How to Manage Pests
UC Pest Management Guidelines
Dothiorella cankers exude reddish sap that dries to a brown and white powder. Bark may be cracked, darkly discolored, or slightly sunken. With older cankers, bark may shed or can be easily removed from the damaged area. Under the canker, inner bark and wood is brown, orangish, or red, instead of the normal pale color. Brown dead leaves remain attached if much of the xylem becomes infected and rapidly kills the entire limb.
Dothiorella canker can be a serious problem in new plantings; stock sometimes arrives from the nursery with latent infections in the graft union. Where infection kills the graft union, the dead scion retains a dry brown canopy, while shoots and green leaves sprout up from the rootstock. The graft union may be unusually swollen and rough before the young tree dies. Cutting inside at the graft union reveals dark, discolored wood that can extend through the entire width of the small trunk.
Dothiorella canker is usually of minor importance in established, older trees. Scattered small branches and sometimes large limbs can die back. Usually the entire tree is not affected and the tree remains productive. In severe cases, the main trunk may be girdled, killing the tree.
Several pathogens cause Dothiorella canker on trunks and large limbs. This disease was formerly attributed to a Botryosphaeria anamorph, Dothiorella gregaria (teleomorph B. ribis), and the disease was known as Dothiorella canker. Neofusicoccum luteum is now known to be the most common cause of Dothiorella canker disease on avocados in California. Botryosphaeria dothidea also causes Dothiorella canker in California. Several other Botryosphaeria, Diplodia, or Fusicoccum spp. cause DOTHIORELLA FRUIT ROT, DOTHIORELLA LEAF AND STEM BLIGHT, and cankers and these fungi can not be reliably distinguished during certain stages of their growth, except with molecular tests such as PCR (polymerase chain reaction).
Dothiorella cankers closely resemble Phytophthora canker. Dothiorella cankers usually occur higher above the ground, beginning around the first main branch crotch or higher. Dothiorella can infect much smaller limbs, such as twigs and small branches, as well as the upper trunk and large limbs. When cut into with a knife, Dothiorella cankers sometimes extend deep into wood, while Phytophthora canker discolors only a shallow layer of outer wood. Except when trees are young, Dothiorella canker is usually not as serious as diseases caused by Phytophthora spp.
Botryosphaeria spp. can infect only through wounds. Heavy rainfall causes increased spore production and infection. Spores spread in air and water. Trees that are stressed are much more susceptible to this disease. Common stresses include poor irrigation, low quality irrigation water, nutritional deficiencies, or severe insect and mite feeding. Drought stress especially promotes symptom development and triggers latent infections to develop into disease. Mexican rootstocks are generally much more resistant to this disease than are Guatemalan cultivars.
Look for diseases and disease-promoting conditions regularly throughout the grove using recommended monitoring methods. Consider planting rootstock cultivars that have some resistance to this disease. Where Dothiorella canker is a problem, rely primarily on sanitation and good cultural practices to control it. Prune out dead limbs and twigs, where the pathogen pycnidia (spore-forming structures) and spores persist. Dispose of dead wood and old fruit well away from avocado trees. Prune and harvest only during dry conditions. Correct environmental and nutritional stresses, and minimize other pest problems. Appropriate amount and frequency of irrigation is especially important. Leach soil periodically and use low salinity water if salt toxicity is a problem. Nurseries should use stringent sanitation measures, disinfest propagation material, and consider treating graft unions with fungicide.
UC IPM Pest Management Guidelines: Avocado