Eutypa dieback—Eutypa lata
This disease is also known as Cytosporina, gummosis, and limb dieback. It commonly affects apricot and grape and sporadically damages cherry. The cause is generally Eutypa lata, but other fungi in the Diatrypaceae family can be involved. For example in grapes in the North Coast, Eutypa leptoplaca also causes Eutypa dieback.
Spring growth of leaves with faded color, undersized leaves, and wilting are commonly the first-observed symptoms of Eutypa dieback. Affected limbs have cankers where bark can be dark and roughened. Cutting off bark adjacent to old pruning wounds can expose cankered wood, which is dark and may be cracked, swollen, or both. Discolored wood may extend beyond the edge of the canker and into heartwood. If limb cankers are cut in cross-section, the darkly discolored wood is commonly U shaped or wedge shaped. Brown gum can exude from bark at the margins of cankers.
Several years after the initial infection, a layer of dark fungal tissue (stroma) forms on the surface of cankered wood where bark has sloughed off. Stroma give limbs a blackened, burnt appearance. In wet locations spore-forming structures (perithecia) form in stroma on wood that has been dead for at least 2 years. The perithecia are black and spherical and can be observed by cutting into stroma and examining the fungal tissue with a hand lens. Perithecia are produced if annual rainfall is at least 14 inches. Perithecia also occur at sites of lower rainfall if hosts are irrigated with sprinklers that wet infected wood.
Eutypa spores are spread by splashing irrigation sprinklers, rain, and wind. The spores infect hosts through wounds, commonly pruning wounds. Eutypa lata develops within hosts' vascular tissue, killing cambium, invading heartwood, and decaying and weakening limbs.
After an initial infection, several growing seasons may elapse before the symptoms of cankers, dieback, and stunted shoots develop. Once a limb has been killed, it takes several more years before spores are produced on the old infected host tissue and then only under conditions of high moisture.
Ascospores are the only known infective stage of Eutypa lata. The spores are discharged during periods of rain, primarily fall through spring, and when infected limbs are wetted by sprinklers. The spores can spread long distances with wind and infect hosts when they land on tissue that was wounded relatively recently. During cool weather pruning wounds can remain susceptible to infection for 6 or more weeks after the cuts were made. During warm weather pruning wounds remain susceptible to infection for about 2 weeks after the injury.
Eutypa lata can infect hosts other than grape and stone fruits. Eutypa dieback generally is not a significant problem in these other hosts, but they can be sources of spores that initiate infection and serious disease in grape and stone fruits. Other reservoirs for the pathogen include almond, apple, blueberry, crabapple, honeysuckle, kiwi, oleander, pear, and certain native plants (e.g., big leaf maple, California buckeye, ceanothus, and willow).
Eutypa dieback is a serious disease of apricot and grape and occasionally affects cherry. On newly infected hosts generally the first symptoms of Eutypa dieback are foliage that fades in color, is undersized, and wilts on individual branches during spring or summer. Limbs die back and dead leaves commonly remain attached through the winter. The disease is progressive and if not effectively managed an increasing number of limbs die over time. Eventually the entire plant can be killed.
Eutypa dieback is most serious in coastal growing areas because weather there favors dispersal and infection of the fungus. Eutypa dieback also occurs at inland growing areas, such as San Joaquin and Yolo counties. The disease is more important in older hosts because they have more pruning wounds, and infections and dieback accumulate over time.
Prune apricots and cherries only during July and August in inland areas and during August near the coast. This can allow wounded tissue to no longer be susceptible to infection before the rainy season arrives. If hosts must be pruned outside of summertime, flaming wounds with a propane torch for 5 to 10 seconds after making the cut can cause the wound to become immune to infection by Eutypa spores. In grapes, which are commonly pruned during winter, delaying pruning to as late as possible in the dormant season (February or later) reduces the risk of infection.
Sanitation can help limit the pathogen's spread. Prune off infected limbs during summer. Make cuts at least 1 foot below the site of any canker. Make sure no discolored, diseased wood remains below the cut. If wood is discolored, make another cut further down the limb. Dispose of infected wood away from hosts, such as at a landfill.
Provide plants with good growing conditions and proper cultural care, such as fertilization if warranted and especially appropriate irritation to promote vigorous growth. Hosts stressed from drought or other poor growing conditions appear more extensively affected by this disease. If hosts are sprinkler irrigated, direct the spray low enough so that limbs are not wetted or use deflectors to prevent wetting of host wood.
Painting or spraying fresh pruning wounds with the fungicides myclobutanil or thiophanate-methyl can reduce the risk of wound infection by Eutypa lata. When pruning is done during the summer, fungicide treatment is generally not warranted.
Adapted from Grape Pest Management Third Edition from the University of California Agriculture and Natural Resources also available as an eBook, and Integrated Pest Management for Stone Fruits, Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Grape limb with stunted foliage (right) due to Eutypa dieback.
Dark, rough bark on apricot with Eutypa dieback.
Gum oozing from margins of bark cankered by Eutypa lata.
Eutypa canker exposed under bark at the site of an old pruning wound.