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Cherry leaf spot

cherry-leaf-spot

Cherry leaf spot on 'SK Carmine Jewel' tart cherry

Thaddeus McCamant, Central Lakes College

Cherry leaf spot, caused by the fungal pathogen Blumeriella jaapii, can be a devastating disease for tart cherries in parts of southern Minnesota. Leaf spot has a scattered distribution in Minnesota. Disease pressure can be quite high in some sites, while a few miles away, the disease is absent. Where the disease pressure is high, cherry trees can be defoliated during humid summers. Trees that are defoliated in late summer are more susceptible to winter injury and grow slowly the following spring. Yields on trees defoliated in August will be reduced the following summer due to poor flower bud formation.

Identification

Cherry leaf spot starts out as purple spots on the upper surface of older leaves in early summer. The spots grow to about 1/4 inch in diameter and turn reddish brown. The spots eventually turn brown, and several spots may grow together into larger dead patches. Yellow areas form around the leaf spots and the leaves start to die and fall off the tree about one month after the infection started. Leaf spot infections on plum and other Prunus species often dry up and fall out of the leaves, resulting in small holes scattered across the leaf, or a "shothole" appearance. Cherry leaf spot occasionally forms on leaf or fruit stems but infection on the fruit is rare.

Important biology

Cherry leaf spot fungi overwinter in infected leaves that have fallen to the ground. Although the leaves are dead, the fungi inside the leaves survive until the following growing season. The fungi start to grow in early spring in response to warm weather and produce spores that are discharged into the air. Once the spores land on susceptible leaves, the spores will only germinate if the leaves are wet. The optimum temperature for infection is between 58 and 73°F. Young leaves are resistant to leaf spot until they become completely unfolded. Ten to 15 days after infection, the spots appear on the leaves. Fungal spores are produced on the lower surface of leaf spots. These spores are spread by wind and splashing water, resulting in secondary infections. In years with frequent rains in May and June, the fungi can spread extremely quickly. By the end of July, most cherry trees stop forming new leaves. Since all the leaves are mature in late summer, all the leaves are susceptible to cherry leaf spot at that time, and entire trees can be defoliated.

Management

In Minnesota, leaf spot can be found in plums and tart cherries. European plum is less susceptible to disease. All varieties of tart cherry are susceptible to cherry leaf spot. If cherries are losing their leaves due to leaf spot, aggressive measures should be taken to control the disease.

Sanitation is a critical component of cherry leaf spot control. Collect and destroy fallen cherry leaves in September or October to remove the leaves that harbor the fungus through winter.

Fungicides

If trees have a history of leaf spot infection severe enough to result in significant leaf loss before September, fungicides can be used to protect leaves. Fungicide applications should be started two weeks after bloom, when leaves are completely unfolded and repeated at the interval specified on the fungicide label through the growing season, including one application after harvest. Fungicides with an active ingredient of myclobutanil or captan will protect leaves from infection with cherry leaf spot when applied properly. The leaf spot fungi may develop resistance to myclobutanil if this fungicide is applied too frequently. To avoid fungicide resistance, alternate between myclobutanil and captan when making repeated fungicide applications. Fungicides with an active ingredient of copper may provide some protection against leaf spot infection and some formulations acceptable for use in organic production are available. Fungicides work best if combined with sanitation.

The name of the plant being treated MUST BE LISTED on the fungicide label or the product cannot be used! Some products are registered for use on ornamental Prunus species but are not safe to use on stone fruit intended for human consumption. Always completely read and follow all instructions on the fungicide label.


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