How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Phytophthora cankers usually originate at or below ground level but can occur higher above ground, especially where trunks or lower limbs have been wounded. The canker appears as a region of dark bark that often exudes red resin, which becomes brownish to white and powdery as it dries. Cutting away the superficial canker reveals an orange-tan to brown lesion, instead of the normal white or cream-colored tissues. The lesion may have a fruity odor when exposed. The lesion infects the inner bark and outer layer of wood, killing cambium and phloem. Discoloring rarely extends deeper into wood than the outer woody layer. Depending on the local conditions and rootstock, the tree may ward off the disease and the lesions may heal.

Affected trees show a gradual loss of vigor and decline of the top. With advanced disease, foliar symptoms of Phytophthora canker differ some from symptoms caused by avocado root rot (Phytophthora cinnamomi). With Phytophthora canker, leaves retain their normal size, there is a gradual loss of canopy, and branch dieback (staghorning) is less typical. Unlike root rot, canker and collar rot affects the major tree roots and the smaller feeder roots are usually still present. Occasionally, in advanced stages, trees will die suddenly, with leaves turning brown within a short period of time. Confirmation of P. citricola is achieved by laboratory tissue isolations onto selective media for Phytophthora.

COMMENTS ON THE DISEASE

Phytophthora canker is the most important of several canker diseases infecting avocado and is second only to root rot in severity among diseases of avocado. Phytophthora citricola infects the root crown and lower trunk and limbs of older trees, causing diseases called Phytophthora canker, Citricola, Citricola canker, Phytophthora canker and collar rot, or Phytophthora canker and crown rot. Phytophthora citricola also causes PHYTOPHTHORA FRUIT ROT. It has a wide host range, including cherimoya, cherry, fir, and walnut.

Phytophthora citricola damages trunks and limbs and only the larger roots, while P. cinnamomi, which cause avocado (Phytophthora) root rot damages small roots. P. citricola occurs innocuously on the feeder roots of many or most avocados, but disease occurs on only some of these trees. Disease develops after crowns, limbs, or trunks become infected through wounds, such as injuries from equipment, pruning, vertebrate chewing, and wind damage. Spore spread and disease development are favored by excess soil moisture and wet conditions. Cankers often occur on the side of trunks wetted by irrigation sprinklers. Phytophthora citricola produces oospores, which ooze from wounds and spread in splashing water or anything that contacts ooze. Contaminated equipment and tools that wound healthy trees can cause a new infection.

MANAGEMENT

Look for diseases and disease-promoting conditions regularly throughout the grove by MONITORING DISEASES AND DISEASE-PROMOTING CONDITIONS. In California, the diseases caused by Phytophthora spp. (root rot and canker) are increasingly found together. Hence, integrated approaches to the control of both need to be followed including sanitation, selection of tolerant rootstocks, good water management, and wound prevention.

Phytophthora citricola can easily be spread in contaminated nursery stock and on irrigation equipment, vehicles, and people. Follow the same sanitation procedures as described in the section AVOCADO ROOT ROT.

Certain rootstock cultivars are more resistant to or tolerant of Phytophthora canker or Phytophthora root rot. Consider planting more than one rootstock in a grove with a history of Phytophthora. Seedling rootstocks are much more sensitive than most of the clonal cultivars to trunk cankers. In University of California field trials, Toro Canyon, Duke 7, Duke 9, and Barr Duke have shown moderate tolerance, as compared to other, more susceptible rootstocks such G1033, G6, and G755B. Thomas rootstock has tolerance to root rot, but is quite susceptible to canker and collar rot and other problems such as excess salinity.

Do not keep the lower trunks wet for long periods, as this increases the chances of infection. Place drippers away from trunks, aim sprinklers to avoid wetting trunks, or switch from sprinkler to drip irrigation where feasible. Avoid wounding major roots and trunks, especially avoid pulling suckers so the bark below ground is injured. Do not stack cut wood against trunks. Rake mulch several inches back from the trunk.

Chemical Control
Consider promptly treating fresh wounds with fungicide, such as wounds from pruning. Remove suckers only by cutting them above ground, then treat the wound. Periodically disinfect pruning tools, such as after finishing work on each tree. If cankers are detected in an early stage before much of the trunk is invaded, they can sometimes be controlled by cutting out the infected tissue and spraying the wound with an effective fungicide. Where cankers extend below ground, a combination of aboveground application and soil drench with fungicide may be warranted. There is little documentation of fungicide efficacy for managing Phytophthora canker and crown rot in avocado. See AVOCADO ROOT ROT for discussion of Phytophthora fungicide application.

IMPORTANT LINKS

• Photos of causes of fruit damage

PUBLICATION

UC IPM Pest Management Guidelines: Avocado
UC ANR Publication 3436
Diseases
B. A. Faber, UC Cooperative Extension, Santa Barbara/Ventura counties
A. Eskalen, Plant Pathology, UC Riverside
G. S. Bender, UC Cooperative Extension, San Diego County
Acknowledgment for contributions to Diseases:
H. D. Ohr (emeritus), Plant Pathology, UC Riverside
J. A. Menge, Plant Pathology, UC Riverside
L. J. Marais, Plant Pathology, UC Davis
R. Hofshi, Hofshi Foundation, Fallbrook, CA
J. S. Semancik, Plant Pathology, UC Riverside
J. A. Downer, UC Cooperative Extension, Ventura County
U. C. Kodira, Plant Pathology, UC Davis

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