Wooster Unit 2 May/June Updates

By: Maria Smith, HCS-OSU

May weather:

Precipitation is the story so far.

Rain is an expected feature of spring in Ohio, and that’s not much of a debate. In fact, the 30-year average cumulative precipitation for May in Wooster is 3.73”. This year, we’ve received nearly 1” above that average with 4.59” of total cumulative precipitation (https://newa.cornell.edu Wooster Hort 2 station). The real uncanny part to me is the number of days that it has rained this month: 22 of 31 days. That’s right, we logged measurable precipitation for 71% the month.

…and the fun hasn’t stopped. June to date has logged an additional 2.18” across 4 total days of rainfall so far, which is nearly 57.6% of the 30-year average monthly rainfall, with more forecast over the next 7 days.   

Temperature and Growing Degree Days (GDD)

Not only was May rainy, but it was also cool. The average daily temperature in May was 57.6 F, which is 2.6 F cooler than the 30-year average. Despite below average temperatures, cumulative annual GDD (720, base 50 F, January 1 to June 12) remains similar to, albeit slightly below, the 30-year average (787). 

Phenology

Reports across the state place phenology anywhere from pea-size (Southeast Ohio) to pre-bloom and early bloom (Northeast Ohio). 

In Wooster, we are moving through bloom this week, with some early hybrid cultivars (e.g., Itasca) near fruit set, while others (e.g., Cab Franc) are just beginning bloom (Figure 1, Table 1). 

Figure 1. Clarion approaching post-bloom (top) and Regent at trace-bloom (right). 10 June 2025. 

Table 1. 50% bloom date and GDD for select cultivars at Hort 2, Wooster, OH. 

Canopy management

With the rain and recent warming temperatures, shoots have reached nearly 3’ in length (Figure 2). Shoot and wire positioning Vinifera for VSP and suckering/thinning have been ongoing over the past several weeks. We are waiting to comb the high-wire shoots on the hybrids until the fruit starts to weigh the shoots down and shoot tips become less prone to breaking (Figure 3). 

Figure 2. Regent canopy, VSP, 10 June 2025

Figure 3. Clarion canopy, High-wire cordon, 10 June 2025.

Disease updates

Cool, rainy conditions in May meant Phomopsis in several cultivars at Unit 2 (Figure 4). In more southern regions, many are also seeing Anthracnose stem and leaf infections (Figure 5). Reminder: we are still well-within the critical window for flower and fruit protection for all major diseases. This period over the next several weeks are the most important time of the season to ensure broad spectrum coverage using your best fungicides for preventative protection. For fungicides registered for grapes and disease efficacy, see: Midwest Fruit Pest Management Guide. 

Figure 4. Phomopsis on shoots and leaves, Clarion, 27 May 2025

Figure 5. Anthracnose on Vidal blanc. Note how the lesions are gray in the center and sunken compared with Phomopsis. Leaf lesions of Anthracnose also fall out as the leaf tissue dies, leading to a “shot hole” appearance. June 2025. (Photo used with grower permission)  

Insect management

It is right about time to start turning our attention to Grape Berry Moth (GBM). This is one of the handful of insects that can cause direct crop loss from fruit damage (Figure 6). 

The other major insect to begin to start monitoring for is Japanese beetles. These foliage feeders can get out of hand quickly if allowed to accumulate large populations. 

Some minor pests to monitor for include gall makers (grape cane gallmaker, grape tumid gallmaker), flea beetle larvae, and leafhoppers. 

Scouting and monitoring (using GDD-based tools and traps) are the best means for deciding when insect control is warranted. In Wooster, we typically use fewer than 5 insecticide applications all season (this includes pre-harvest sour rot management sprays) by regular monitoring and providing well-timed and appropriate insecticide applications. Excessive use of insectcides may cause secondary pest infestations, including mites and mealybugs.  

Figure 6. Berry damage from GBM in July 2024 (and some more Phomopsis on the stem). 

Figure 7. Japanese beetle adult (top). Foliar feeding damage from Japanese beetles (bottom). July 2023.