Diplodia Canker and Shoot Dieback, or Diplodia Blight

Various Diplodia species can infect broadleaves, conifers, and palms. The fungi canker, discolor, and kill bark and wood and cause the death of shoots terminal (outward or upward) from the infection site.

Identification

The bark of Diplodia-infected limbs becomes cankered; the infected bark areas become slightly sunken and discolored reddish brown. Infected bark may become rough, and the cankers can become several feet long.

Black, pimplelike, fungal fruiting bodies (pycnidia, Oregon State University photograph) erupt from bark cankered by Diplodia. Yellowish gum may exude from cankered bark. Cutting off bark reveals that infected cambial tissue has become discolored brown (Oregon State University photograph) to yellowish.

Life cycle

Diplodia overwinters in cankers. During wet weather in late winter and spring, fungal spores ooze out of tiny, black fruiting bodies. The spores are spread by splashing irrigation water and rain, wind, and pruning tools that contact infected tissue. The spores cause infection when they contact young buds, succulent shoots, and bark wounds such as those caused by frost, pruning, and sunburn.

Diplodia species can live in infected branches for years in a dormant state without producing any disease. Then if the plant becomes stressed or weakened, the fungi become active and cause cankers and shoot dieback.

Damage

Diplodia-infected bark becomes discolored, sunken, rough, or all of these. Leaves discolor and die on stems outward or upward from the cankered bark. Fruit, leaves, and nuts can also be directly killed by Diplodia fungi. Disease is most prevalent in wounded or stressed trees that are growing poorly (e.g., atypically slowly). Hosts include almond, apple, bird of paradise, citrus, oak, palm, pine, rhamnus, and walnut.

Solutions

Grow plants that are well adapted to the conditions at that site. Provide them with a good growing environment and appropriate cultural care to keep them growing vigorously. Periodic irrigation during the dry season to provide roots with adequate soil moisture is especially important. Protect aboveground parts and roots from injury. For example, apply several inches of coarse mulch around the trunk to control weeds and keep string trimmers and mowers back from the trunk. Immediately adjacent to the trunk, keep mulch back a few inches or thin.

Prune off cankered limbs and dispose of the wood away from valued plants. Use sharp tools and do not prune when conditions or plants are wet. Make each cut just outside the branch collar and branch bark ridge. After working on an infected plant, clean tools and dip or spray them with a 10% bleach solution, 70% isopropyl rubbing alcohol, or a commercial disinfectant as directed on the product label.

Grow resistant cultivars if available. Do not fertilize affected plants because increased nitrogen can increase disease development. For more information on Diplodia canker and blight of pines, see this video from Iowa State University.

Adapted from Apple (Malus spp.)-Diplodia Canker, Pacific Northwest Pest Management Handbooks; Diplodia Shoot Blight and Canker, University of Minnesota; and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).