European brown rot control in tart cherries for 2015

Predicting if growers need to apply a fungicide to Montmorency tart cherries can be difficult with changing weather conditions.

European brown rot is a disease whose infection has been hard to predict and control in the past few years. This disease caused by the pathogen Monolinia laxa has traditionally been a problem in Balaton cherries, but in 2013 this disease had devastating effects on Montmorency blocks that had slow-drying conditions, such as orchards surrounded by wind breaks or in low spots within an orchard. This disease almost always impacts Montmorency trees. Trees that become infected with European brown rot are almost always located in northwest Michigan regions that commonly have spring fogs that settle over these orchards. For example, Northport, Michigan growers regularly treat for European brown rot in Balaton and Montmorency trees.

At this time, we have associated European brown rot with cool and wet conditions around bloom time. However, because we know so little about the European brown rot life cycle and its optimal development conditions, applying fungicides for control of this disease has been challenging. For example, in 2013 we had warm and dry conditions during the popcorn stage of development. Under our current thinking, European brown rot does not develop optimally under warm and dry conditions, and as a result, most growers did not apply a fungicide at this popcorn timing in 2013. However, these conditions quickly changed, and the warm and dry weather turned to cold and wet. Consequently, many Montmorency blocks suffered atypically high levels of European brown rot infection in 2013.

Conversely, in 2014 we had cold and wet conditions leading up to bloom, but during bloom the weather was hot and dry. In this case, we had very few European brown rot infections, even with the high inoculum levels from the 2013 epidemic. In a nutshell, determining when and if we should spray for European brown rot is a challenging decision for growers.

Unfortunately, we are working our way toward that difficult decision later this week (week of May 4, 2015). Most orchards in west central and northwest Michigan will be approaching the popcorn stage of development toward the end of this week, and conditions are predicted to be warm and dry (high 70s and low 80s). Rain is predicted to move into the regions this weekend, and temperatures are forecasted to be close to 80 degrees Fahrenheit. In short, conditions leading up to bloom, including popcorn, will be warm and dry, but will be replaced by warm and wet conditions during the start of bloom. Therefore, growers need to make a decision to apply a fungicide at popcorn under conditions that would not typically be optimal for European brown rot development.

Our preliminary laboratory data and the European brown rot literature suggest the European brown rot pathogen will produce spores under warm conditions, but infection is much more favorable under cool conditions. The wetting period for this weekend is predicted to last throughout the weekend and into Monday, May 11. Because this wetting period is forecasted to be so long (over three days), there is potential for European brown rot infection, particularly if the temperatures do not get as warm as predicted.

In a nutshell, with the predicted forecast of initial warm weather for the remainder of the week, we could see the European brown rot pathogen produce lots of spores. With an increased spore count and extended wetting period over the weekend, we have a chance of European brown rot infection. Tyre Proffer, Michigan State University Department of Plant, Soil and Microbial Sciences, is currently conducting trials to determine the optimal temperatures for European brown rot infection, but those data are forthcoming.

Based on our current European brown rot thinking and the predicted weather forecast, Michigan State University Extension recommends growers treat for European brown rot following the traditional two applications timings: popcorn and one week later. At this time, Indar is the best control option for European brown rot. Two applications should be applied at the above timings at a rate of 6 fluid ounces per acre. Vanguard has also shown some efficacy against the European brown rot pathogen under non-optimal conditions. All Balaton growers should be applying Indar at these two timings regardless of predicted weather conditions.

We are currently screening other fungicides for control of European brown rot at the Northwest Michigan Horticultural Research Center.

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

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