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By: Maria Smith and Lisa Dunlap – HCS-OSU

2026 cannot catch a break. Winter freeze injury, spring freeze injury, heat, wildfire smoke, unstoppable rainfall… and it’s still August. We are currently on track across the state for an early harvest with tough management decisions heading into the home stretch of the growing season. It has been a warm year, with about 200 to 400 higher GDD to date from the long-term average. Most notably, however, the whole state has seen above average rainfall, with central and southern half of the state receiving nearly 300% of annual monthly precipitation over a handful of days in early August (Figure 1). 

 

Figure 1. GDD departure from 1 April to 16 August (using modified max temperature value of 86F) and 30-day percent depature of precipitation from historical average from 18 July to 16 August. Maps from https://mrcc.purdue.edu.  

Current Vineyard Management Decisions (Pre-harvest)

The grapes are all well into ripening across the entire state, including Ashtabula County, which is currently about 10-14 days ahead of the 2025 growing season. With both an early ripening year and a very rainy year, routine monitoring of fruit conditions becomes even more critical. But also, as recently mentioned, disease management and PHI becomes increasingly important. The following sections will highlight some of the recent rain-related issues that we have encountered through scouting at Hort 2 and regional site visits. 

As always, consult management guides and product label restrictions prior to making any pesticide application.

Downy Mildew

Despite best seasonal management of DM, it is popping out following our past couple of weeks of persistent rainfall (Figure 2). How can you manage DM close to harvest? It’s not easy, and some may elect to tolerate some foliar infections to avoid use of fungicides close to harvest. Keep in mind, though, that DM is one of the few diseases that can very quickly defoliate a canopy if left uncontrolled for the remainder of the season. Below are options and supplemental resources to help guide your decisions. More options become available during the post-harvest period to carry the foliage through to the first hard freeze. 

• Captan, Phosphorus Acids, and Copper all have a 0-day PHI, however, there are many considerations to make when using low-PHI options. These include residue and potential fermentation inhibitions from Captan and Copper, fungicide tank-mix incompatibilities, and emerging DM resistance to Phos. Acids

Additional resources discussing DM and low-PHI fungicides:

• https://extension.psu.edu/veraison-to-harvest-vineyard-and-winery-considerations
• https://extension.umd.edu/resource/pre-harvest-disease-management-and-late-season-bunch-rots/

 

Figure 2. Recent downy mildew outbreak and active sporulation from the front (top) and back (bottom) of leaves. 24 August.  Photo credit: Diane Kinney.

Powdery Mildew

In warmer regions of the state, PM is emerging as a late-season issue (Figure 3). There are more options to manage PM than DM when it comes to PHI, including the following suggestions from UMD:

• Select FRAC 3 and FRAC 7 fungicides, Quintec (FRAC 13), Vivando (FRAC U8), and Torino (FRAC U6). 

Rotation is key for PM management, where excessive and repetitive use of a single FRAC code increases risk of PM resistance. 

Figure 3. Powdery mildew and berry splitting in Vidal blanc. August 2026. 

Late-season Rots (Sour Rot, Botrytis, Ripe Rot)

Sour rot is typically the worst of the three when it comes to rainy conditions due to berry splitting caused from excessive rainfall. Split berries provide opportunities for fruit fly and other insects that vector bacteria and wild yeasts, leading to berry discoloration and characteristic “sour” vinegar (acetic acid) aromas (Figure 4).

Botrytis infections may also arise from infections that occured earlier in the growing season during bloom or from veraison through to harvest (Figure 4). For cultivars that are highly susceptible to Botrytis, notably highly compact cluster cultivars, there are effettive cultural and chemical management options avaialble. When using Botrytis-specific fungicides, rotation is necessary to limit resistance issues.   

• Conventional chemical management for sour rot is to apply one combination of low-PHI insecticides targeted for fruit flies (e.g., Mustang Maxx, Malathion) combined with peroxyacetic acid (e.g., Oxidate) at 15 brix. *Again, it should be noted that insecticide-resistance for fruit flies has been documented in neighboring states (Michigan, New York), which may contribute to sour rot control failure. Judicious use of insecticides for sour management should be practiced. 

Figure 4. Botrytis (left) and sour rot (right) in Sauvignon blanc. Botytis is characterized by fuzzy gray spores, while sour rot is typically characterized by tan berries with an acetic (vinegar) odor.

When is your fruit ready to pick?

Fruit maturity

Measuring basic fruit chemistry (total soluble solids, titratable acidity, and pH) provide basic indicators of fruit maturity, yet do not tell a complete story. Flavor, seed development, and other characteristics aid in identifying peak maturity. Ripening parameters are not always a linear trend and will vary by precipitation amounts (i.e., sugar levels decrease with pulp dilution and rainfall). In some cases, sugar concentration may increase by continuing to hold the fruit on the vines post-rain, but in other instances where harvest maturity may be close, taking a risk-averse approach to harvest prior to any subsequent rain event may be preferable to avoid significant yield loss.

