Fruit Phenology Update April 26, 2024
This past week, we have experienced several frost events. On 4/20 and 4/21, temperatures across the state dropped to 25˚ F in central and northern Wisconsin and 29-30 ˚ F in southern Wisconsin. Yesterday, on 4/25, the Door Peninsula experienced a hard frost, with temperatures reaching 24 ˚ F for several hours in places like Egg Harbor. These frosts are of particular concern for apple growers since the crop is at tight cluster and king bloom. It is critical for growers to go out to their orchards and scout for damage, especially in low spots. This information will help you determine the best thinning strategy for this season. Please refer to the companion article in this newsletter for frost damage assessment.
Based on the weather forecast for this weekend and early next week, minimum temperatures will be above the freezing point, which is a relief given that apples, blueberries, and strawberries in the southern part of the state are close to blooming. Next week, we will also have rain during the beginning of the week. Based on the accumulated Growing Degree Days (Table 1), the southern part of the state is very close to where we were at this time of the year in 2023, while central and northern Wisconsin are behind compared to 2023.
| Location | 2023 | 2024 |
| GDD base 45 ˚ F (50 ˚ F) | GDD base 45 ˚ F (50 ˚ F) | |
| South central (Madison) | 154 (102) | 181 (83) |
| Southwest (Gays Mills) | 145 (97) | 181 (126) |
| Southeast (Rochester) | 159 (102) | 159 (123) |
| Central (Hancock) | 142 (101) | 108 (34) |
| Central West (Eau Claire) | 137 (98) | 102(63) |
| Central East (Sheboygan) | 147 (96) | 93(25) |
| Door peninsula | 116 (86) | 83(15) |
| Spooner | 87 (59) | 65 (9) |
Apples
Early cultivars such as Zestar are at king bloom, and later cultivars such as Liberty and Galas are at pink in Southern Wisconsin. At the Peninsular Ag Research Station (PARS) in Sturgeon Bay, apple cultivars are at tight cluster. This is the time, at the early pink stage, to finish any precision pruning for Honeycrisp, as well as the Prohexadione Calcium application to reduce bitter pit (see last week’s article)
Blueberries
At the West Madison Agricultural Station (WMARS), blueberries, the most advanced cultivars were at pink bud, and the later cultivars at tight cluster. The recent frost events might have damaged earlier cultivars in the pink stage. Here are some resources to evaluate frost damage:
How to quickly evaluate blueberry bud damage
Grapes
At the research vineyards in WMARS, early cultivars like Marquette and LaCrescent are at E-L* Stage 3 (“Wooly bud with green showing”), while late cultivars like Petite Pearl are at E-L* Stage 2 (“Bud scales opening”). For those growers that are concerned about potential frost damage in early cultivars, you should scout for damage first in the lowest spots of your vineyard. Here’s a previous article with information about assessing frost damage.
