Jun 30, 2015

Controlling cherry leaf spot in orchards with existing symptoms

Cherry leaf spot is the most important fungal disease of tart cherries in Michigan. The leaf spot fungus, Blumeriella jaapii, typically infects leaves and symptoms first appear on upper leaf surfaces as small, purple spots. As lesions accumulate on leaves, the leaves turn yellow and fall from the tree. However, the number of lesions required to cause leaf yellowing and drop is variable. Sweet cherries can tolerate quite a few more lesions than Montmorency tart cherries before leaf drop occurs. Cherry leaf spot also impacts Balaton tart cherries. Effective leaf spot management is especially critical to prevent defoliation in tart cherries.

At the start of infection, leaf spot can be difficult to see on the leaves. The first symptom of cherry leaf spot are small (1-3 millimeters), red to purple spots on the upper leaf surface. These spots can be more easily detected if back-lighting is used when looking at leaves. Eventually, lesions produce visible white sporulation in the middle of the lesion on the bottom side of the leaf. The presence of these spore masses can be a definitive method to determine the lesion is indeed due to cherry leaf spot infection. Spores produced on these masses are the source of inoculum of new infections and will spread to adjacent tissue with each rain event.

In heavy cherry leaf spot infections, lesions can coalesce and produce larger areas of dead leaf tissue. Cherry leaf spot is evident in most tart cherry blocks across the state, likely as a result of our wet conditions in May and June. Additionally, we have observed a few orchards with high levels of cherry leaf spot infection, and in these orchards, leaves are already turning yellow and dropping from trees. Early defoliation will increase the potential for tree mortality in a hard winter. In some cases of orchards with larger crop size, early defoliation will prevent ripening of the fruit. As there is also bacterial canker in orchards, growers should look for the white spore growth to identify leaf spot. Bacterial canker symptoms on the leaves are dark brown, circular to angular and often have a yellow halo; canker lesions are also larger in size than leaf spot lesions.

Cherry leaf spot is usually effectively controlled early in the season with proper fungicide application timing. In years with extended dry weather, leaf spot symptoms are not visible in most orchards until August-September. However, in years when we receive significant rainfall early in the season with extended wetting periods, early infections of leaf spot can occur. This year was a particularly challenging year for keeping tissue covered during multiple days with rain and long infection periods. Most growers have been scouting their orchards for leaf spot infections, and Michigan State University Extension encourages growers to continue looking for lesions on newer tissue. If cherry leaf spot symptoms are evident, this is an indication that spore loads in orchards will soon be very high and this disease will be difficult to control through September.

If cherry leaf spot symptoms are currently present in the orchard this early in the season, tart cherry leaves will need effective fungicide protection from now until after harvest. In orchards with cherry leaf spot infection already started, there is potential for high disease pressure. Therefore, we recommend growers use the maximum label rates of fungicides and cover entire orchard blocks (i.e., do not use an alternate middle row spray plan). Coverage of all leaves is critical to protect from new cherry leaf spot infections because infected leaves are producing spores that will spread relatively easily to adjacent leaves in the tree during rain. Full cover sprays will minimize the impacts of this fungal disease by reducing the potential of a cherry leaf spot epidemic this season. Growers should also slow down to provide excellent coverage of full tree canopies.

The next two to three fungicide applications should be full cover applications. The goal of this strategy is not to burn out existing infections, but to protect all healthy tissue from new infections. Although existing cherry leaf spot infections are evident on older leaves, we are more concerned with the spread of this disease to uninfected leaves, and as a result the next sprays to prevent this spread are critical. Fungicides with partial systemic activity (e.g., Merivon, Luna Sensation, Syllit) are excellent choices right now mixed with Captan. These fungicides provide the best potential for control.

Leaves that currently exhibit multiple sporulating lesions are almost sure to defoliate, and the goal of this season’s management plan is to limit the amount of infection of currently healthy foliage through protection against subsequent infections. The cherry leaf spot spore load will probably be high in most orchards for the remainder of the season, and these blocks must be intensively managed for the next several months to minimize impacts on overall orchard health.

— By George Sundin, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences, and Emily Pochubay and Nikki Rothwell, MSU Extension