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 quite wet with just over two inches of rain that fell in Southern Wisconsin in the past two weeks. Several secondary insects have been observed, including green apple aphid, woolly apple aphid, phylloxera, mites and leafhoppers and rain has driven disease risk. 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 27-32 (fruit setting to beginning bunch closure). At the Peninsular Agricultural Research Station (PARS) all cultivars are around E-L stage 16-18 (12-14 leaves separated, single flowers 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 18 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. Growers have also reported a wide spread in phenology depending on location.

Growing degree day (GDD) accumulation as of June 18, 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.

Recent rain has driven infection events for black rot and phomopsis. Keep an eye out for developing leaf lesions.

Phylloxera galls have increased significantly in the past week with the warmer temperatures and rain.

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 1.5”+ in diameter while later cultivars (Fuji, Honeycrisp, McIntosh) have reached 1”+ in southern Wisconsin. Varieties at PARS are quite a bit behind compared to West Madison: most varieties are between ½” and 1”. Remember to record petal fall and biofix dates at several blocks/cultivars in your orchard, to use in the NEWA models this summer. Make sure to check codling moth traps daily when nighttime conditions are calm, warm and humid and watch for an influx of captures that may indicate second generation flight.

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 18 is roughly similar to what we observed during the 2024 season. Temperatures in the next week will remain moderate, in the 70s, and wet in Southern Wisconsin. Expect fruit development and degree-day accumulation to steadily increase.

Growing degree day (GDD) accumulation in 2024, 2025 and 2026 as of June 18 (January 1 start date; base 43°F BE*).

Pests and Other Considerations