Cherry leaf spot disease biology and rationale underlying fungicide use

A leaf spot fungicide program will delay the initial infections of trees, and reduce infection to the extent that trees will hold their leaves into September and beyond.

This article summarizes cherry leaf spot biology and indicates the rationale underlying fungicide applications for disease control. The accompanying article, Fungicide resistance management considerations for cherry leaf spot control, summarizes where we are at regarding fungicide resistance management, or fungicide resistance prevention, as many of the fungicides used for leaf spot control are at risk for resistance development.

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 often with yellow halos around the spots on the tops of the leaves. As spots accumulate on foliage, the leaves turn yellow and fall. The amount of lesions required causing leaf yellowing and drop is variable, particularly among Balatons and Montmorency tart cherries. Sweet cherries can tolerate quite a few lesions before leaf drop occurs; however, Montmorency tart cherries will drop with only a few lesions, signifying the importance of proper leaf spot management. Balatons can have more lesions than Montmorency before leaf drop, but Balatons are just as susceptible to leaf spot as Montmorency.

As soon as leaves unfold in the spring, they are susceptible to leaf spot infection. Similar to apple scab, the leaf spot fungus overwinters in infected leaves on the orchard floor, and produces ascospores that serve as the primary inoculum. Ascospore discharge occurs during and shortly after rainfall from early bloom to about six weeks after petal fall. Ascospore discharge is highest over a wide temperature range of 60 to 85°F, and lowest at 41 to 46°F. The optimum conditions for lesion development are temperatures of 60 to 68°F with rainfall or fog. After lesions appear on upper leaf surfaces, examination of the underside of leaves reveals a proliferation of white spore masses. These spores are dispersed by rain and wind within trees and to adjacent trees; such secondary cycles can continue repeatedly under favorable conditions through autumn.

Preharvest defoliation can result in a crop that does not mature adequately in order to produce marketable fruit. Subsequent annual defoliation can cause serious tree damage and greatly reduce winter hardiness. Even late summer – August and early September – defoliation reduces the ability of trees to store photosynthate in roots, which can result in an overall loss of vigor and trees are more susceptible to winter injury or mortality. Early-defoliated trees also typically exhibit reduced flower bud formation and often set less fruit the following season.

The leaf spot pathogen is a prolific sporulator and can spread very quickly, initiating new lesions that will produce more spores. The rapidity of spread combined with the high susceptibility of Montmorency and Balaton trees is what drives leaf spot epidemics. Thus, the goal of a leaf spot fungicide program is to reduce infection to the extent that trees will hold their leaves into September and beyond. The best way to do this is to delay the initial infection of trees starting at petal fall and to maintain this protection up to and continuously around harvest time when temperatures are usually quite favorable for leaf spot infection events. Usually, if trees are well protected through harvest and exhibit little to no infection, these trees will hold their leaves into September and beyond.

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

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