Interaction of basal foliage removal and late season fungicide applications in management of Hop powdery mildew

Authors: Gent, David - Dave; PROBST, CLAUDIA - Washington State University; NELSON, MARK - Washington State University; GROVE, GARY - Washington State University; MASSIE, STEPHEN - Pharsalia Hops; TWOMEY, MEGAN - Oregon State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2016
Publication Date: 6/30/2016
Citation: Gent, D.H., Probst, C., Nelson, M.E., Grove, G.G., Massie, S.T., Twomey, M.C. 2016. Interaction of basal foliage removal and late season fungicide applications in management of Hop powdery mildew. Plant Disease. 100:1153-1160.

Interpretive Summary: Powdery mildew is most damaging to hop cones when infection occurs during bloom and the early tages of cone development. Experiments were conducted over three years to evaluate whether fungicide applications could be ceased after the most susceptible stages of cone development (late July) without unduly affecting crop yield and quality, and how this practice might be influenced by canopy management practices. In experiments with two different varieties, disease levels on leaves was similar whether fungicides were ceased in late July or made through August. Disease levels on leaves were unaffected by the intensity of basal foliage removal, although the intensity of basal foliage removal interacted with the duration of fungicide applications to impact disease levels on cones. In an experiment in a commercial hop yard, application of fungicides into August had a modest, suppressive effect on powdery mildew as compared to ceasing fungicide applications in late July. However, this effect was moderated by how intensively the basal foliage was removed. Removing basal foliage two to three times allowed fungicide applications to be terminated in late July rather than late August without diminishing disease control on cones, yield, or cone quality factors. The additive effect of fungicide applications targeted to the periods of greatest cone susceptibility and canopy management to reduce disease favorability may obviate the need for fungicide applications. This strategy has both economic and environmental benefits.

Technical Abstract: Experiments were conducted over three years to evaluate whether fungicide applications could be ceased after the most susceptible stages of cone development (late July) without unduly affecting crop yield and quality when disease pressure was moderated with varying levels of basal foliage removal. In experimental plots of cultivar Galena, the incidence of leaves with powdery mildew was similar whether fungicides were ceased in late July or made through August. Disease levels on leaves were unaffected by the intensity of basal foliage removal, although the intensity of basal foliage removal interacted with the duration of fungicide applications to impact disease levels on cones. Similar experiments conducted in large plots of cultivar Tomahawk in a commercial hop yard similarly found no significant impact on disease levels on leaves from either the duration of fungicide applications or intensity of basal foliage removal. In contrast, on cones, application of fungicides into August had a modest, suppressive effect on powdery mildew. There was some evidence that the level of powdery mildew on cones associated with fungicide treatment was influenced by the intensity of basal foliage removal. When fungicide applications ceased in late July there was a progressive decrease in the incidence of cones with powdery mildew with increasing intensity of basal foliage removal. Removing basal foliage two to three times allowed fungicide applications to be terminated in late July rather than late August without diminishing disease control on cones, yield, or cone quality factors. Thus, this study further establishes that fungicide applications made during the early stages of hop cone development are the most impactful for suppression of powdery mildew. The additive effect of fungicide applications targeted to the periods of greatest cone susceptibility and canopy management to reduce disease favorability may obviate the need for fungicide applications later in the season.