Wisconsin Orchard Update: Bitter Rot and SBFS Management Reminders
As Wisconsin apple growers head into August and begin preparing for harvest, two persistent fruit diseases may begin to be observed: bitter rot and the sooty blotch and flyspeck (SBFS) complex. These diseases, which thrive under warm and humid conditions, can significantly reduce fruit quality, storability, and marketability. With careful monitoring and timely management, growers can minimize losses and maintain high-quality fruit through harvest.
Bitter rot, caused by several species of Colletotrichum, has become increasingly problematic in Wisconsin, especially in seasons with high humidity and frequent summer rains. It can infect apples at any stage, but symptoms typically appear from mid-summer to harvest. Bitter rot lesions begin as sunken, circular spots that range from light to dark brown. Under humid conditions, concentric rings of orange to salmon-colored spores may appear on the fruit surface. A key diagnostic feature is the V-shaped lesion that extends toward the fruit core when the apple is cut open, which helps distinguish bitter rot from other rots. The pathogen overwinters in mummified fruit, dead wood, cankers, and fire blight-infected shoots. Spores are spread by rain splash. To manage bitter rot, it is critical to remove and destroy sources of overwintering inoculum, such as mummified fruit, dead limbs, and fire blight strikes. Thinning fruit to avoid tight clusters and maintaining an open canopy through summer pruning can improve airflow and reduce humidity around the fruit, both of which help limit disease development.
The sooty blotch and flyspeck (SBFS) complex is another fruit disease that becomes more evident in late summer. Unlike bitter rot, SBFS is a superficial disease that does not rot the fruit but significantly reduces visual appeal—particularly problematic for fresh-market and U-pick operations. SBFS is caused by a complex of fungi that colonize the surface of the apple’s cuticle. Sooty blotch presents as dark, smudgy blotches, while flyspeck appears as small black dots arranged in clusters. Infections begin as early as late May under humid conditions, but symptoms often do not appear until several weeks later. SBFS thrives in dense, humid canopies and during periods of extended leaf wetness. Management begins with proper cultural practices: pruning to open the canopy, mowing and managing ground vegetation, and removing inoculum sources like dead twigs and old fruit. Weather-based decision support tools, such as the SBFS prediction model available through the NEWA platform, can help growers time their fungicide sprays.
Fungicides for managing bitter rot and SBFS:
- Captan
- Use pattern: 7–14 day intervals depending on weather pressure.
- Strengths: Excellent protectant activity against SBFS and bitter rot.
- PHI: 0 days; REI: 24 hours.
- Notes: Use high rates in warm, wet periods.
- Ziram
- Use pattern: 7–10 day intervals; often rotated with Captan.
- Strengths: Good on SBFS and bitter rot.
- PHI: 14 days.
- Notes: Use early-mid season for SBFS.
- Flint Extra (FRAC 11) or Sovran (FRAC 11)
- Use pattern: 10–14-day intervals; rotate with different FRAC groups.
- Strengths: Good on bitter rot; fair on SBFS.
- PHI: 14 days; REI: 12 hours.
- Notes: Avoid overuse due to resistance risk (FRAC 11).
- Inspire Super (FRAC 3 + 9)
- Use pattern: Apply at 14–21-day intervals; alternate with non-FRAC 3 materials.
- Strengths: Excellent for SBFS.
- PHI: 14 days.
- Notes: Good summer option; systemic activity.
- Merivon (FRAC 7 + 11)
- Use pattern: Apply at 10–14 day intervals; rotate with different modes of action.
- Strengths: Excellent on bitter rot and SBFS.
- PHI: 0 days.
- Notes: Avoid back-to-back applications to delay resistance.
Organic disease management:
Products like Serenade, Oxidate, Regalia, and potassium bicarbonate may provide some protection under low disease pressure but require frequent reapplication and have shown inconsistent results in field trials. Organic growers should emphasize sanitation and canopy management to reduce disease pressure and enhance product efficacy.
- Sulfur
- Use pattern: Frequent applications needed; tank mix with other materials.
- Strengths: Suppression of SBFS; minimal bitter rot activity.
- PHI: 0 days.
- Notes: Less effective under heavy disease pressure.
- Serenade Opti (Bacillus subtilis QST 713)
- Use pattern: Apply at 7–10-day intervals; best in rotation or as tank mix.
- Strengths: SBFS suppression; limited on bitter rot.
- PHI: 0 days.
- Notes: Useful late season near harvest.
Application Tips for Wisconsin Growers
- Timing: Begin SBFS coverage shortly after petal fall; maintain coverage through harvest under wet/humid conditions.
- For bitter rot: Focus fungicide applications from fruit set through mid-August, especially during hot, wet weather.
- Rotate fungicides with different FRAC codes to reduce resistance risk.
- Reapply after heavy rains (>1 inch), especially if using protectant fungicides.
- Consider weather-based models (e.g., NEWA SBFS model) to time sprays efficiently.
Additional Resources