Hello fruit growers, we are excited to share apple and grape scouting updates with you this year through the Wisconsin Fruit Newsletter. Recent rain events have pushed along spore maturity and disease infection events for apple scab and fire blight in apples, and Phomopsis cane and leaf spot in grapes. Insects are more active as well, with several growers reporting codling moth trap captures and observations of plum curculio and grape phylloxera.

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 10-11 (3-4 leaves separated) on Brianna, Petite Pearl to E-L stage 12-13 (5-6 leaves separated) in Frontenac. At the Peninsular Agricultural Research Station (PARS) all cultivars are around E-L stage 4-5 (Budburst). 

*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 May 20 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’ve seen in the last couple of growing seasons. 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 May 20, 2026 (April 1 start date; base 50°F BE*) at the WMARS and PARS.

Pests and Other Considerations

Grape phomopsis cane lesions have been observed in West Madison. Read more about phomopsis management in last year’s Wisconsin Fruit News here.

This is a critical time for disease management to limit fruit infections.. Black rot and phomopsis remain the main diseases of concern. Check out Leslie Holland’s article on post-freeze disease management.

Early signs of phylloxera have been observed in West Madison, keep an eye out for galls while scouting.

Shoot thinning reminder: Thin shoots down to ~2 per spur to improve light penetration, aid in air flow, and keep vines healthy. Reminder: Spurs should be spaced between 4-6 inches, aiming for 2 shoots/spur.

Apples

Phenology

Early apple cultivars (Zestar, Ida Red) have reached ~10-12mm in diameter while later cultivars (Fuji, Honeycrisp) have reached 6-8mm in southern Wisconsin. Varieties at PARS are quite a bit behind compared to West Madison: most varieties are still at pink into early bloom. 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 are beginning to set codling moth biofix. 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 May 20 is roughly the same to slightly ahead compared to last year. Once temperatures are sustained around the mid 70’s and 80’s, expect fruit development and degree-day accumulation to increase quickly.

Growing degree day (GDD) accumulation in 2024, 2025 and 2026 as of May 20 (January 1 start date; base 43°F BE*).

Pests and Other Considerations