Chlorothalonil use for pre-shuck split cherry leaf spot control

Chlorothalonil is recommended for cherry leaf spot control early in the season, but under current wet conditions this spring, growers need to maintain the 10-day retreatment interval between chlorothalonil applications.

The goal of early cherry leaf spot sprays is to prevent or delay initial infection events. Based on our previous observations, cherry leaf spot infection levels can rise to epidemic proportions under weather conditions that are both conducive and optimal for rapid disease development and infection. Michigan State University Extension has found that orchards with lower levels of infection early in the season have less inoculum in the tree for subsequent infections, which makes control of cherry leaf spot more easily achieved when conditions favor cherry leaf spot development.

Keeping inoculum low early in the season helps to minimize the potential difficulty of disease control prior to harvest and in some years, after harvest. Additionally, early season cherry leaf spot control helps trees hold leaves into the fall because if infection becomes severe during the season, early defoliation of leaves can occur. Sufficient healthy leaves on the trees are critical to ripen fruit evenly, and early defoliation can impact winter hardiness of trees.

At the open bract leaf stage, trees must be covered to prevent infection prior to rain events that could trigger a cherry leaf spot infection. Prior to shuck split, the recommended fungicide for cherry leaf spot management is chlorothalonil (Bravo and generics). This fungicide is a multi-site protectant that is excellent for leaf spot control and not at risk for fungicide resistance development.

Thus far this spring, we have experienced wet conditions, and forecasts predict more wet weather this weekend and into early next week, May 15-18, 2015. Warm and wet weather favors the development of cherry leaf spot, and growers will be challenged to keep open leaves covered in these conditions. As stated above, chlorothalonil is the optimal fungicide at this time; however, growers need to be sure that applications of this material do not fall short of the 10-day re-application interval. All chlorothalonil labels indicate that applications of this chemistry must be spaced at least 10 days apart, and this directive holds true for full cover sprays and alternate row middle applications. Captan is another efficacious fungicide for cherry leaf spot and can be rotated with chlorothalonil applications to provide good early season control.

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

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