Apple Disease Risks Rise as Wisconsin Harvest Nears

By Blake Jackson

Wisconsin apple growers should remain alert for two fungal diseases bitter rot and Marssonina leaf blotch as warm, humid weather during late summer creates ideal conditions for infections. Maintaining an effective fungicide program through harvest is essential to minimize crop losses and protect fruit quality.

Bitter rot, caused by fungi in the Colletotrichum group, begins as small, sunken spots on fruit that expand into circular lesions with a bull’s-eye appearance. Infected apples often develop a distinctive V-shaped internal rot, and symptoms may not become visible until fruit is in storage, reducing shelf life.

The fungus survives in mummified fruit, fallen apples, bark crevices, and dead wood, making orchard sanitation an important part of disease management. Susceptible varieties, including Honeycrisp and Empire, are especially vulnerable during warm, rainy periods.

Early-season protection from bloom through the first cover spray should include rotating and tank-mixing fungicides from FRAC Groups 3, 7, and 11 to reduce resistance.

As harvest approaches, products combining Groups 7 and 11, such as Luna Sensation, Merivon, and Pristine, provide strong protection while offering short pre-harvest intervals.

Additional products, including Aprovia, Omega, and ziram, can also be effective depending on disease pressure. However, fungicides alone cannot eliminate bitter rot if cankers are already established, so removing infected wood and fruit remains critical.

Marssonina leaf blotch has re-emerged as a concern in Wisconsin orchards, particularly where fungicide applications decline later in the season.

The disease produces grayish-brown leaf spots with purple borders, followed by yellowing and premature leaf drop. Warm temperatures between 68°F and 77°F combined with rainfall favor infection.

To manage the disease, growers should begin fungicide applications around late bloom or early petal fall and continue through early to mid-June. FRAC Groups 3 and 7, combined with rainfast mancozeb or captan, provide the best control, while Group 11 products should be reserved for later fruit rot management.

Organic growers can suppress the disease with micronized sulfur, and practices such as removing fallen leaves, pruning for better airflow, and applying urea in the fall help reduce future infections.

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