‘Candidatus Phytoplasma prunorum’, is the cause of European stone fruit yellows (ESFY). Other common names include peach decline, peach yellows, peach vein-clearing, and apricot dieback. The ESFY phytoplasma is found throughout Europe, north Africa, and western Asia. Though related to the apple proliferation phytoplasma (AP), it is a distinct organism. It is also separate from the North American “X-diseases”, also caused by phytoplasmas.

The primary insect vector of ESFY phytoplasma is the plum psyllid (Cacopsylla pruni). Plum psyllids overwinter as adults on conifers, moving to Prunus spp. to reproduce. They only feed on Prunus, and only produce one generation per year. New adults leave as soon as they are developed, to fly to their overwintering host. Their preferred summer hosts, blackthorn (Prunus spinosa) and mahaleb cherry (Prunus mahaleb) are not reported in Alaska, but they are found in British Columbia. Psyllids are infected for life, and the phytoplasma multiplies in the insect. Psyllids require 1-2 days of feeding to transmit the ESFY phytoplasma. The specific vector species has not been reported from Alaska, but many other Cacopsylla species have been, along with several specimens identified only to genus level. Other possible vectors include leafhoppers in the genera Euscelis and Empoasca, both of which have member species in Alaska.

two small green jumping bugs on the underside of a green leaf

Cacopsylla sp. psyllid, Whitney Cranshaw, Colorado State University, Bugwood.org

a small 1mm green bug with large wings

Empoasca sp. leafhopper, Charles Olsen, USDA APHIS PPQ, Bugwood.org

Trees may not show symptoms for 4-5 months after infection. In contrast to the AP phytoplasma, ESFY phytoplasma can persist in Prunus stems during winter dormancy. It is graft transmissible, and capable of rapid spread within individual plants, as well as between plants.

Hosts and Symptoms

In general, ESFY symptoms are strongly influenced by host species, cultivar, rootstock, environmental conditions, and phytoplasma isolate. Often, only a few branches show symptoms, but the whole tree may be affected. Leaves are often chlorotic late in the season. Shoots may be shortened, with small deformed leaves. Premature leaf drop may occur. Fruit production is reduced, with poor development, and may drop early. In late winter, leaf buds may break early, increasing the risk of frost damage. In sensitive varieties, young trees may die within 1-2 years, though older plants are also susceptible.

In apricots and Japanese plum, leaves become chlorotic, rolling upward longitudinally (along the long axis), followed by reddening. Normal dormancy is reduced or suppressed. Severe and progressive necrosis occurs, followed by decline and possible death of the tree.

ESFY phytoplasma, leaf roll, apricot, G. Morvan, INRA, Montfavet, Bugwood.org

ESFY phytoplasma, phloem necrosis, apricot, G. Morvan, INRA, Montfavet, Bugwood.org

In peach, leaves may show severe longitudinal upward rolling, with early reddening. Midribs and primary veins may become thickened and corky. Leaves may become brittle, and fall early. In the fall, latent buds may form, producing tiny chlorotic leaves and sometimes flowers.

Other hosts include multiple species of cherry, including European bird cherry. Cherry trees may bloom abundantly, flowers are malformed with poor fruit set. Fruit are small with short peduncles and drop prematurely. Leaves are small and deformed with rosetting (shortened internodes), as well as phloem and bark necrosis.

Similar symptoms can be caused by Phytophthora root rot, which can cause yellowing and stunting, though generally all branches are affected unlike ESFY, which doesn’t usually affect all branches. Digging up the roots of a Phytophthora-infected tree will show red to brown rotting of the root system, with sharp lines separating healthy and diseased tissue. ESFY has some symptoms in common with the North American “X-diseases” , also caused by phytoplasmas. Leaves may yellow, roll, and veins may swell. X-diseases also cause yellow or red, irregular shaped leaf spots. These spots appear water-soaked, later drying and dropping out. Proper diagnosis requires laboratory testing.

wilted plant with brown spots on edge of leaves

X-disease, yellow leaves with rolled, swollen veins, peach, F. Dosba, INRA, Bordeaux, Bugwood.org

brown spots on leaves

X-disease, red leaf spots, peach, Mid-Atlantic Orchard Monitoring Guide

Most of the continental U.S. has a fairly low risk for establishment of ESFY phytoplasma, based on host availability. Due to Alaska’s low density of susceptible hosts, the risk of establishment is expected to be low.

Long-distance dispersal of the ESFY phytoplasma is by planting materials (plants, scions, and rootstock).

Suspected phytoplasma samples of symptomatic leaf tissue should be collected in the late summer or early fall, when phytoplasma concentrations are at their highest. At least 5 samples per plant should be taken from different parts of the canopy.