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ARS Home » Pacific West Area » Corvallis, Oregon » Forage Seed and Cereal Research Unit » Research » Publications at this Location » Publication #322065
Title: Pre- and post-infection activity of fungicides in control of hop downy mildew

Author
item Gent, David - Dave
item TWOMEY, MEGAN - Oregon State University
item WOLFENBARGER, SIERRA - Oregon State University
item WOODS, JOANNA - Oregon State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2015
Publication Date: 6/30/2015
Citation: Gent, D.H., Twomey, M.C., Wolfenbarger, S.N., Woods, J.L. 2015. Pre- and post-infection activity of fungicides in control of hop downy mildew. Plant Disease. 99:858-865.

Interpretive Summary: Optimum timing and use of fungicides for disease control are improved by an understanding of the characteristics of fungicide activity. We conduct greenhouse and field studies to quantify the degree of control of hop downy mildew from various fungicides when applied at varying times before or after a single infection event by pathogen. Control of downy mildew on leaves was similar among fungicides tested when applied preventatively, but varied depending on both the fungicide applied and the timing of the application post-infection. In field studies, dimethomorph, fosetyl-Al, and trifloxystrobin suppressed development of shoots with systemic downy mildew to the greatest extent when applied near the timing of inoculation, although the duration of preventative and post-infection activity varied among the fungicides. Trifloxystrobin had a brief period of pre-infection activity, and limited post-infection activity, with maximal disease control when applied within 4 to 5 days before inoculation. Dimethomorph had the longest duration of protective activity. Percent disease control diminished progressively with increasing time between inoculation and application of dimethomorph, being intermediate to the curative activity of fosetyl-Al and trifloxystrobin. These findings provide guidance to use of fungicides when applications are timed with forecasted or post hoc disease hazard warnings, as well as guidance on tank-mixes of fungicides that may be suitable both for resistance management considerations and extending intervals between applications.

Technical Abstract: Optimum timing and use of fungicides for disease control are improved by an understanding of the characteristics of fungicide physical mode of action, although this information is lacking for hop downy mildew. Control of downy mildew on leaves was similar among fungicides tested when applied preventatively, but varied depending on both the fungicide applied and the timing of the application post-infection. Disease control decreased progressively as applications of copper were made later after inoculation. In contrast, cymoxanil, trifloxystrobin, and dimethomorph reduced disease with similar efficacy when applied 48 hours after inoculation compared to preventative applications of these fungicides. In field studies, dimethomorph, fosetyl-Al, and trifloxystrobin suppressed development of shoots with systemic downy mildew to the greatest extent when applied near the timing of inoculation, although the duration of preventative and post-infection activity varied among the fungicides. There was only a small decline in efficacy of disease control when fosetyl-Al was applied 6 to 7 days after inoculation as compared to protective applications. Trifloxystrobin had a brief period of pre-infection activity, and limited post-infection activity, with maximal disease control when applied within 4 to 5 days before inoculation. Dimethomorph had the longest duration of protective activity. Percent disease control diminished progressively with increasing time between inoculation and application of dimethomorph, being intermediate to the curative activity of fosetyl-Al and trifloxystrobin. These findings provide guidance to use of fungicides when applications are timed with forecasted or post hoc disease hazard warnings, as well as guidance on tank-mixes of fungicides that may be suitable both for resistance management considerations and extending intervals between applications.