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Ultraviolet Light as an Integrative Pest Management Tool for Grape Powdery Mildew

Oregon Wine Research Institute — The concerning emergence of fungicide resistant grape powdery mildew (Erysiphe necator) to numerous fungicides, has led to a need for new integrative pest management strategies in grape production. Ultraviolet spectrum C (UVC) radiation has been used for over a century to kill or disable microorganisms by damaging their DNA. However, prolonged exposure times have been required because the microbes have very efficient DNA damage repair machinery. A recent discovery showed that this machinery is shut off at night to conserve energy, which indicates that UVC light applied at night might be effective with shorter exposure times (e.g., strawberry powdery mildew and wheat powdery mildew spores). In collaboration with David Gadoury at Cornell University and Michelle Moyer at Washington State University, we began testing whether UVC could be used to manage grape powdery mildew and bunch rot this past growing season.

Figure 1. The tractor mounted UVC array in operation.

Our research tests were conducted in a 22-year-old block of vertical shoot positioned (VSP) trellised Pinot noir trained at the Oregon State University (OSU) Botany and Plant Pathology Farm in Corvallis, Oregon using an over-canopy array of UVC light banks (Figure 1). UVC treatments were applied once per week, one hour after sundown at a speed of two or three miles per hour, which relates to a theoretical dose of 120 and 80 joules per square meter (J/m2), respectively. We also applied fungicide programs to subplots within rows of 5 lb/A sulfur on 7- to 14-day intervals, 10 fl oz/A Azoxystrobin on a 14-day interval, an alternation of sulfur and a QoI on a 14-day interval, or untreated control. Powdery mildew incidence ratings were performed every other week starting in mid-May and ending at veraison with leaf and cluster mildew severity ratings completed just before veraison. Grape powdery mildew leaf incidence was significantly (TukeyHSD, < 0.05) reduced with weekly UV treatments (Figure 2). The UVC treatments did not lead to a significant reduction of mildew severity on clusters, mildew colonies, or rater’s gloves to monitor the amount of mildew and presence of QoI resistance of each of the plots.

Figure 2. Area under disease progress curve for the powdery mildew epidemic based on foliar mildew incidence ratings. Error bars represent the standard error of the mean.

Botrytis cluster disease severity and incidence was measured at harvest after incubating clusters for 48 hours at 68°F and high humidity. There was no significant difference in Botrytis incidence between treatments. Due to the heavy mildew pressure, many of the clusters with high mildew severity had clusters too desiccated or decayed at harvest to be colonized by Botrytis. Botrytis isolates collected from diseased clusters are being tested for fungicide resistance.

These results suggest that UVC treatments, in conjunction with fungicide programs, has the potential to improve disease management of grape powdery mildew, but the frequency or dose of the application need to be increased. Future field studies at the BPP field site will examine increasing dose and/or UVC application frequency. In collaboration with Willamette Valley Vineyards and Saga Robotics, we will begin exploring the use of an autonomous drive base to apply the treatments on a commercial vineyard scale. Using UVC as part of an integrative pest management tool for powdery mildew will hopefully reduce costs and environmental impacts of disease management by reducing the amount of chemical inputs for disease control.  — By Alexander Wong, Graduate Research Assistant, Dept. of Botany and Plant Pathology, OSU; and Dr. Walt Mahaffee, Research Plant Pathologist, USDA-ARS & FRAME Networks group (USDA-NIFA-SCRI)

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