Aug 2, 2011

Post-harvest sprays for cherry leaf spot

Cherry leaf spot is the most important fungal disease of tart cherry in Michigan. The leaf spot fungus Blumeriella jaapii infects leaves with symptoms first appearing on upper leaf surfaces as small, purple spots. As spots accumulate on leaves, the leaves turn yellow and fall. The amount of lesions required causing leaf-yellowing and drop is variable. Defoliations that begins before early September reduces the ability of trees to store photosynthate in roots, leading to an overall loss of vigor and leaving trees more susceptible to killing by winter injury. Early-defoliated trees also typically exhibit reduced flower bud formation and often set less fruit the following season.

As harvest will be winding down for cherries, many growers will be considering post-harvest applications for cherry leaf spot. Considering the substantial amount of precipitation early this season, disease pressure is likely higher in most blocks throughout the state. We have observed defoliation in many sites, and our test blocks at the Northwest Michigan Horticultural Research Center have considerable leaf drop. Growers should be applying a post-harvest application for cherry leaf spot.

Leaves typically fall from branches a few weeks after they begin to show disease symptoms. Thus, the goal of a cherry leaf spot management program is to maintain a healthy canopy on trees at least through the end of September. This recommendation ensures an adequate amount of leaves on trees into late October and beyond. Because of the wet weather this season, most growers, particularly those with cherry leaf spot symptoms already present in the orchard, should be using a post-harvest spray to prevent early defoliation.

The fungicide of choice for leaf spot control after harvest is chlorothalonil (Bravo or some equivalent product). Chlorothalonil provides excellent leaf spot control and is a broad spectrum fungicide, making it a good choice for mitigating fungicide resistance.

By Nikki Rothwell, Erin Lizotte, and George Sundin, Michigan State University Extension