Becky Wheeler-Dykes, Orchard Researcher, CSU Chico
High humidity, high temperatures and tightly clustered fruit create the perfect environment for brown rot. In recent years, heavy smoke cover after wildfires has increased orchard humidity, making it especially important to monitor for disease presence. Rain in the forecast prior to harvest also indicates a need for brown rot treatment.
Brown rot treatment sprays, where deemed necessary based on current season observations and block history, should begin 4-6 weeks before harvest and are only useful on uninjured fruit. Recent UC research on timing and material efficacy found that fungicides containing FRAC 3 chemistry (brand name examples include1 Tilt, Teb, Tebucon, Indar, etc.) provided the best possible fruit brown rot control. For a complete list, consult the UCIPM 2022 Fungicide Efficacy Tables.
Including 2% (v/v) summer (415) oil in the tank and 140 gallons per acre spray volume improved disease control over using non-ionic surfactants and spraying 80 gallons per acre. Make sure there is no residual sulfur in the tank before adding the oil to the tank. Oso® fungicide1 is the best rated organic fungicide for fruit brown rot management. Check with your processor regarding fungicides approved for use and your PCA for rates and materials.
Pre-harvest spray efficacy can be particularly challenging due to dense canopy and branches weighed down by the crop. Slow tractor speeds will deliver the best spray coverage under these conditions. Slow down and do the best job possible. Aerial application will provide less control than careful ground application. Control of brown rot in fruit is difficult, and complete control may not be possible.
1 Mention of a pesticide is not a pesticide recommendation, merely the sharing of research results. Always read the pesticide label, the label is law.