Wisconsin Orchard Update: Stay Ahead of Bitter Rot
As we head into July, Wisconsin apple growers should be on high alert for bitter rot, a disease that continues to increase in prevalence—even in seasons with relatively dry conditions. While it may seem counterintuitive, this disease tends to appear most aggressively later in the season, especially following summer rain events and during periods of high humidity and warm temperatures. This timing coincides with the weeks leading into harvest, making late season monitoring and fungicide management essential.
The fungi responsible for bitter rot are especially problematic because they do not require fruit wounds to infect—the pathogen can directly penetrate healthy skin. Varieties such as ‘Honeycrisp’ and ‘Empire’ are particularly susceptible, and infection can lead to significant losses both pre- and post-harvest. Bitter rot lesions can rapidly expand under warm, moist conditions, and once symptoms appear, it’s too late to stop the infection.
Recent research (Martin and Peter, 2023) has provided new insight into the biology of this disease. Spores of bitter rot fungi are present in the orchard year-round, residing in buds, branch tissue, and mummified fruit. In their study, spores were detected in rainwater from bud break to leaf drop, with peak spore activity occurring during summer and early fall—precisely when temperatures and moisture levels create ideal conditions for infection. This finding confirms that infection timing is driven more by weather conditions than by the stage of fruit development.
Even when early infections occur during bloom or fruit set, they may remain latent and asymptomatic until the right environmental conditions activate them. Late summer rains, particularly in August and September, are often the trigger. As rain splashes spores throughout the canopy, bitter rot can spread quickly, especially in high-density systems where close tree spacing creates humid microclimates.
Scouting and Symptom Development
Growers should begin scouting now, paying close attention to mid- and lower canopy fruit where moisture tends to linger. Look for small, sunken, light brown lesions that may develop concentric rings. Under humid conditions, these lesions often exude orange to salmon-colored spores, which can spread rapidly with wind and rain.
Fungicide Timing and Coverage Matter
Because bitter rot infection can happen any time from fruit set through harvest, a season-long fungicide strategy is key. Early-season applications—starting at bloom through first cover—should include single-site fungicides from FRAC groups 7 or 11. These products are effective at preventing initial infections.
From early to mid-summer, continue with a rotational spray program using FRAC 7, and 11 materials. Mancozeb may be included in the first or second cover spray (77-day PHI), taking advantage of its extended use under the apple scab program, which also provides some collateral protection against bitter rot.
As harvest nears, consider FRAC 7 + 11 premix fungicides, such as Merivon or Pristine. These products offer strong protection against bitter rot and have short pre-harvest intervals, making them ideal for late season use. Be mindful that heavy or frequent rain events can wash off fungicide residues, so growers should monitor weather forecasts closely and reapply if intervals between sprays are extended or if cumulative rainfall is high. Research from Penn State determined that fungicides should be applied before, rather than after rain events because once the bitter rot fungus penetrates the apple surface it is very difficult to kill even with systemic fungicides.
Avoid gaps in fungicide coverage, particularly in late July and August, as these can open the door to new infections that not only damage fruit in the field but also increase the risk of storage rots post-harvest.
Tips for Wisconsin Growers
- Scout weekly for early signs of bitter rot, especially in Honeycrisp and Empire blocks.
- Prioritize fungicide coverage ahead of hot, humid, and rainy weather.
- Reapply fungicides following heavy rains and monitor intervals closely.
- Keep orchard floors clean of mummified fruit, which serve as inoculum sources.
- Rotate fungicide chemistries and avoid overreliance on a single mode of action to prevent resistance.