Late-season fungicide sprays in grapes and potential effects on fermentation

Knowing what spray to use, the rate applied and rainfall amount are factors to consider when concerned if applying a late-season fungicide spray in grapes will lead to problems with fermentation.

Most pesticides leave detectable residues on fruit and, for safety reasons, have a restriction on when the last spray can be applied prior to harvest. This is called the pre-harvest interval (PHI). Fungicides differ greatly in their PHI’s, with some allowed on the day of harvest (i.e., Oxidate, Serenade) and some having a 66-day PHI (i.e., Manzate, Ridomil MZ). The PHI is related to the residue tolerance (amount of active ingredient allowed on the harvested portion of a crop), how fast the residue is known to degrade, and how the crop is used, although sometimes it is difficult to understand how the same fungicide can have radically different PHI’s depending on the crop. Late-season disease outbreaks, such as powdery mildew, downy mildew and bunch rots may necessitate fungicide sprays close to harvest. Even after harvest, keeping leaves relatively healthy for as long as possible is important for the proper hardening off of the grapevine prior to winter. In wine grapes, the application of bird netting may impede additional spray applications or reduce fungicide coverage. From a winemaking standpoint, one of the primary concerns about late-season fungicide sprays is that potential residues may inhibit fermentation. Since yeasts are basically single-celled fungi, they can be killed or inhibited by fungicide residues, leading to so-called “stuck” fermentations.

The most common explanation for stuck fermentations and sulfur problems, such as hydrogen disulfide (H2S) or “rotten egg” smell, in wine is the excess or late application of sulfur in vineyards to control powdery mildew. Yeasts have the ability to convert elemental sulfur into H2S. More than 5 mg/L of residual elemental sulfur in grape must is likely to cause problems, although amounts as low as 1 mg/L have been implicated. Consequently, a late application of sulfur, particularly if no rain occurs between the spray and harvest, may lead to problems. Thus, the rule is not to use sulfur sprays in the vineyard within several weeks of harvest (up to five or six weeks in California). However, even though sulfur is a likely culprit, it is not the only explanation of fermentation problems.

Copper sprays may also be a problem. Copper is toxic to yeast and even non-lethal doses of copper can cause stress on the yeast, potentially leading to incomplete fermentation and release of undesirable metabolites, all leading to wine aroma defects. Copper can also be inhibitory to bacteria responsible for malolactic fermentation. Generally, this occurs only with very high residual copper concentrations. Since there are currently many different and more benign fungicide options for control of downy mildew later in the growing season, copper really isn’t needed late in the season. However, organic growers having fewer fungicide options may still consider using copper for downy mildew control. The biofungicide Serenade (Bacillus subtilis) or compost tea are other control options in this case.

The broad-spectrum fungicide Captan (captan) is used by some growers late in the season to specifically control a variety of bunch rots as well as downy mildew. In studies done in California in the 1950s, captan residues of at least 1 mg/L delayed alcoholic fermentation by 20 to 40 hours, but did not prevent the proper completion of fermentation once it had started. The degree of delay was related to the amount of residue, with smaller amounts only causing slight delays. However, wines prepared from captan-treated grapes retained a definite cloudiness after six months of storage, but the cloudiness could be removed by filtration. Another study showed that Captan was degraded in grape must and was only found as its metabolite (THPI) in wine. The degradation of captan to THPI was due to the acidity in must and wine. This metabolite was present at low levels on grapes, and, unlike captan, had no negative effect on the fermentative process.

Other possible late-season sprays include potassium bicarbonate (Kaligreen, Armicarb, Milstop), monopotassium phosphate (Nutrol), hydrogen peroxide (Oxidate), JMS Stylet Oil and the phosphorous acid products. The first two contain potassium, but there is no evidence that must pH is raised by these products. It is probably prudent, however, to avoid a heavy application shortly before harvest. No issues are known to occur with hydrogen peroxide, which dissipates rapidly after application. JMS Stylet Oil is actually a very good late-season spray to reduce powdery mildew and European red mite. There is some evidence that late applications depress Brix (sugar) accumulation due to a temporary reduction in photosynthesis. Research conducted in California indicated that Stylet Oil had no effect on fermentation nor are there any obvious issues with the phosphorous acid products.

With synthetic fungicides such as the strobilurins (i.e., Sovran, Abound) and sterol inhibitors (i.e., Elite, Rally), there appears to be no issues from a fermentation stand-point. Late-season use of strobilurins is not recommended anyway due to possible fungicide resistance issues when spraying them on actively sporulating infections. The powdery mildew fungicide Quintec (quinoxyfen) has also been studied and no negative effects were found on fermentation. In addition, the Botryticides Elevate, Vangard and Scala are safe for both alcoholic and malolactic fermentation. In general, these fungicides are quite specific for Botrytis and inactive against most other fungi, including yeasts. The older fungicide Rovral (iprodione) also has no known effects on fermentation, vinification or organoleptic qualities of wine. Leaf pulling around clusters generally is more effective than fungicides in reducing bunch rots, but should have been done earlier, like at bunch closure or veraison. Late-season sprays for Botrytis bunch rot may be needed as the disease can increase rapidly under the right conditions in the weeks before harvest, but are not likely to lead to fermentation problems.

A number of factors can influence whether a late fungicide spray will lead to problems with fermentation or not. Obviously the type of product used, but also the rate applied (higher rates can lead to higher residues at harvest) and whether rain occurs between the spray application and harvest. One to 2 inches of rain usually removes at least 50 percent of fungicide residues from plant surfaces, however, many clusters are shielded to some extent from rain by the canopy. When spraying sulfur or copper fungicides, residues on grape clusters may be reduced by turning nozzles off in the cluster zone. Some enologists call for at least 30 days between the last spray and harvest. However, this probably applies to the West Coast where little rainfall occurs and residues remain on fruit for longer periods. There, fungicides may be reduced more by UV light degradation. There is also evidence that the fermentation process reduces fungicide residues in wine. For instance, fenhexamid (Elevate) residues were reduced by fermentation on the skins.

In general, it would be best to minimize fungicide sprays within one to two weeks before harvest where possible, not only with a view to fermentation, but also to the presence of fungicide residues in wine and juice products for human consumption.

Based on articles by A. Wise, Cornell University in “Wine grape information for Pennsylvania and the region,” 2006; J. Castor et al., American Journal of Enology and Viticulture, 1957; and R. Church, Understanding and eliminating sulfur-related aroma defects in wine, 2004.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.

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