Disease Management on cherry leaf spot

Low power image of sporulating acervuli on the underside of a tart cherry leaf;
Close up of the formation of an acervulus on the underside of a tart cherry leaf.

Resistance

There are no resistant varieties available on the commercial market yet. However, researchers have found the a wild type gene linked to the resistance. They are currently crossbreeding the wild lines with commercial cultivars and beginning to carry out field trials. No data is available yet.

Small or backyard growers

For small or backyard growers, collecting and destroying all leaf debris on the ground is an absolute necessity due to the potency of this disease because the fungus overwinters in this leftover leaf litter. This is its main form of survival. By removing and destroying these leaves, a grower can significantly decrease the amount of primary inoculum available in the spring. It will greatly decrease the apparent infection rate. There has also been a study done on the addition of a straw mulch bedding to the ground after all the leaves have been picked up. The addition of this mulch further reduced the spring infection rate. Leaf litter removal is not very practical for large commercial growers due labor needs and number of trees but if at all possible, a majority of the old leaves should try to be collected.

When planting, growers should select locations which have a large amount of direct sunlight such as a south facing slope. Proper pruning should also be completed to increase the amount of sunlight penetration and air circulation through the canopy. Any practice that increases the faster drying of leaves will help reduce the risk of infection. Growers may also consider making an after harvest fungicide application using a combination of Benomyl (50% WP)and Captan (50% WP) at rates of 1/4 Tablespoon and 2 Tablespoons respectively per gallon of water. This will help reduce the rate at which pathogens may develop resistance to Benomyl products. Prior to shuck split, the recommended fungicide for cherry leaf spot is chlorothalonil (Bravo and generics).This fungicide is a multi-site protectant and is excellent for leaf spot control and is not at risk for fungicide resistance development. At least two applications of chlorothalonil should be made before shuck split with the goal to minimize the potential of infection at this early stage.

Commercial growers

For commercial growers, the disease is primarily controlled by use of fungicide sprays. Fungicides are much more effective when applied early in the season when the inoculum load is low as cherry leaf spot is a prolific, unrelenting, tireless disease.

Fungicide applications should begin at petal fall or shortly after leaves have unfolded. These sprays should continue on a schedule of every 7–10 days until harvest. Upon harvest, one or two postharvest applications should be administered, beginning 2–3 weeks after harvest. It is suggested that spraying alternate sides of trees on a 7-day program is more effective than spraying both sides of trees on a 10-day schedule.

Michigan State University suggests getting an early start on protection before the fungus starts infecting for the production year. This means that growers should spray the at the bract leaf stage with chlorothalonil (Bravo and generics). These bract leaves open prior to bloom, which means bract leaves could be infected early, before petal fall. Typically the first fungicide application is recommended around petal fall, but due to the early and epidemic levels of infection in found in 2012, the first application should be applied earlier.

Significant infection was also found in the bract leaves in mid- to late June 2012. This was particularly surprising because the weather conditions were not notably conductive to the super development of cherry leaf spot infection. These early and significantly strong infection rates indicate that the fungus is evolving and becoming resistant to certain fungicide treatments. Control programs will need to be altered to keep up with the genetic advancement of the fungus. These earlier infections are a concern because once infection occurs; more spores will be produced from the lesions (conidia) than the leaf debris (ascospores) on the ground. These conidia are much more potent than ascospores in terms of infection rate.

In addition, spores from the lesions are much closer to new uninfected leaves than the spores from the leaf debris on the ground. Due to the smaller distance, infection will occur much quicker. Dr. George Sundin, a professor and fruit extension specialist from Michigan State University advocates that the new chemistries of succinate dehydrogenase inhibitors (SDHIs) are also effective in controlling cherry leaf spot. Pristine was registered in 2004. It is a premix of boscalid (SDHI) and pyraclostrobin (strobilurin). This has been indicated effective at a rate of 10.5 oz/acre. Other SDHIs that may be effective in cherry leaf spot control include fluopyram (a pyramide manufactured by Bayer AG under the name “Luna”) and fluxapyroxad (a pyrazole-carboxamide manufactured by BASF SE under the name Merivon).

Sterol demethylation inhibitor (DMI) fungicides including fenarimol, fenbuconazole, myclobutanil, and tebuconazole were used immensely in the 1980s and 1990s. The efficacy of DMI fungicides has decreased dramatically in recent years and have not been used greatly since 2002.

In an effort to keep a high level of diversity in the cherry fungicide programs and reduce the amount of resistance building up to the DMI fungicides, copper based fungicides can be used with great efficacy to battle the fungus. However, the copper application is associated with noticeable leaf bronzing. There has been great concern that this bronzing causes a highly negative effect on the photosynthetic integrity of the leaves which in turn decreases the number of fruits per shoot, fresh fruit weight, and soluble solids concentration of the mature fruit. It has been scientifically proven that the standard application of copper based fungicides does not have any adverse effects on the developing fruit.

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