American brown rot management during bloom

Forecasted weather conditions may lead to infection during bloom, resulting in blossom blight symptoms.

American brown rot (ABR) is caused by the fungus Monilinia fructicola. This fast-growing fungus is an important pathogen on cherry (sweet cherry in particular), peach, apricot, nectarine and plum. The fungus attacks fruit, blossoms, spurs and shoots; under ideal infection conditions, the fungus can rot individual cherry fruit within 24 hours. The fungus sporulates from infected fruit, continually increasing inoculum for further infections. Under ideal conditions, sporulation can be initiated within three days after infection. American brown rot causes fruit rot before and after harvest, greatly reducing the quality and quantity of the yield, particularly in heavily bunching sweet cherry varieties.

Our concern at this time is with the potential for infection during bloom resulting in blossom blight symptoms. The potential for blossom infection in Michigan is typically held in check by cooler weather as the optimal temperature for infection is between 72 and 77°F, and spore production is greatest between 59 and 74°F. Since ABR symptoms on fruit were prevalent in many sweet and tart cherry orchards in 2010, there is a risk of higher inoculum loads in orchards this year. However, remember that favorable conditions for ABR blossom infection include relatively warmer temperatures with some rain.

The American brown rot fungus overwinters in fruit mummies from the previous season and will sporulate from these mummies, producing conidia that are disseminated by wind and rain. Although blossom blight caused by ABR can be severe enough to cause yield loss, this mostly occurs in other more southern regions of the United States. However, another important aspect of blossom infection is this enables the fungus to re-establish itself in orchards for the current season. Since the ABR fungus is a prolific sporulator, blossom infection will serve to increase orchard inoculum loads to critical levels.

The most efficacious fungicide for ABR control is the sterol-inhibitor (SI) fungicide Indar. SI fungicides (Indar, Elite, Rally, Orbit) are at risk for developing fungicide resistance and should not be used at this timing. The best timings for use of Indar for ABR control are closer to harvest when we are working on reducing ABR infection of fruit. We will discuss fruit rot management in a later article.

The best fungicide for American brown rot management during bloom is the dicarboximide fungicide Rovral. This is an effective ABR material during bloom because inoculum levels are lower and can be controlled. Also, since this fungicide has a different mode of action than SI’s, it is an excellent alternate tool for SI fungicide resistance prevention. A maximum of two applications of Rovral can be used, and do not use this fungicide after petal fall.

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

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