Home Industry News Economics To Use or Not Use? Dealing with Fungicide Resistance

To Use or Not Use? Dealing with Fungicide Resistance

Fungicide resistance in both grape powdery mildew and botrytis bunch rot is making growers question what fungicides are and aren’t efficacious. Like with many things in life, the answer is… “it depends.”

Botrytis bunch rot. Alex Wong, during his PhD in my group, monitored fungicide resistance in Botrytis cinerea across Oregon and Washington production regions. He observed that over 50% of the Botrytis isolates tested had some level of tolerance to at least one fungicide class with 3% having tolerance to four or more fungicide classes. All fungicide classes labeled had at least one Botrytis isolate with tolerance to that class. Additionally, Alex observed that resistant isolates can be found in the rachis and cane debris on the vineyard floor and in nearby blackberries. These data indicate that growers should rotate fungicide classes across growing seasons and never use the same class more than once in a season. This also means that the fungicide class last used for bunch rot management the previous season should not be the first class used for bunch rot management this season.

Alex also used DNA from our spore trapping work to examine when Botrytis spores are available and noticed two interesting trends. First, Botrytis inoculum was always available. This coupled with the fact that spores survive dormant on plant surfaces for several weeks means that if the right weather conditions occur, there is always potential for Botrytis infection, and we need to have the clusters protected. Second, Alex observed that Botrytis inoculum concentrations peaked right around bloom time, reaching tens of thousands of spores per day (see the data http://gall-id.cgrb.oregonstate.edu:3838/grunwald/wonga4/). These data reinforce our understanding that mid-bloom to fruit set is the most critical time to protect against Botrytis bunch rot. It also indicates that this period is when a newer fungicide class should be applied to reduce the chance of fungicide resistant isolates infecting or colonizing flower debris in the clusters.

Grape powdery mildew. My group has spent the past several years examining fungicide resistance development in grape powdery mildew. In 2017, we showed that over 90% of the late season mildew population has resistant to QoI (FRAC 11, quinone outside inhibitor/strobilurin) fungicides, and 73% of the population had tolerance to DMI (FRAC 3, dimethyl esterase inhibitors) fungicides. These results meant that over 65% of the population had tolerance to both DMI and QoI fungicides. Based on these data and outreach efforts, many growers stopped using both fungicide classes and turned to SDHIs (succinase dehydrogenase inhibitors, FRAC 7) and other new fungicide chemistries. Unfortunately, we are now starting to find isolates with tolerance to these newer fungicides, but we have not yet seen field failures. To avoid future field control failure, fungicide classes need to be rotated as much as possible and premixed fungicides (e.g., those with two or more active ingredients premixed together) used very carefully and both fungicide classes in the premix rotated. Many of these premixes use a new chemistry that is likely prone to developing resistance with an older chemistry to which our grape powdery mildew population may already have tolerance (e.g., FRAC 50 with FRAC 11). The issue with using products like this is that only the FRAC 50 is efficacious, and we are now selecting for isolates that have both FRAC 50 and FRAC 11 resistance. Remember that over 65% of the mildew population was known to have resistance to both DMI and QoI. Thus, by using a QoI, we are likely also selecting for DMI resistance and isolates with resistance to all three fungicide classes. An analogy for selecting fungicide resistance would be to sort through pocket change for a coin that has Oregon stamped on it. That coin will most likely be worth 25 cents and have George Washington on it, even though you did not select for these characteristics. You selected for one trait and others came along for the ride.

Now, what do you do since we are starting to see resistance to so many fungicides? What fungicides are there left to rotate into your management program? That is a very difficult question, and Sarah Lowder (now with University of Georgia, Athens) spent a good bit of her PhD in my group examining whether we could mitigate QoI tolerance in vineyards. Her work showed that the frequency of the mutation for QoI tolerance decreases over the winter in chasmothecia (over-wintering structures) on grape bark. This reduction results in QoI resistance being barely detectable in the spring. The results made her wonder if we could integrate QoI fungicide use back into our management program. So, Sarah set up field scale trails in Oregon and California (in cooperation with Paul Walgenbach at Bayer) to examine whether QoIs can be used during the season. Blocks of 4 to 9 acres were split, and each side treated identically except for either a QoI or a SDHI application at the same time around bloom. Across all nine locations, there was no significant difference in disease levels or the frequency of QoI resistance in the population. However, the frequency of QoI resistance increased with each sampling post QoI application, and at harvest nearly 100% of the samples had detectable QoI resistance. These data indicate that we can reintegrate QoIs into powdery mildew spray programs if the frequency of resistance is low, but we can only use them one time that season.

The above research results indicate that fungicide resistance is a problem currently and will continue for many years to come. However, it also indicates that resistance can be mitigated by being diligent in fungicide rotations within and across season, limiting the frequency of each fungicide class used, and making informed decisions by using monitoring to assess for resistance presence. For information about available fungicides for Botrytis bunch rot and powdery mildew management, see the Pacific Northwest Disease Handbook or the Pest Management Guide for Wine Grapes in Oregon— By Walt Mahaffee, USDA-ARS Horticultural Crops Disease & Pest Management Research Unit (Oregon Wine Research Institute)

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