2018-19 OWB Funded Research | Persistence of Fungicide Resistance in Grape Powdery Mildew

In its 2018-19 fiscal year, the Oregon Wine Board of Directors granted $437,500 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, which receives funding from the Oregon Wine Board, is part of a larger effort to develop improved methods for managing, assessing and mitigating fungicide resistance. The larger effort has also received funding from USDA-NIFA-SCRI, Northwest Center for Small Fruits Research and American Vineyard Foundation. This work is built upon prior funding from OWB for grape powdery mildew monitoring.

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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 to rejuvenate efficiency of cost effective fungicides; specifically Quinone outside inhibitors (QoI) and Demethylation inhibitors (DMI).

The specific objectives are:

1. Assess fitness (rate of reproduction and over-wintering) of necator isolates that are sensitive and resistant QoI and DMI fungicides;
2. Conduct mating studies of QoI and DMI sensitive and resistant necator isolates to determine inheritance of the resistance genotype;
3. Determine if correlation exists between frequency of the Y136 genetic mutation and fungicide resistance.

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

In 2016 and 2017, there were widespread control failures in Oregon, Washington and California. Recently, there has been a shift to using FRAC groups 7 and 13 fungicides but unfortunately we have now found powdery mildew isolates resistant to both of these fungicide classes. Thus, 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 is being examined on two fronts: field populations and control lab experiments. Both air sampling and swabbing of worker gloves, which collect spores, have a significantly lower proportion of resistant spore detection compared to colony samples taken from the vineyard. These results indicate that under field conditions, QoI-resistant mildew is less fit since it seems to sporulate less and will therefore spread more slowly.

QoI-resistant chasmothecia—the mildew’s over-wintering structure—were not detected as long into the spring season as wild-type chasmothecia, indicating that QoI resistance affects the ability of mildew to overwinter. The frequency of QoI-resistant mildew in vineyards that have stopped using DMI and QoI fungicides has also significantly declined to very low levels. All these data indicate that QoI-resistant isolates are less fit, indicating that we may be able to effectively use QoI fungicides again.

There appears to be good correlation between 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. These results indicate that air or glove sampling (i.e. samples that aggregate several isolates) are not suitable to detect the presence or absence of the Y136F mutation  and can only be useful for isolate testing. (For more details, see Detecting DMI resistance in powdery mildew.)

Next steps:
Even though this project is just starting, the results offer promise that the QoI mutation reduces the rate of reproduction and overwintering and that we can alter management programs to mitigate QoI resistance and, potentially, reduce resistant populations to levels where QoI fungicides are an effective management tool again. In the next 2.5 years, we will test this potential by understanding how resistance is inherited and factors that influence reproduction of resistance to understand when these fungicides would be most effectively used.