Postharvest fungicide application for cherry leaf spot control on tart cherries
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
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 defoliate. The amount of lesions required to cause leaf yellowing and drop is variable. Late summer (August, early September) defoliation 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.
Symptoms of cherry leaf spot infection are typically first seen in the tops of trees with the observance of yellow leaves (see photo). Infection usually occurs in the tops of trees first, because this foliage may not be covered effectively by airblast fungicide application, and also because newly-growing, unprotected leaves occur in the tops of trees as well. Since the leaf spot fungus sporulates out of the underside of leaves, and these spores are carried downward via rain and wind, the fungus can very quickly infect all of the leaves on a tree and leaves on adjacent trees.
Each new infection results in the production and spread of additional spores (termed a secondary cycle of the disease). These secondary cycles will continue throughout the rest of the growing season. Leaf loss through defoliation is what is critical. The earlier leaves fall, the more profound impact on tree health and winter hardiness. The first appearance of yellow cherry leaf spot-infected leaves usually results in initial defoliation within four to six weeks and significant defoliation within six to eight weeks, depending on the season.
Thus, trees that currently contain large numbers of yellow leaves are at risk of becoming defoliated before the beginning of September. We usually regard mid-September as kind of a magic target date for tart cherry trees in terms of holding their leaves. If trees retain a significant number of green leaves by September 10-15, they should be in good shape entering the winter because they should retain an adequate amount of leaves into October and beyond. However, if trees are defoliated prior to this time, they are at risk of significant winter injury, including tree death, again depending on winter conditions.
Growers should consider a postharvest fungicide application to slow down the cherry leaf spot fungus and to continue to protect leaves, particularly in orchards exhibiting large numbers of yellow leaves currently. The best fungicide for postharvest application is chlorothalonil (Bravo and generic equivalents). Chlorothalonil is a broad spectrum fungicide that is not at risk of fungicide resistance development.
Two formulations of Bravo available are Bravo Ultrex (82.5% WDG) and Bravo Weather Stik. For Bravo Ultrex, the label states a range of 2.8 to 3.8 pounds per acre. In our field trials conducted yearly at the Northwest Michigan Horticultural Research Station, we use a rate of 3.0 pounds per acre and typically observe excellent leaf spot control. Do not apply more than 18.8 pounds of Bravo Ultrex per season.
For Bravo Weather Stik, a liquid formulation containing 6.0 pounds of chlorthalonil per gallon, the label states a range of 3 1/8 (3.125) to 4 1/8 (4.125) pints per acre. Note: in the 2010 Michigan Fruit Management Guide, there is a listing for Bravo 720 indicating a maximum rate of 5.5 pints per acre. This rate is not applicable to Bravo Weather Stik, whose maximum rate per application is 4.125 pints per acre. Do not apply more than 20.5 pints of Bravo Weather Stik per season.