Overview of EPA Proposed Cancellation of Mancozeb in Grape Production

In the eastern US, grapes are typically grown in a wet, humid climate and subjected to many diseases caused by fungal pathogens. Therefore, fungicides are an essential part of grape production in the eastern US, and fungicides containing mancozeb are applied by commercial growers virtually every season, for a variety of reasons.

Mancozeb is a “multi-site inhibitor”, that controls fungal diseases by impairing or inhibiting multiple essential biological processes of the targeted pathogens. In so doing, mancozeb is very effective at controlling several very serious diseases like black rot, Phomopsis, and downy mildew; the most serious of which is downy mildew. These diseases are not typically encountered in drier climates like California, but they can cause devastating crop loss for eastern grape growers if not controlled adequately in every season, particularly in wet seasons conducive to the development of fungal pathogen populations. The fact that mancozeb inhibits pathogens at multiple levels, makes it very unlikely that those pathogen populations will develop resistance to mancozeb, and mancozeb has therefore been used very effectively to control grape diseases in the east, for decades. This makes mancozeb an extremely important tool for eastern grape growers.

In contrast, more modern fungicides are “single-site inhibitors”, that control diseases by inhibiting just a single essential biological process of the pathogen life cycle. This makes the single-site inhibitors much more vulnerable in terms of the pathogen being capable of developing resistance to that single-site fungicide active ingredient: a single mutation in the pathogen population can quickly render that fungicide ineffective at controlling that pathogen on a commercial scale. Metalaxyl is a classic case of an excellent single-site inhibitor fungicide (very specific and very effective at controlling downy mildew of grapes) that is, unfortunately, very vulnerable to the development of resistance by the downy mildew pathogen. For example, in Europe, metalaxyl provided excellent control of downy mildew of grapes when first registered for use in 1977. However, after just a few years (by 1983, just 6 years after it was introduced), populations of the downy mildew pathogen had developed resistance to it. We have maintained its effective use here in the United States for decades. How? By limiting its use (because we had several other options), and by tank mixing it or rotating applications of it, with multi-site inhibitors, primarily mancozeb.

With the impending loss of ziram through EPA review, our reliance on mancozeb will be even more important for the control of grape diseases as well as managing resistance to single-site inhibitors. If we lose the use of mancozeb in grapes, we will be left with just two, less effective, multi-site inhibitors (captan and copper) to take on the role that mancozeb now most prominently serves in the eastern grape industry. Unfortunately, captan is not as effective as mancozeb on black rot, and copper is not very effective at all on Phomopsis and black rot, in relation to mancozeb. Also, captan has been recently subject to EPA review and impending, additional label restrictions (mainly an extension of the re-entry interval).

Perhaps most alarming about the potential loss of mancozeb by EPA review, is that other single-site inhibitor options for downy mildew control are already being lost to resistance. Recent research has confirmed that several single-site inhibitors for downy mildew (Fungicide Resistance Action Committee [FRAC] 11, P07, and 40) are failing to control this disease in Pennsylvania, New York, and Maryland vineyards, due to resistance. Earlier research in Virginia resulted in similar conclusions, with strobilurin resistance (FRAC 11) detected as early as 2005 (just 8 years after its introduction), and carboxylic acid amide resistance (FRAC 40) detected as early as 2016, in the downy mildew pathogen. Indeed, new evidence of downy mildew resistance to FRAC 11, FRAC 40, and FRAC P07 chemistries represent the loss of efficacy for half of our important single-site inhibitor fungicides for grapes. This has occurred over a period of about 15-20 years, a timeframe likely only achievable due to the current heavy reliance on mancozeb as an important resistance management tool. That leaves us with single-site inhibitor fungicides in FRAC groups 4, 21, and 45, making the retention of mancozeb registration even more essential for the economic sustainability of the eastern grape industry. If we lose mancozeb, we will be relying solely on single-site inhibitors for effective black rot control, and we may lose the remaining single-site inhibitors for downy mildew control, over a span of time too short for the industry to replace them with new, effective single-site inhibitors, with different modes of action. The loss of mancozeb will pose a serious threat to the sustainability of the eastern grape industry, as mancozeb is arguably the most important grape fungicide for much of the acreage in the eastern grape industry.

For more information on this matter, be sure to check out Dr. Katie Gold’s “Urgent Update on EPA Mancozeb Proposal”. In her article, she distills all the important details of the EPA’s proposal and how the affected members of the eastern grape industry can respond. Please click on the “survey” link at the beginning of her article and complete the brief survey (it’ll take you 5 minutes and it’s time well spent), as this survey is relevant to the Pennsylvania grape industry.