In its 2019-20 fiscal year, the Oregon Wine Board of Directors granted $417,000 to researchers for nine projects with the potential to advance quality grape growing and winemaking in Oregon. The update below is part of a series to let you know about the status of these projects.

Dr. Walt Mahaffee is a research plant pathologist with the USDA-Agriculture Research Service and courtesy faculty with the Department of Botany and Plant Pathology and core member of Oregon Wine Research Institute at Oregon State University. He has prepared the update below.

This project is part of a larger effort to develop improve methods for managing, assessing and mitigating fungicide resistance. The larger effort has received funding from USDA-NIFA-SCRI program, Northwest Center for Small Fruits Research, American Vineyard Foundation, and the Oregon Wine Board. This work is built upon prior funding from OWB for grape powdery mildew monitoring.

Persistence of Fungicide Resistance in Grape Powdery Mildew

Project objectives:
The long term goal of this research is to understand how fungicide resistance persists among grape powdery mildew (E. necator) in vineyards, and how to mitigate resistance development and therefore rejuvenate the efficiency of cost-effective fungicides, specifically Quinone outside inhibitors (QoI) and Demethylation inhibitors (DMI).

The specific objectives are to:

  • Assess the fitness (rate of reproduction and over-wintering) of necator isolates that are sensitive and resistant QoI and DMI fungicides.
  • Conduct mating studies of QoI and DMI sensitive and resistant necator isolates to determine inheritance of the resistance genotype.
  • Determine if correlation exists between mutation frequency and fungicide resistance.

Importance to the Oregon wine community:
In 2015, mildew control failures in numerous vineyards in Oregon were associated with mildew populations resistant to both QoI (FRAC group 11) and DMI (FRAC group 3) fungicides. Surveys of all Oregon appellations demonstrated that more than 90% of mildew samples had resistance to one or both fungicides classes.

In 2016 and 2017, widespread control failures were reported in Oregon, Washington and California. Recently, we have detected powdery mildew isolates resistant to both FRAC 7 and FRAC 13 fungicides, indicating these fungicide classes are also at risk. These data indicate there is a clear need to understand how fungicide resistance persists in a population in order to develop strategies to mitigate resistance.

Progress so far:
The fitness of fungicide-resistant mildew was examined in the field and controlled lab experiments. In most vineyards, samples from spore traps and glove swabs indicate that there is a significantly lower frequency of QoI resistant mildew in the early part of the season each year that transitions to a higher frequency as the season progresses. Lab studies showed that resistant isolates germinated more quickly at 50-60oF but behaved similarly at all others, which could partially explain why the frequency of resistance increases as the season progresses and indicate that use of QoIs early in the season could give resistant isolates an increased advantage.

QoI resistance in chasmothecia (the overwintering structure) and released ascospores decreases in frequency as the winter progresses such that it is very rare once bud break occurs.  The frequency of QoI resistant mildew in vineyards that have stopped using DMI and QoI fungicides has also significantly declined to very low levels since 2016. All these data indicate that QoI resistant isolates are less fit (lower reproduction and survival rates) and indicate that we may be able to effectively use QoI fungicides again.

There appears to be good correlation between the frequency of the Y136F genetic mutation and the rate of DMI fungicide tolerated by most powdery mildew isolates. However, there are isolates for which the presence of the Y136F mutation is not diagnostic for DMI tolerance. For more details see Detecting DMI resistance in powdery mildew.