Overwintering of Sphaerotheca humuli, the cause of hop powdery mildew

Abstract

SUMMARYThe 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 18oC 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 24oC. 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.

The ecological and evolutionary trajectory of oak powdery mildew in Europe

“…Most tropical powdery mildew species lack chasmothecia (Piepenbring et al, 2011). Conversely, for hop and apple powdery mildews, very low temperatures have been shown to have a deleterious effect on fungal survival in buds (Liyanage & Royle, 1976;Spotts & Chen, 1984), and there is also evidence of better oak powdery mildew survival in buds during mild winters (Marçais, et al 2017). Despite this sensitivity, pathogen hibernation in buds has the advantage of providing a "green bridge" between two growing seasons in temperate climates; the pathogen experiences no discontinuity in host availability and can resume its development in perfect synchrony with that of the host as soon as weather conditions are favorable for leaf development.…”

Association of Spring Pruning Practices with Severity of Powdery Mildew and Downy Mildew on Hop

“…In an analogous but biologically distinct process, Podosphaera macularis may infect crown buds during the preceding season, persisfing as asexual mycelia or generally less common, sexually derived cleistothecia in and on buds (19). After dormancy and resumption of host growth in the following early spring, some proportion of these infected buds give rise to shoots colonized by tbe fungus and are referred to as "flag shoots" (19).…”

Identification and distribution of mating‐type idiomorphs in populations of Podosphaera macularis and development of chasmothecia of the fungus

“…Colonies resulting from ascosporic infection were observed in laboratory experiments. Liyanage & Royle () described powdery mildew on hop early in the growing season that appeared to be the result of ascospore infection based on circumstantial evidence. However, their attempts to demonstrate ascospore infectivity experimentally failed.…”