Cherry leaf spot: A damaging fungal pathogen of 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 arguably the most damaging fungal pathogen of tart cherry. Cherry leaf spot primarily infects foliage and reduces the photosynthetic ability of a tree through tissue degradation and early defoliation. When significant defoliation occurs before harvest, fruit may be soft and immature, have low soluble solids, and ripen unevenly. For a point of reference, at least two leaves are needed to effectively ripen each cherry and defoliation that reduces leaf numbers to below that threshold can cause damage to the current year’s crop. Blossom production may also be reduced in subsequent years and following severe defoliation, trees are more susceptible to winter injury because of the loss of stored carbohydrates in the roots.
Cherry leaf spot overwinters in fallen leaves on the orchard floor and produces apothecia (sexual spore-bearing structures) in the spring. Ascospore (sexual spore) dispersal occurs during the drying period that follows a wetting event and temperatures between 60-85°F. Infection occurs through the leaf stomata, which remain susceptible throughout the growing season. The primary infection period may last two to six weeks depending on conditions. Following infection, acervuli (asexual spore-bearing structures) develop on the underside of the leaf and produce a visible mass of asexual spores called conidia. Spores are dispersed from leaf to leaf by wind or rain and this secondary infection cycle can be repeated several times within a season, depending on conditions. All commercial cherry cultivars are susceptible to cherry leaf spot. The disease is resistant to sterol inhibitor fungicides (Indar, Elite, Orbit) in all the major fruit producing areas of Michigan.
The table below lists season long recommendations for management; we are currently heading into the timing for a petal fall application in northern Michigan. Remember to alternate the use of fungicide classes during the season to manage against resistance development. To monitor for cherry leaf spot using a disease forecasting model visit www.enviroweather.msu.edu.