Managing Fungicide Resistance and Cucurbit Powdery Mildew.

Editor’s Note: In recent years, a number of new fungicide chemistries have become commercially available for vegetable growers to control diseases. Many of these new chemistries have site-specific modes of action and target a single site in fungal growth and development. Because of this specificity, some chemistries have a high risk for fungicide resistance development.

A few years ago industry personnel established the Fungicide Resistance Action Committee, or FRAC, to address fungicide resistance management. FRAC helps identify existing and potential resistance problems, develops resistance management guidelines, and in the process, stimulates collaboration among universities, government agencies, crop advisers, extension workers, distributors, and growers. Most importantly, this committee established what are known as FRAC codes for different fungicide chemistries. Fungicides with similar modes of action (MOA) are placed into the same FRAC code group.

By Meg T. McGrath and Andy Wyenandt

Powdery mildew is the most common disease of pumpkin, squash, and other cucurbit crops, occurring every year throughout the U.S. Cucurbit powdery mildew can affect leaves, petioles, vines, and stems of fruit causing premature defoliation resulting in poor yields and fruit quality. Regular foliar applications of fungicides are needed, even with resistant varieties, and it is especially important to control powdery mildew on the lower surface (underside) of leaves where conditions are most favorable for disease development. Fungicides that are mobile provide the most effective control on lower surfaces, but are at risk for resistance development due to their single-site mode of actions.

Cucurbit powdery mildew has a long history of adapting to fungicide chemistry. Resistance to Benlate (FRAC code 1), was first detected in 1967 before this product was even registered for cucurbit powdery mildew, and was one of the first documented cases of fungicide resistance in the US. Bayleton, the first DMI (demethylation inhibiting) fungicide (FRAC code 3), was registered for powdery mildew control in 1984 and the first reported control failure occurred just two years later. Bayleton failure became widespread during the early 1990s. Since DMI resistance is quantitative, it was not surprising that Nova (myclobutanil), a new DMI being developed then, was highly effective when Bayleton failed in experiments. Nova was registered for this use in 2000 and another new DMI, Procure (triflumizole) was registered in 2002. Unfortunately, soon thereafter Nova at the lowest label rate exhibited reduced efficacy. Recently, both Nova and Procure at high rates have exhibited poor contr!
ol in some areas. Quadris was the first QoI fungicide (azoxystrobin, FRAC code 11) used commercially in 1998 in some states under an emergency exemption granted because of control failures due to Bayleton resistance. Quadris was registered in 1999 and resistance to QoIs was first detected in the U.S. in 2002.

Guidelines
Reduce the need for fungicides by using an integrated program consisting of a few timely fungicide applications to powdery mildew-resistant varieties.

Use mobile fungicides only when needed and not curatively. Scout to ensure fungicides are applied very early in powdery mildew development. Examine both leaf surfaces of 5 older, crown leaves in at least 10 locations throughout a field. Start applying mobile fungicides when powdery mildew is found at a very low level on at least 1 of 50 leaves. Apply only protectant fungicides late in the growing season when a high level of control is no longer essential.

It is critically important to examine the underside of leaves, especially where protectant fungicides have been used.

Alternate among mobile fungicides with different modes of action and tank mix with protectant fungicides. Alternate among effective mobile fungicides in at least two different FRAC code groups on a seven-day schedule, with a protectant fungicide included in every application. All at-risk fungicides should be tank-mixed with a protectant fungicide because protectant fungicides have multi-site activity and thus a low resistance risk and will help control pathogen strains resistant to the mobile fungicide.

Specific fungicide recommendations can vary from year to year and even among locations due to development of resistance and registration of new fungicides. For 2007, Pristine and Procure at the highest label rate are considered the best choices. Resistance to FRAC code 1 and 11 fungicides was common where examined in the northeast in recent years, thus these fungicides are no longer recommended.

Maximize control obtained with protectant fungicides by selecting a product with good efficacy and ensuring good spray coverage.

Assess efficacy on the lower surface of leaves following fungicide applications. Any disease control problems should be reported promptly to local extension specialists. Mobile fungicides should no longer be used once powdery mildew is severe; use only protectant fungicides.

Lessons Learned about Resistance

1. The cucurbit powdery mildew pathogen has a high potential of developing resistance to certain fungicides and has developed resistance to every chemical class labeled at-risk for resistance following repeated use somewhere in the world. Presence of resistant strains has been associated with control failure. Therefore a resistance management program needs to be implemented each season beginning the first year the product is used.
2. It is expensive to manage a disease when resistance is an issue. For example, if resistance was not a possibility, powdery mildew could be controlled by applying Nova at the lowest rate (about $11/acre) and/or Cabrio ($19) rather than Procure at the highest label rate ($23) alternated with Pristine ($39) plus a protectant fungicide with both ($13-20 for Bravo).
3. While we have learned a lot about resistance, it can still be difficult to predict. For example, when QoIs were developed, it was thought that the cucurbit powdery mildew pathogen would be the first to develop resistance, and resistance was predicted to be quantitative and slow to develop. However, gummy stem blight pathogen developed QoI resistance first and resistance was qualitative.
4. Detecting resistance based on fungicide performance in a commercial field can be difficult when guidelines for managing powdery mildew are followed because other fungicides may sufficiently cover up control failure. University fungicide efficacy evaluations play a critical role in resistance monitoring.

McGrath is associate professor at Cornell University and Wyenandt is Extension specialist in Vegetable Pathology with the New Jersey Agricultural Experiment Station, Rutgers University; mtm3@cornell.edu.