Hello fruit growers, we are excited to share apple and grape scouting updates with you this year through the Wisconsin Fruit Newsletter. It has been dry and hot in the last week, which has resulted in lower disease pressure. Rain in the next week may push disease incidence along. Insects are more active, with several growers reporting codling moth trap captures and seeing impacts of plum curculio, grape phylloxera, thrips and aphids. As a reminder, this newsletter will cover apple and grape phenology, general scouting observations, and includes an overview of degree day accumulation. Growers can reference the Midwest Fruit Pest Management guide for further information on pest management best practices.

Grapes

Phenology 

At the West Madison Agricultural Research Station (WMARS) in Madison, WI, buds average around E-L* stage 12-14 (5-7 leaves separated) on Brianna and Petite Pearl to E-L stage 16-17 (10-12 leaves separated) in Marquette and Frontenac. At the Peninsular Agricultural Research Station (PARS) all cultivars are around E-L stage 9-11 (2-4 leaves separated). 

*E-L stands for Eichhorn-Lorenz growth stages scale to describe grapevine development.

Growing Degree Day (GDD) Accumulations

Below displays the GDD accumulations from April 1 through June 4 for the current and last two seasons. This data is collected using the NEWA website. You can visit their “About degree days” page to learn more about the concept of degree days as well as the formulas utilized for calculations. We have similar degree-day accumulation at both locations that we observed in the 2024 growing season. Growers have reported signs of cane death, likely due to cold temperatures in January with no to very little snow cover.

Growing degree day (GDD) accumulation as of June 4, 2026 (April 1 start date; base 50°F BE*) at the WMARS and PARS.

Pests and Other Considerations

Pre-bloom through bloom is a critical time for disease management to limit fruit infections. During this window, leaves, flowers, and developing fruit are highly susceptible to infection by several important fungal pathogens, including black rot, Phomopsis cane and leaf spot, powdery mildew, downy mildew, and Botrytis gray mold. Check out Dr. Leslie Holland’s article on Protecting Grape Clusters During the Critical Pre-Bloom to Post-Bloom Period.

Petiole nutrient sampling during bloom: collect the leaves opposite the first or second flower cluster of a shoot. Aim for about 50 percent bloom and try to hit the same stage each year, since nutrient levels can shift over the bloom period. Check out Dr. Amaya Atucha’s article on Petiole Sampling for Determining Fertilization Needs in Cold Climate Grapes.

Apples

Phenology

Early apple cultivars (Zestar, Ida Red) have reached ~¾”+ in diameter while later cultivars (Fuji, Honeycrisp, McIntosh) have reached ~½”+ in southern Wisconsin. Varieties at PARS are quite a bit behind compared to West Madison: most varieties are still in bloom or just hitting petal fall. Remember to record petal fall dates at several blocks/cultivars in your orchard, to use in the NEWA models this summer. Plum curculio is out in full force, and growers in southern Wisconsin have hit codling moth biofix in the last two weeks. Make sure to check codling moth traps daily when nighttime conditions are calm, warm and humid.

Growing Degree Day (GDD) Accumulations

We are tracking growing degree days (DD 43 °F BE) from January 1 to monitor bud development in apples. This data is collected using the NEWA website. You can visit their “About degree days” page to learn more about the concept of degree days as well as the formulas utilized for calculations.  Similar to grapes, degree day accumulation (Base 43°F BE*) between January 1 and June 4 is similar to what we observed during the 2024 season. Temperatures in the next week will remain warm, hitting 90°F+ in Southern Wisconsin. Expect fruit development and degree-day accumulation to increase quickly.

Growing degree day (GDD) accumulation in 2024, 2025 and 2026 as of June 4 (January 1 start date; base 43°F BE*).

Pests and Other Considerations