Virus diseases of cherry—Various pathogens
Cherries in California are affected by at least seven viral diseases or viruslike plant pathogens. Most of these are discussed here. Tomato ringspot virus that affects almond, cherry, peach, and plum is discussed separately.
Identification
Viral diseases are commonly difficult to distinguish from one another based on symptoms alone. Submitting samples to a plant diagnostic laboratory may be necessary to confidently identify the cause of unhealthy trees. The cause of some viruslike diseases of cherry is unknown.
Cherry mottle leaf. Cherry mottle leaf virus infections are usually symptomless except on highly susceptible sweet cherry cultivars. The susceptible cultivars develop leaves with chlorotic mottling, distortion, and puckering. In comparison with other cherry viruses that cause chlorosis, the margin between yellow and green leaf tissue is not as distinct when Cherry mottle leaf virus is the cause. Fruit on infected limbs may remain undersized and ripen later than normal.
The virus is spread by budding and grafting with infected wood. The bud mite Eriophyes inaequalis also can spread the virus from infected bitter cherry (Prunus emarginata) and infected but symptomless peach trees to sweet cherry. However, in the field in California spread of this virus by the mite is rare. Cherries grown in coastal or foothill locations where bitter cherry naturally grows are at greater risk of a mite-vectored infection.
Cherry necrotic rusty mottle. This malady may be caused by a virus but the pathogen is unknown. Brown lesions develop on affected leaves about one month after full bloom. Tissue with lesions may drop out, leaving shot holes in leaves. Affected leaves may drop prematurely. Shortly before harvest, infected leaves may develop a dark green and yellow mottling. Terminal buds may be killed. Dead patches on bark may develop blisters and gum.
The disease is transmitted by budding and grafting with infected wood. Spread from tree to tree in the field has not been observed in California. Note that shot hole fungus (Wilsonomyces carpophilus) also kills buds and causes discolored lesions and holes in leaves. However, shot hole generally develops on cherry only after an unusually rainy winter or spring when the tree grows near more susceptible hosts such as apricot, nectarine, and peach.
Cherry rasp leaf. Cherry rasp leaf virus causes leaves to develop prominent, distorted or leaflike growths (enations) on the underside along the midvein. Affected leaves are distorted but remain green and develop no chlorosis. The green color distinguishes cherry rasp leaf from the cherry rugose mosaic strain of Prunus necrotic ringspot virus that causes chlorotic leaves.
Cherry rasp leaf symptoms begin on the lower part of the tree and move upward as the virus spreads. Because fewer leaf buds develop on infected wood, limbs become bare near the base of the tree while leaves higher up develop leaf enations. Shoots may be stunted, and bark of the trunk and limbs may develop cankers.
Cherry rasp leaf virus is spread by budding and grafting with an infected host and by soil-dwelling dagger nematodes, Xiphinema species. If an infected cherry or peach tree grew at the site then was removed and another cherry or peach tree was planted there, the new tree would also become infected because the nematode vector of the virus persists in soil.
Cherry rugose mosaic. Prunus necrotic ringspot virus infects Prunus species and certain other Rosaceae such as roses. The several strains of this virus cause symptoms that vary according to the particular host. The most important strain on cherry is rugose mosaic that causes necrotic ringspots (photograph by Oregon State University) on leaves early in the growing season. Leaves later turn yellow, distort at the tip, and become ragged. Discolored areas on leaves may turn brown and leaves may drop prematurely, such as early in summer. Enations (leaflike growths) may develop on the underside of leaves near the midvein. Fruit may be deformed and ripen later than normal. In the field the virus can be spread by thrips.
Cherry stem pitting. The cause of this graft-transmissible disease has not been identified. It is believed to be a soilborne virus that is spread by soil-dwelling nematodes. In groups of cherry trees, symptoms commonly spread from tree to tree in a circular pattern, which is a characteristic pattern of spread by soilborne vectors. Symptoms resemble those of some other viruses and root diseases.
Buds on infected trees with cherry stem pitting open later than normal. Trees have a more open canopy because leaves are smaller and fewer than normal. Fruit may be small and pointed with short stems, similar in appearance to fruit affected by X-disease (cherry buckskin). In some cases, foliage and fruit symptoms develop on one or two branches of a tree. Cherry stem pitting is characterized by pits and grooves in the wood of the trunk just underneath the bark.
Dwarf, or stunt. Prune dwarf virus mostly affects peach and prune. Infection of cherry causes the leaves to be narrower than normal and have a rough, wrinkly texture. The virus is spread by budding and grafting with infected wood. If thrips have fed on virus-infected pollen, such as that deposited by bees, when these thrips feed on cherry they may also spread the virus.
Solutions
Virus-infected trees in the field cannot be cured. Measures to prevent virus infection and tree replacement are the available management methods.
Buy and plant virus-free and certified planting stock obtained from a reputable commercial supplier. Provide trees with a good growing environment and proper cultural care to increase their tolerance for disease damage.
If a group of cherries are being grown, promptly remove those that are virus infected and dispose of the wood away from the growing site. If the viral cause is believed to be spread by soil-dwelling nematodes, after cutting down infected trees consider promptly treating the stumps with a systemic herbicide registered (labeled) to kill stumps, which includes certain glyphosate formulations such as Roundup Weed and Grass Killer Super Concentrate. This prevents the roots from continuing to support the nematode vectors.
Replace trees that are not performing satisfactorily. Where cherry viruses have been a problem, consider replanting with Colt rootstock because it provides some resistance to virus diseases. Or replant with something other than a stone fruit.
Additional controls can be used for a few cherry viruses. Cherry mottle leaf virus is controlled partly by removing bitter cherry trees near those grown for their sweet fruit. Cherry rasp leaf virus and cherry stem pitting may be avoided in part by preventing the movement of soil from around infected trees and preventing surface water from moving from around infected trees to those that are healthy; this is because the nematode vectors of these maladies move with soil and water that is moved. Cherry rugose mosaic virus and Prune dwarf virus can be spread by thrips; controlling nearby weeds before they flower can reduce the abundance of thrips and perhaps reduce the spread of these viruses.
Techniques used to manage Tomato ringspot virus probably are effective against cherry rasp leaf. Colt rootstock appears to slow the development of symptoms in cherry scions.
Adapted from Prunus necrotic ringspot virus, Cherry rugose mosaic (PDF), California Department of Food and Agriculture, and Integrated Pest Management for Stone Fruits and Pest Management Guidelines: Cherry, University of California Statewide Integrated Pest Management Program (UC IPM). |
