleistocarps, and germination of ascospores in laboratory tests was greater, at 18 than at 4, 8 or 24^oC. Colonies of S. humuli arose on leaves of potted plants exposed to overwintered cleistocarps in the hop garden and were observed microscopically to originate from ascospores. However, a Burkard spore trap, operated amidst the cleistocarps in this garden in 1972 and 1973, failed to detect ascospores. Ascospores, discharged onto susceptible leaves in the laboratory, germinated but failed to produce colonies. It was demonstrated that S. humuli can perennate in aerial, dormant buds on hop rootstocks. Examination of buds in autumn revealed mycelium external to and between the bud scales. At budburst the mycelium was still present internally. Cleistocarps were occasionally associated with hibernating mycelium. Primarily infected shoots arose from plants bearing infected buds in conditions which precluded chance infection. Some evidence was obtained that conditions during the winter determine the success of survival in buds. The fungus appeared to be incapable of infecting a selection of weeds common to hop gardens and their vicinity.