August/September Hort 2 Updates

By: Maria Smith, HCS-OSU

An abrupt change in weather patterns in late-July has changed the outcome for this growing season for fruit quality and maturity… but not before making its impacts on yield. 

August/September weather

Precipitation

Genuinely, a tale of two very different halves to the season. What started out as unending rain has transitioned into mild to moderate drought conditions through mid-September (Fig. 1). From 1 Aug to 15 Sep, Hort 2 in Wooster has only seen 1.03” of cumulative rainfall, more than half of which fell in a single day (0.66” on 19 Aug). That is in contrast to the approximately 7.45” of cumulative precipitation typical of August and September long-term (30-year) cumulative precipitation (https://newa.cornell.edu/ and https://mrcc.purdue.edu).

This pattern is effectively due to a reverse of the same “Omega Block” pattern that lent itself towards our cool and very rainy spring.  When will it change? I’m unsure. The 10-day outlook continues forecasting dry, warm, and sunny weather. I will discuss more on the implications for ongoing harvests and fruit quality below.   

Figure 1. Current drought map for Ohio, as of 9 September 2025. Figure from https://droughtmonitor.unl.edu/.

Temperature and GDD

August average temperatures were on-the-mark this year at 70.1F (30-year average at 70.5F). So far, we are on track for near to above average (63.6F) September temperatures at 64.1F. However, it’s been a bit of a wild ride so far, with a 5-day stretch averaging 60.4F. For the grapes, a late-September warm-up to summertime temperatures bodes well for reaching better maturity across the back-half of harvest. 

Growing degree days (GDD, base 50F) from 1 Jan to 15 Sep is currently 2574. The long-term, 30-year average cumulative GDD for 15 Sep is 2745, indicating an overall cooler year in Wooster. Interestingly, we are behind in temperature accumulation compared to the Kingsville vineyard (AARS), which is uncharacteristically ahead of Wooster and above average in heat accumulation for this year (currently 2738 vs 2524 for the 30-year average).

Fruit set, yield, and fruit maturity trends 

The bloom period this year was marked by very cool and rainy conditions, leading to poor fruit set (Fig. 2) in several cultivars at Unit 2 in Wooster. The general trend for overall yields is lower this year, however, it’s unknown if this is purely due to poor fruit set or if there were carry-over effects stemming from the 2024 herbicide drift injury.  

Figure 2. Poor fruit set and small cluster sizes in Regent at 100% veraison on 11 August 2025 (Top). Fruit set and cluster size was highly variable across several Vinifera and hybrid cultivars, including table grape Vanessa (Bottom).

With good diurnal temperature flucuations and dry conditions, overall fruit quality is high so far with only a few notable pre-harvest issues. This year, sugar accumulation is high, although with the cool September days, acid has been slower to degrade and is stubbornly high in the early- to mid-season cultivars (Fig. 3, for all reported cultivars, visit the 2025 Weekly Fruit Maturity Report at https://go.osu.edu/grapes). The warmer forecast should aid in reducing acidity moving over the next couple of weeks. 

Figure 3. Weekly total soluble solid accumulation (top) and titratable acidity (bottom) for Regent (harvested 9/3/25) and Marquette (harvested 9/10/25).

Sure, we have some minor issues here and there, such as minimal foliar downy mildew in August (remember, it did rain at least a few days) and the occasional Phomopsis infection from the early season (Fig. 4). The real frustration for us this fall, because there’s always at least one, is the birds. Despite netting ahead of veraison this year, birds have been a persistent problem for continuing to hang fruit. Have you noticed that the grapes are there one day and gone without a trace the next, but there’s still a rachis? That’s likely bird damage (Fig. 5). 

Several growers across Ohio have reported significant bird injury this year. If you are starting to see significant yield loss (>20%) as a result of bird damage, it’s better to harvest pre-maturely than to risk losing an entire crop. 

UNH has one of the most extensive factsheets on managing birds in fruit crops. While netting is the most tried and reliable protection method, mesh size, shape, and timing of application are important variables to consider. In some cases, deploying multiple tactics may be necessary depending on the level of pressure and species causing damage. Some of the more common netting supplements and alternatives that I have come across over the past few years include Bird Gard auditory devices, laser deterrent systems, and raptor nesting for increasing natural predators. Adopting any practice comes with pros, cons, costs, and benefits. It is important to weigh these prior to adopting new protection methods for your operation.

Figure 4. Phomopsis infection in the rachis and berry in Vanessa at harvest (21 Aug).

Figure 5. Bird damage to Vanessa (top, 21 Aug) and Regent (bottom, 9 Sep) at harvest.