Phomopsis canker, or Diaporthe stem canker and dieback—Phomopsis =Diaporthe spp.
Phomopsis canker affects a wide variety of conifers and woody broadleaves in part because there are many Phomopsis (=Diaporthe) species pathogens. The fungi mostly infect and kill branches less than 4 inches in diameter, but occasionally kill larger branches or the main trunk.
Identification
Symptoms of Phomopsis canker as described below under Damage indicate this or a similar plant pathogen is present. Infections and symptoms are commonly most abundant on the lower portion of the plant. For example, sunken cankers that usually have sharply defined borders with healthy bark develop on Phomopsis-infected branches of current-year growth during spring and summer. Cutting away bark at the edge of cankers reveals healthy green or whitish tissue adjacent to reddish brown, diseased, vascular tissue.
Once a Phomopsis species becomes established in a stem, black, pimplelike fruiting bodies (pycnidia) can be observed erupting through the bark. Each pycnidium produces a curling tendril of asexual spores (conidia) that are gelatinous, stringlike, and cream colored to white.
Life cycle
The fungi overwinter in bark. As temperatures warm in late winter, spores are produced from infected sites. The fungi can continue to produce spores throughout the growing season. The infectious spores are spread between nearby plants or within a plant by splashing irrigation water and rain. Long distance spread of the spores is by air movement.
The spores infect bark wounds, such as those caused by frost, pruning, or sunburn. The fungi can also infect young buds then move down into the stem. Otherwise, healthy woody plants may harbor a minor infection that can go undetected until the shrub or tree becomes stressed, then the fungus becomes more active and noticeable disease symptoms develop. Hot summer weather suppresses the development of the Phomopsis fungi and disease symptoms. The fungi become active again in late fall and winter.
Damage
The most obvious symptoms of Phomopsis canker are wilting and browning of leaves and browning and dieback of young shoots (blight). Dark and sunken areas (cankers) develop on infected stems and sometimes ooze resin or sap. When cankered bark is scraped away or infected wood is cut into, vascular tissue that is discolored reddish brown can be observed. Portions of the plant canopy, the top of the shrub or tree, or occasionally the entire plant can die. On some hosts such as grape the fungus causes leaf spots.
Phomopsis canker primarily affects hosts that are growing poorly, are stressed from other causes such as drought, or have been wounded. Hosts include almond, avocado, cedar, cypress, dogwood, Douglas-fir, elderberry, eucalyptus, fig, golden-chain tree, grape, honey locust, honeysuckle, incense-cedar, madrone, strawberry tree, pear, Port Orford cedar, walnut, and wisteria.
Solutions
During dry weather, prune off and dispose of all blighted stems and cankered branches. Dispose of prunings away from valued plants because infected wood can continue to produce spores after it is cut off. Annual pruning of dead stems and especially of interior branches of trees with thin bark and dense canopies can help to prevent the disease from becoming more extensive.
Avoid pruning during wet weather as this can spread the pathogen. Sanitize pruning tools after working with plants that are known or suspected of being infected with Phomopsis. For example, clean the tool then dip or spray it with 70% alcohol, 10% bleach, or a commercial disinfectant as directed on the product label.
Provide plants a good growing environment and appropriate cultural care. Periodic irrigation during the dry season to provide sufficient soil moisture is especially important.
Where disease has been severe, prune to remove dead and diseased stems and then spray plants promptly with Bordeaux mixture to prevent or reduce the development of new infections. Pruning is the only method that eliminates disease in already infected stems.
Adapted from Phomopsis Canker, University of Massachusetts and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM). |
Shoot tip dieback due to Phomopsis canker.
Brown Phomopsis canker with sharply defined borders with healthy green tissue.
Black fruiting bodies of the Phomopsis canker fungus, some producing tendrils of pale spores (bottom).
Bark removed revealing reddish brown discolored vascular tissue due to Phomopsis canker.
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