Avocado root rot, or Phytophthora root rot—Phytophthora cinnamomi
Phytophthora root rot is the most serious disease of avocado. It is caused by P. cinnamomi, which is a funguslike oomycete (water mold).
Identification
Foliar symptoms of Phytophthora root rot include small and pale green or yellowish leaves. The leaves wilt and develop brown necrotic tips even when the soil under the tree is well irrigated and wet. Foliage is sparse and new growth is rare, or new leaves may be small and pale. There may be little leaf mulch under infected trees. Small branches die back in the tree canopy, exposing other branches to sunburn because of the lack of shading foliage. Fruit yield declines, but diseased trees in many cases produce a heavy crop of small fruit before dying from root rot.
Small blackish to brown lesions encircling the smaller (feeder) roots are an early symptom of infection. In advanced stages of this disease, the small, fibrous feeder roots are scarce. Where present, the small roots are black, brittle, and dead from infection. Affected trees decline and die, some rapidly and others more slowly.
Life cycle
Root rot thrives where there is excess soil moisture and poor drainage (waterlogging). The pathogen infects mostly the small feeder roots. Roots pencil size or larger are seldom attacked by P. cinnamomi. Because infected trees have relatively few healthy feeder roots, nutrient and water uptake is greatly inhibited and aboveground parts decline and die. Trees of any size and age may be affected.
The pathogen is easily spread between its many host plants. Phytophthora cinnamomi spreads through the movement of contaminated nursery stock (avocado and other plants), on equipment and shoes, in seed from fruit that have lain on infested soil, or through any human or animal activity that moves pathogen-infested soil from one place to another.
Phytophthora produces zoospores, which are specialized to spread by swimming from tree to tree in wet soil. Zoospores move easily and rapidly in water as it moves over or through the soil. Avocado root rot also produces resting spores that persist for years in infested soil and readily infect hosts that are later planted there.
Damage
Infection, decay, and death of small (feeder) roots by Phytophthora cinnamomi causes the aboveground parts of the tree to decline and die. Before the tree dies, foliage becomes pale and wilted with brown tips. Leaves drop prematurely and new foliage growth is absent or sparse and discolored and undersized. Limbs die back then the entire tree dies.
Phytophthora cinnamomi also causes Phytophthora fruit rot. A different species of this oomycete, Phytophthora mengei =P. citricola, causes Phytophthora canker, or citricola canker, of the lower trunk and root crown. All these diseases can occur together on the same tree.
Solutions
To avoid and manage avocado root rot, use an integrated pest management (IPM) approach that emphasizes prevention. Use certified disease-free nursery stock (if available) and root rot-resistant cultivars obtained from a reputable, commercial supplier. Inspect the roots of new trees before planting them. If root health appears questionable, seek advice from the local UC Cooperative Extension office before you decide whether to plant the tree.
Employ stringent sanitation measures to avoid introducing Phytophthora via infected plants and infested soil and water. Phytophthora cinnamomi has more than 1,000 hosts, including many annual flower crops, berries, deciduous fruit trees, ornamentals, and vegetables. If one of these hosts was infected with P. cinnamomi and avocado grows nearby or is later planted at the site, it is likely to become infected with the pathogen because P. cinnamomi persists and moves in soil.
Provide Favorable Soil Conditions. In new plantings, avoid soil conditions that are favorable to root rot development, including poor drainage and salinity. Plant on well-drained soil or improve the soil’s drainage before planting. Methods include planting on a soil mound or raised bed. Do not work around trees when the soil is soggy as this can compact soil, which reduces drainage and injures roots.
Plant Resistant Rootstocks. Certain rootstock cultivars are resistant to or tolerant of root rot, including Dusa and Latas. Be aware that resistant rootstocks are not immune to root rot. If you plant or maintain them under adverse conditions, they may be killed by a combination of the adverse conditions and the pathogen.
Apply Gypsum and Mulch. Create soil conditions that suppress Phytophthora root rot by applying gypsum under the canopy of each tree, perhaps 25 pounds beneath a medium-sized avocado tree. Apply at least 4 to 6 inches of coarse organic mulch on soil beneath the tree canopy, but keep the mulch several inches away from the trunk itself. Use organic mulch such as composted green waste (yard trimmings) or hardwood chips.
Mulching promotes the development of beneficial microorganisms antagonistic to Phytophthora cinnamomi. Mulch can also reduce salinity by reducing water evaporation that concentrates salts near the soil surface. Adding gypsum supplies calcium, which suppresses the formation of Phytophthora spores. For best results apply mulch and gypsum when the tree is first planted. As the tree grows it produces its own mulch that should be left under the canopy. As the gypsum disappears and mulch decompose, apply more.
Irrigate Carefully. Appropriate irrigation is the single most critical practice for preventing the disease, improving the health of affected trees, and managing avocado root rot. Avoid overirrigation and do not create prolonged soggy soil. Avoid watering soil that is already wet. Schedule the frequency and amount of irrigations based on sophisticated methods, such as according to local evapotranspiration (ET) rates or the use of soil moisture monitoring devices such as tensiometers. Careful irrigation helps to avoid the disease and can often prolong the life of infected trees.
Irrigate with high-quality water such as that used for drinking. Irrigation water with high overall salinity or an excess of boron, chloride, or sodium promotes Phytophthora infection of roots.
Provide Appropriate Nutrition. Moderate amounts of nitrogen fertilization promote good growth that helps make avocado trees more tolerant to root rot. But avoid applying excessive amounts of fertilizer. Avoid applying large amounts of animal manures or other products high in ammonia or salts. Avocado roots are sensitive to ammonia and salts, which can increase root susceptibility to Phytophthora.
Certain systemic oomyceticides (fungicides) can control Phytophthora root rot. Products that can provide some control are available only to professional pesticide applicators who must be hired to make the treatment. To get the services you want consult Pest Notes: Hiring a Pest Control Company.
Adapted from Integrated Pest Management for Avocados, Pest Management Guidelines: Avocados, and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM).
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