Botryosphaeria and Phomopsis Cankers

The initial symptom of branch infection is often wilting and flagging of leaves on branches distal to the canker. Cutting away the bark reveals the discolored, brown to black cortical and cambial tissues. If the infection is old enough, peeling away the bark will reveal well-developed embedded black pycnidia (asexual fruiting bodies that contain fungal spores) or occasionally perithecia (sexual fruiting bodies) of the Botryosphaeriaceae species, Phomopsis species, or sometimes both. In young trees, infected stems usually turn black, while in older trees major branches can be killed. Killing of large branches is often associated with shading and heavy numbers of walnut scales and possibly other scales.

Infected green fruit do not immediately show symptoms. Fruit develop disease symptoms when mature in August and September. Infections start as decay lesions on the hull of a fruit and decay can spread to the neighboring fruit. The lesions spread and invade the peduncle (the stalk that bears the fruit) and subsequently the spurs, resulting in black cankers and dead buds. Hulls of infected fruit turn black initially and then brown to beige after drying. Areas of fruit with pycnidia are light beige. Infected fruit may fall prematurely. Cankers on spurs continue growing during fall and are covered with a dense layer of pycnidia and sometimes perithecia of both Botryosphaeriaceae and Diaporthe (Phomopsis) spp. fungi. If sprinkler irrigation wets leaves and shoots, necrotic lesions can form on the leaves.

Since the late 1990's, Botryosphaeria and Phomopsis species have been isolated from a large number of blighted shoots of Chandler, Howard, Payne, Tulare, Vina and other walnut varieties from Butte, Colusa, Glenn, Fresno, Kern, Kings, Placer, Sacramento, San Benito, San Joaquin, Solano, Stanislaus, Sutter, Tehama, Tulare, Yolo and Yuba counties.

The fungi overwinter on dead branches and shoots, and can develop both water-splashed and airborne spores. Pycnidia of Phomopsis spp. and fungi in the Botryosphaeriaceae family can be found year-round on diseased branches and may sometimes occur together on the same branch. The same fungi also occur on woody trees and shrubs in riparian areas next to walnuts, thus serving as inoculum sources. Some of the Botryosphaeriaceae species that cause panicle and shoot blight of pistachio and band canker of almond also cause Botryosphaeria canker of walnuts.

Fungi in the family Botryosphaeriaceae and in the genus Phomopsis generally indirectly infect through wounds such as pruning or sunburn wounds, leaf scars, and peduncle scars. Pruning wounds are susceptible to infection for four months. These fungi can also directly infect walnut fruit. Under favorable environmental conditions, Botryosphaeriaceae fungi can directly infect shoots as well.

Reduce Botryosphaeria and Phomopsis cankers in walnut orchards with fungicide sprays applied in mid-May, mid-June, and mid-July. Evidence is inconclusive of the efficacy of bloom and postharvest sprays.

Ensure good sanitation and use cultural practices to reduce inoculum.

• Prune out diseased limbs, cutting back into healthy wood (where the wood is no longer discolored); at least 2 to 3 inches beyond the lower canker is sufficient to remove the pathogen.
• Remove dead wood and destroy it (shred, chip, or burn) during a dry period. Remove all large dead branches out of the orchard when there is not severe Botryosphaeria blight, but shred branches in the orchard floor when there is very heavy Botryosphaeria blight.
• Walnut scale, and probably other species of scales attacking walnuts, predisposes branches to infection from Botryosphaeria fungi. Control scale pests to minimize entry points for the fungi.

Chemical management of this disease depends on the application of protective sprays to prevent infections of pruning wounds and developing fruit, leaves, leaf and bud scars, and buds.

• Consider the presence of riparian woody vegetation next to the orchards (a source of inoculum) and adjust disease management approaches accordingly.
• In orchards with high inoculum levels (a lot of dead wood and pycnidia of Botryosphaeria and Phomopsis are commonly found), apply protective fungicides in mid-May, mid-June, and mid-July.
• In orchards where the inoculum is light, follow a lighter program when conditions favor disease development (temperatures above 50ºF and at least 0.25 inch of rain). Disease is favored during frequent rains, when sprinkler irrigation water wets the foliage, or when orchards are next to rivers or creeks since this can result in higher humidity and dew formation.