Other indicators may dictate that fruit should be harvested whether chemical parameters indicate full maturity. These can include “shelling” or “shatter”, where individual berries easily detach from the pedicel, and various types of berry shrivel disorders (e.g., bunch stem necrosis) that have associations with abiotic stress. In cases where overall cluster and berry conditions are a factor, fruit should be harvested to preserve yield regardless of our desire to hit certain quality thresholds. This is also where decisions in the winemaking process become integral to achieving quality wine from sub-optimal grapes. 

Fruit quality and rot thresholds

Following a season characterized by frequent rainfall and elevated disease pressure, wineries may be faced with fruit containing varying levels of sour rot, Botrytis, and other bunch rots. While there is no universal threshold at which fruit should be accepted or rejected, it is important to recognize that the severity and distribution of rot can significantly impact wine quality, processing requirements, and production costs.

The decision to harvest fruit should not be based solely on soluble solids, pH, or titratable acidity. Cluster integrity, berry splitting, evidence of sour rot, and overall fruit condition must also be evaluated. In some cases, harvesting fruit slightly earlier may preserve more quality than waiting for additional ripening while disease pressure continues to increase.

From a winery perspective, the question is often not "Can wine be made from these grapes?" but rather "What level of intervention will be required to produce acceptable wine, and is that effort justified?" As rot incidence increases, the risk of elevated volatile acidity, microbial spoilage, oxidation, and fermentation difficulties also increases.

Fruit Processing Considerations

Sour rot presents challenges beyond the acetic acid already present in infected berries. The larger concern is introducing acetic acid bacteria (AAB), spoilage yeasts, and other undesirable microorganisms into the winery. Once these organisms enter the production process, they can continue to increase volatile acidity and negatively impact wine quality.

When processing compromised fruit, wineries should consider the following practices:

1. Sort aggressively. Remove visibly damaged, split, or sour-rotted clusters whenever possible. This is often the most effective step for reducing microbial load and lowering the amount of preformed acetic acid entering the winery.
2. Process fruit promptly. Fruit affected by rot can continue to support microbial activity after harvest. Minimize delays between picking and crushing.
3. Minimize oxygen exposure. Acetic acid bacteria require oxygen for growth and metabolism. Avoid excessive fruit handling and limit exposure of crushed fruit to air. The use of carbon dioxide or nitrogen may be beneficial when feasible.
4. Maintain sanitation. Equipment, bins, presses, pumps, and hoses should be properly cleaned and sanitized throughout processing to reduce opportunities for microbial contamination.

Options for juice management pre-fermentation

For fruit exhibiting signs of sour rot or elevated microbial pressure, early intervention can help reduce the risk of spoilage during fermentation.

Sulfur dioxide additions at crush remain one of the most important tools available to wineries. Potassium metabisulfite additions help suppress spoilage organisms and slow the development of additional volatile acidity. However, sulfur dioxide does not remove acetic acid already present in the fruit. The goal is microbial control rather than correction. Depending on fruit condition and juice pH, additions of 50 mg/L or greater may be appropriate for compromised fruit.

Clarification and settling can help reduce microbial populations and remove particulate matter that may harbor spoilage organisms. Where appropriate, winemakers may consider settling juice prior to inoculation to improve fermentation conditions.

Prompt yeast inoculation is strongly recommended for compromised fruit. Allowing desirable wine yeast to rapidly dominate the fermentation reduces opportunities for spoilage organisms to proliferate. Cold soaking and uninoculated fermentations generally increase risk when processing rot-affected fruit.

Nutrient management should also receive additional attention. Diseased fruit can create a less favorable environment for yeast growth, increasing the likelihood of sluggish or stuck fermentations. Monitoring and correcting YAN levels may be necessary to support a healthy fermentation.

Finally, wineries should routinely monitor volatile acidity (VA) through both sensory and analytical evaluation. Measuring VA at the completion of primary fermentation provides an important baseline for future cellar decisions.

Remember: you cannot ferment away acetic acid. The most effective strategy is preventing additional volatile acidity development through careful fruit sorting, quick processing, microbial control, and quality fermentation management.

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Links to additional OSU Factsheet Resources:

• Sour rot grapes: https://ohioline.osu.edu/factsheet/plpath-fru-50
• Powdery Mildew grapes: https://bpb-us-w2.wpmucdn.com/u.osu.edu/dist/b/28945/files/2026/08/Grape-Powdery-Mildew-Print-Version-2026.pdf
• Downy Mildew grapes: https://ohioline.osu.edu/factsheet/plpath-fru-33