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X-Disease

Phytoplasma

This disease is caused by a mycoplasma and infects many varieties of stone fruits. On cherry, infected trees tend to develop a dieback and a generally unthrifty appearance (A). Infected trees decline, but the rate of decline is dependent on the rootstock. Cherry trees on resistant rootstock, such as Mahaleb, decline rapidly because of a hypersensitive response at the graft union. On susceptible rootstock, such as Mazzard or Colt, decline occurs over several years. Cherry fruit on these trees tend to be small, flattened, pointed, and pale-colored and are confined to a few branches, but mixed with some normal fruits (B). On peach, leaves curl inward after several months. Water-soaked spots turn red, become necrotic and drop out, giving leaves a tattered appearance (C). Localized areas or the entire canopy defoliates, leaving foliage only at the tips. The entire tree may show symptoms 2 to 3 years after the initial infection (D).

Distribution

In eastern US, primarily a problem in MI, the Hudson Valley of NY, and southern New England, with occasional reports from the Cumberland-Shenandoah region.

Similar Species

Nitrogen deficiency can cause red spotting on peach leaves. Bacterial canker can cause a dieback in older cherry trees that might be confused with X-disease. Over-cropping of sweet cherries on Gisela rootstocks can cause uneven fruit ripening.

Management

Chokecherry (Prunus virginiannae L.) is a natural host/reservoir for the pathogen. The disease is spread from infected chokecherry, wild sweet cherry seedlings, and cultivated sweet cherry by leafhoppers. The only effective control measure for this disease is the eradication of chokecherry and infected cherry trees within a 150-m radius of all peach, nectarine, and cherry orchards.

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