Symptoms and Signs
Early symptoms of branch wilt are yellowing and withering of leaves on the outermost branches typically with a southwest exposure during midsummer to early fall. Next, leaves on larger infected limbs suddenly wither, turn dull green, then dark brown, and then they become dry. These leaves remain attached well after healthy leaves have fallen. Blighted fruit due to branch wilt can also be colonized by the pathogen (frequently the pathogen can be isolated from the shriveled kernel of these fruit). Portions of the thin outer layer of bark peel away, exposing a black sooty, dusty mass of fungal spores (arthrospores). Diseased limbs have a gray to dark brown discoloration in the shape of a cylinder or wedge extending into the center of the wood.
Comments on the Disease
Branch wilt develops only under warm temperatures. It affects English walnut throughout the interior valley of California but not in the coastal areas of the state. All commercial cultivars of walnut grown in California are susceptible to the disease. The fungus invades branches only through splits, frost damage, and sunburn injury from spores spread by windblown rain. The branch wilt pathogen has also been found on almond, apple, apricot, chestnut, citrus, fig, peach, poplar, grape, and black walnut and can cause similar wilt symptoms in some of these hosts. Frequently, additional Botryosphaeriaceae fungi colonize branches killed by the branch wilt fungus, so sections of these branches may be covered with pycnidia of other Botryosphaeriaceae species. Also, the pathogen can infect sunburned trunks of newly planted trees and kill them.
- Remove infected limbs when the weather is dry to avoid infection by Botryosphaeriaceae fungi through pruning wounds. Prune out diseased limbs, cutting back into healthy wood (where the wood is no longer discolored); at least two inches beyond the lower canker is sufficient to remove the pathogen. Where permitted, burn these branches.
- Avoid sunburn by maintaining vigorous trees through adequate irrigation, fertilization, and pest control.