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United States Department of Agriculture

Agricultural Research Service

Research Project: Reducing the Impact of Diseases on Hop Production

Location: Forage Seed and Cereal Research

Title: Crop damage caused by Powdery Mildew on Hop and its relationship to late season management

Authors
item Gent, David
item Grove, Gary -
item Nelson, M -
item Wolfenbarger, S -
item Farnsworth, Joanna -

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 31, 2013
Publication Date: June 30, 2014
Citation: Gent, D.H., Grove, G.G., Nelson, M.E., Wolfenbarger, S.N., Farnsworth, J.L. 2014. Crop damage caused by Powdery Mildew on Hop and its relationship to late season management. Plant Pathology. 63:625-639.

Interpretive Summary: Powdery mildew of hop (Podosphaera macularis) may cause economic loss due to reductions in cone yield and quality. Understanding damage from the disease is fundamentally important to developing rational and appropriate management strategies. In this research, damage from powdery mildew was measured over three years of field experiments in Washington State. The amount of cones with powdery mildew was directly related to yield of cones, bittering acids, and accelerated cone maturation. However, the impact of powdery mildew on aroma characteristics and bittering acid content were less substantial than cone yield. The damage caused by powdery mildew to cone color and alpha-acid yield, as well as the effectiveness of fungicide applications made to manage the disease, appears inseparably linked to the maturity of cones at harvest. Realising achievable yield potential requires control of the disease through stages the juvenile stages of cone development and harvest before cone maturity exceeds approximately 25% dry matter.

Technical Abstract: Powdery mildew of hop (Podosphaera macularis) may cause economic loss due to reductions in cone yield and quality. Quantitative estimates of crop damage from powdery mildew remain poorly characterised, especially the effect of late season disease management on crop yield and quality. Field studies in Washington State evaluated cone yield, bittering acid content, and quality factors when fungicide applications were ceased at different stages of cone development. The incidence of cones with powdery mildew was linearly correlated with yield of cones, bittering acids, and accelerated cone maturation. In cultivar Galena, the cumulative effect of every 1% increase in cones powdery mildew incidence was to reduce alpha-acid yield by 0.33%, which was due to direct effects on cone yield but also indirect effects mediated by dry matter. In the more susceptible cultivar Zeus, alpha-acid yield was increased 20% by controlling powdery mildew through stage II of cone development compared to ceasing fungicide applications at bloom: additional applications provided only modest improvements in alpha-acid yield. In both cultivars, the impact of powdery mildew on aroma characteristics and bittering acid content were less substantial than cone yield. The damage caused by powdery mildew to cone color and alpha-acid yield, as well as the effectiveness of fungicide applications made to manage the disease, appears inseparably linked to dry matter content of cones at harvest. Realising achievable yield potential in these cultivars requires control of the disease through stages I and II of cone development and harvest before cone maturity exceeds approximately 25% dry matter.

Last Modified: 9/10/2014
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