|PROBST, CLAUDIA - Washington State University|
|NELSON, MARK - Washington State University|
|GROVE, GARY - Washington State University|
|TWOMEY, MEGAN - Oregon State University|
|Gent, David - Dave|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2016
Publication Date: 8/1/2016
Citation: Probst, C., Nelson, M.E., Grove, G.G., Twomey, M.C., Gent, D.H. 2016. Hop powdery mildew control through alteration of spring pruning practices. Plant Disease. 100(8):1599-1605. doi: 10.1094/PDIS-10-15-1127-RE.
Interpretive Summary: Powdery mildew is an important disease of hop, potentially reduce cone yield and quality. Disease management practices often involve preventative fungicide applications, but alternative approaches are needed. We evaluated simple changes to the timing of standard growth cultural practices in spring, and their impact on suppression of the disease, yield, and cone quality factors. A four week delay in the timing of spring pruning operations consistently suppressed the disease, especially on leaves. This response was not dependent on the method of pruning. Importantly, cone yield and quality was never reduced with delayed pruning. Delayed pruning may offer a low-cost means of reducing both the incidence of powdery mildew and early season fungicide inputs in certain cultivars.
Technical Abstract: Since 1997, Podosphaera macularis, the causal agent of hop powdery mildew, has become a recurrent threat to hops in the Pacific Northwest because of the potential to reduce cone yield and quality. Disease management practices often involve preventative fungicide applications, but alternative approaches are needed. Removal of early season shoot growth (pruning) is a common practice in hop production for horticultural reasons. Studies were conducted over a three year period in a commercial hop yard to quantify the impact of pruning method (chemical versus mechanical) and pruning timing on development of powdery mildew, yield, and cone quality factors. A four week delay in the timing of pruning reduced the incidence of leaves with powdery mildew from 10 to 46% and cones from 1 to 9 %, with the specific effect being season-dependent. Pruning using chemical desiccants rather than by mechanical means had similar impacts on disease levels on leaves. On cones, though, chemical pruning had a small but significant reduction in the incidence of powdery mildew compared to mechanical pruning, with the magnitude of the reduction season-dependent. Cone yield, levels of bittering-acids, and color were not negatively impact in any individual year or cumulatively over three seasons when pruning treatments were applied repeatedly to the same plots during the study period. Delayed pruning may offer a low-cost means of reducing both the incidence of powdery mildew and early season fungicide inputs in certain cultivars.