Considerations for pre-bloom and bloom sprays for American brown rot

The potential for American brown rot blossom blight infection is high this spring if infected fruit mummies from last year overwintered in the orchard. Conditional requirements for infections are listed.

American brown rot is caused by the fungus Monilinia fructicola and is an important problem on peaches, sweet cherries, tart cherries, plums and prunes, and other stone fruit. American brown rot infection is typically initiated within about three weeks of harvest as fruit begin to color and their sugar content increases. However, in some years, American brown rot infection can be initiated pre-bloom (white or pink bud stage) through bloom with blossom blight infections that can result in invasion into woody tissue.

The American brown rot fungus overwinters in mummified fruit. Mummies are fruit that were infected the previous season and then shriveled up and either remained on the tree or fell to the ground. These mummies serve as the initial inoculum source for American brown rot in the subsequent spring. Mummies are the source of conidia (spores), which develop at a temperature range of 55-77 degrees Fahrenheit and are disseminated by wind and rain. The spores have the potential to infect flowers, particularly at the optimal temperature range of 72-77 F.

The risk for American brown rot infection at bloom will be directly related to the amount of brown rot-infected fruit in your orchard last year. Overall in Michigan, American brown rot levels have been low the past two years, but levels of inoculum can vary in individual orchards. If American brown rot levels were at or above 3-5 percent in an individual orchard and a significant number of mummified fruit remained after harvest, the risk of blossom blight infection is fairly high.

Blossom blight infection only occurs with a wetting event accompanied by prolonged conditions of high humidity. Typically, sustained relative humidity values of 90 percent or above for 24 hours or more are associated with optimal infection events. We have observed significantly reduced blossom blight infections if the relative humidity is 60 percent or lower. Therefore, the highest risk conditions for American brown rot blossom blight infection are as follows:

  • Presence of overwintering mummies.
  • Warm temperatures in the 70s during bloom.
  • Rain event during bloom followed by a sustained period where the relative humidity remains high.

If these conditions are not met, a fungicide application targeting the blossom blight phase of American brown rot will not be needed.

Possible chemical control options for American brown rot blossom blight include the sterol inhibitor Indar 2F, other sterol inhibitors such as Elite or Orbit, and Rovral 4F. However, the optimal fungicide choice at this timing is Rovral at a rate of 2 pints per acre. We have recently documented that populations of the American brown rot fungus are shifting towards resistance to sterol inhibitor fungicides, and Michigan State University Extension is recommending alternative modes of action should be used when possible, particularly at this early timing. At the pre-bloom and bloom stages of stone fruit, American brown rot inoculum levels are lower than at pre-harvest. Thus, the efficacy of Rovral should be more than adequate for blossom blight control. This fungicide is rated as excellent against American brown rot, and since Rovral cannot be applied after petal fall, application at bloom timing is recommended. Only two applications of Rovral are allowed.

Dr. Sundin’s work is funded in part by MSU‘s AgBioResearch.

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