Symptoms and Signs
Phytophthora trunk canker and crown rot usually originates at or below ground level but can occur higher above ground, especially where trunks or lower limbs are wounded. The canker is 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 pathogen infects the inner bark and outer layer of wood, killing cambium and phloem. Discoloration rarely extends deeper than the outer woody layer. Depending on the local conditions and rootstock, the tree may overcome the disease and the lesions may heal.
Affected trees show a gradual loss of vigor and decline of the top canopy. As progress of the disease advances, foliar symptoms of Phytophthora trunk canker differ from symptoms caused by phytophthora root rot (Phytophthora cinnamomi). With Phytophthora trunk canker, leaves retain their normal size and there is a gradual loss of leaves causing canopy thinning, whereas 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. mengei is achieved by laboratory tissue isolations onto Phytophthora selective media.
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 mengei infects the root crown and lower trunk and limbs of older trees, causing diseases called Phytophthora trunk canker and crown rot (formerly citricola canker). Phytophthora mengei also causes Phytophthora fruit rot.
Phytophthora mengei damages trunks, limbs and larger roots (although it is sometimes found on the feeder roots of diseased trees as well), while P. cinnamomi, damages smaller roots causing Phytophthora root rot. Disease develops after crowns, limbs, or trunks become infected through wounds, such as injuries from equipment, pruning, vertebrate injury, and wind damage. Spore spread and disease development are favored by wet soil conditions. Cankers often occur on the side of trunks wetted by irrigation sprinklers. Phytophthora mengei produces oospores and sporangia on the wounds that are spread by splashing water. Contaminated equipment and tools that injure healthy trees can cause a new infections.
Look for disease symptoms and use the guidelines for MONITORING DISEASES AND DISEASE-PROMOTING CONDITIONS and the UC IPM video Monitoring for Root and Crown Diseases in Avocado Orchards. 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 mengei can easily spread in contaminated nursery stock and on equipment, vehicles, and people. Follow the same sanitation procedures as described in the section PHYTOPHTHORA ROOT ROT.
Certain rootstock cultivars are more resistant to or tolerant of Phytophthora trunk canker or Phytophthora root rot. Consider planting more than one rootstock in a grove with a history of Phytophthora diseases. Seedling rootstocks are much more sensitive to trunk cankers than most of the clonal cultivars. 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 as 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.
Cultural control methods to prevent disease include irrigation management and wound prevention.
- Do not keep the lower trunks wet as this increases the chance of infection. Place sprinklers 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 to prevent the bark below ground from injury.
Do not stack cut wood against trunks. When adding mulch to orchards, keep it several inches back from the trunk.
Consider promptly treating fresh ground-level wounds such as pruning wounds with a fungicide. 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 at 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 a fungicide may be warranted. There is little documentation of fungicide efficacy for managing Phytophthora trunk canker and crown rot in avocado. See PHYTOPHTHORA ROOT ROT for discussion of Phytophthora fungicide application.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|A.||ALUMINUM TRIS PHOSPHONATE|
|(Aliette WDG)||5 lb||12||0.5 (12 hours)|
|COMMENTS: Apply as a trunk spray. Make the first application at the start of the growing season and repeat every 60 days. Repeat applications at 60 days are important (up to 4 applications/year); a single trunk spray is not sufficient to arrest the disease. Do not exceed 20 lb/acre per year.|
|(Agri-fos, Fosphite)||Label rates||4||0|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33)|
|COMMENTS: Do not apply with copper-based fungicides or fertilizers; allow 10 days before applying copper-based compound after phosphorous acid treatment or 20 days before applying phosphorous acid after copper treatment. Do not apply to dormant or heat- or moisture-stressed trees.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.|