Susceptibility of Hop crown buds to powdery mildew and its relation to perennation of Podosphaera macularis

Authors: Gent, David - Dave; CLAASSEN, BRIANA - Oregon State University; TWOMEY, MEGAN - Oregon State University; WOLFENBARGER, SIERRA - Oregon State University; WOODS, JOANNA - Oregon State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2018
Publication Date: 7/31/2018
Citation: Gent, D.H., Claassen, B.J., Twomey, M.C., Wolfenbarger, S.N., Woods, J.L. 2018. Susceptibility of Hop crown buds to powdery mildew and its relation to perennation of Podosphaera macularis. Plant Disease. 102(7):1316-1325.

Interpretive Summary: In the Pacific Northwestern U.S. the hop powdery mildew fungus survives overwintering periods in association with living host tissue, most importantly buds formed in the previous year that developed near the soil surface (crown buds). This aspect of the disease cycle severely limits when, where, and how much of the pathogen will be available to initiate powdery mildew in a given field or year. Because of the importance of this process to disease development, we evaluated when infection of crown buds is most likely to occur so that control can be reconciled to the periods of greatest risk for infection and potential survival of the fungus. The presence of susceptible crown buds, favorable environmental conditions for infection, and the occurrence of the pathogen were coincident during early July to late September. In the following year, the pathogen emerged over an extended period of time as the plants broke dormancy and initiated growth in late winter and spring. However, the amount of the pathogen surviving from year-to-year was dependent on when infection occurred in the previous year. Given that bud infection occurred over a 10 week period and common cultural practice applied in spring, management efforts seem best directed to both preventative measures to reduce the likelihood of bud infection and remedial practices to physically eliminate infected crown buds in the ensuing year.

Technical Abstract: In the Pacific Northwestern U.S. the hop powdery mildew fungus, Podosphaera macularis, survives overwintering periods in association with living host tissue because the ascigerious stage of the pathogen is not known to occur in this region. Field experiments were conducted over a 5-year period to describe the overwintering process associated with crown bud infection and persistence of P. macularis. Surface crown buds increased in abundance and size beginning in early July and continuing until mid-September. Buds of varying sizes remained susceptible to powdery mildew until late September to early October in each of three years of experiments, with susceptibility decreasing substantially thereafter. Potted plants were inoculated sequentially during early summer to autumn, and then evaluated in the following year for development of shoots colonized by the powdery mildew fungus (termed flag shoots) due to bud perennation. Emergence of flag shoots was asynchronous and associated with shoot emergence and elongation. Flag shoots emerged over a protracted period from late February to early June, year dependent. In all four years of experiments some infected buds broke and produced flag shoots after chemical desiccation of shoots in spring, a common horticultural practice in hop production conducted to set training timing and eliminate initial inoculum. Flag shoots were most numerous when plants were inoculated with P. macularis in early summer and, consequently, when powdery mildew was present throughout the entire period of crown bud development. The number of flag shoots produced was reduced from 6.8 to 46.6-fold when comparing the latest versus earliest inoculation dates. However, all inoculation timings yielded flag shoots at some level, suggesting that bud infection that occurs over an extended period of time in the previous season may allow the fungus to perennate.