Phytophthora Root Rot

(View photos to identify root and crown diseases)

Foliar symptoms of Phytophthora root rot include small, pale green or yellowish leaves. Leaves often wilt and have brown, necrotic tips. Foliage is sparse and new growth is rare. There may be little leaf litter under infected trees. Small branches die back in the tree top, exposing other branches and fruit to sunburn because of the lack of shading foliage. Fruit production declines, but diseased trees frequently set a heavy crop of small fruit.

Small, fibrous feeder roots are scarce at advanced stages of this disease. Where present, small roots are black, brittle, and dead from infection. Foliage is wilted even when soil under diseased trees is wet. Affected trees will decline and often die either rapidly or slowly.

Phytophthora root rot is the most serious and important disease of avocado worldwide. The causal agent, Phytophthora cinnamomi, has over 1,000 hosts, including many species of annual flower crops, berries, deciduous fruit trees, ornamentals, and vegetables.

Root rot thrives in areas of excess soil moisture and poor drainage. Trees of any size and age may be affected. The pathogen is easily spread through movement of contaminated nursery stock of avocado and other plants, on equipment and shoes, in seed from fruit lying on infested soil, or by any activity by people or animals that moves moist soil from one place to another. Phytophthora produces four different spore stages that are involved in disease development and survival: sporangia, zoospores, chlamydospores, and oospores. They spread easily and rapidly in water moving over or through the soil. Entire areas can readily become infested. Phytophthora species are not true fungi but have many fungal-like attributes.

Look for diseases and disease-promoting conditions regularly throughout the grove by MONITORING DISEASES AND DISEASE-PROMOTING CONDITIONS and the UC IPM video Monitoring for Root and Crown Diseases in Avocado Orchards. Use an integrated approach that emphasizes prevention.

• Purchase certified disease-free nursery stock and root rot-resistant cultivars.
• Inspect roots before planting and if their health appears questionable seek advice from a farm advisor or private consultant before planting trees.
• Employ stringent sanitation measures, good cultural practices, and appropriate chemical controls. The most important control of this disease is good irrigation management. For example, where new trees are interplanted among older trees, separate irrigation lines are needed to ensure appropriate irrigation timing and amounts for the different aged trees.

Use cultural practices that promote healthy growth of the tree while discouraging growth of the pathogen.

Provide favorable soil conditions

In new plantings, avoid soils and soil conditions favorable to root rot development, including poorly drained, saline, or pathogen-infested soils. Plant on well-drained soil, or improve drainage by planting on a soil berm, deep-ripping impervious subsoils, or installing subsurface drains. In established plantings, manage soils carefully so that excess moisture does not accumulate.

Use certified disease-free nursery stock

Request certified, disease-free plants, especially when planting new areas, because disease is especially damaging to young trees. Nurseries should disinfest propagation material, such as by immersing seed in water at 120 to 122°F for 30 minutes and then quickly cooling it. Nurseries should also use pasteurized soil mix, clean irrigation water from deep wells or disinfested surface water, and stringent sanitation to prevent pathogen introduction and spread. Nurseries that rely only on fungicides for disease prevention can promote fungicide resistance and produce symptomless plants with infections that develop after planting.

Plant resistant rootstocks

Certain rootstock cultivars are more tolerant of root rot, including Dusa, Latas, and others. Newer recommended cultivars such as Uzi and Zentmyer may also be available. Barr Duke, Duke 7, and Duke 9 can also be good rootstocks but have less Phytophthora-resistance than some newer cultivars. To obtain rootstocks with maximum resistance to Phytophthora root rot, choose rootstocks produced by a nursery using the clonal method because clones of recommended cultivars are more resistant than seedlings. Be aware that resistant rootstocks are not immune to root rot; if they are planted or maintained under adverse conditions, they may be killed by the combination of adverse conditions and the pathogen.

Prevent soil or water movement from infested areas

Excluding P. cinnamomi from an uninfested grove is the most economical control method.

• Install water-tight drains to divert surface runoff if a diseased area lies above a healthy grove.
• Control gophers, as their burrows can provide means of moving the pathogen in water.
• Do not work in infested groves when the soil surface is wet; Phytophthora is readily spread by activities such as walking or driving on infested wet soil.
• Bring only clean bins and equipment into groves.
• Begin harvesting and other activities in healthy areas of the grove; work in diseased areas last to minimize pathogen movement.

Soil solarization

Soil solarization can be effective for treating infested soil following tree removal in warm inland areas of California through a process in which radiant heat from the sun is trapped under clear polythene sheets laid on the surface of the soil. Solarization is effective when soil temperatures in the top 2 inches of soil reach between 108° to 131°F.

Establish a barrier

If Phytophthora root rot occurs in only one area of the grove and cannot spread downhill in surface runoff or drainage water, erect a physical barrier and post warning signs to prevent people and activities from spreading the fungus into protected areas. Establish the barrier around healthy sections of the grove, at least two tree rows beyond where tests indicate the fungus is present.

Irrigate carefully

Appropriate irrigation is the single most critical practice for improving tree health and managing root rot. Schedule irrigation frequency and amount using sophisticated methods, such as based on local evapotranspiration or by installing soil moisture monitoring devices, such as tensiometers. Good irrigation management is especially important where trees are diseased, near the margins of diseased areas of groves, and beneath thick mulch. It may be necessary to replace irrigation emitters around unhealthy trees by installing lower output sprinklers to avoid saturating the soil. Install valves for irrigation lines for infected portions of the grove because infected trees do not use water at the same rate as the healthy portion of the grove. Do not water soil that is already wet because it will become waterlogged and accelerate disease.

Use high-quality irrigation water

Irrigation water with high overall salinity or an excess of boron, chloride, or sodium promotes infection of roots by Phytophthora. Phytophthora can contaminate irrigation water, such as surface water that is runoff from infested soil. The extra cost of purchasing high quality water can often be justified by reduced disease and increased crop quality and yield.

Apply gypsum and mulch

Create soil conditions that suppress development of Phytophthora root rot.

• Apply, wood chip mulch and gypsum when the orchard is being established. Consider periodically applying additional mulch. Reapply gypsum as the old material dissolves from view.
• Apply gypsum under the canopy of each tree, perhaps 25 lb beneath a medium-size tree. Gypsum supplies calcium, which suppresses the formation of Phytophthora spores.
• Apply at least 4 to 6 inches of coarse wood chip mulch onto soil beneath canopies, but keep mulch several inches away from the trunk. Use coarse organic mulch such as avocado trimmings, composted greenwaste (yard trimmings), or hardwood chips which provides better Phytophthora control than naturally dropped leaves. Mulching promotes root growth into the mulch, enhances the development of beneficial microorganisms antagonistic to P. cinnamomi, and reduces the adverse effects of saline soil and water.

Provide appropriate nutrition

Moderate amounts of nitrogen promote good growth that helps avocado better tolerate root rot. Avoid excess amounts of fertilizer, especially avoid large amounts of animal manures or other products high in ammonia or salts. Avocado roots are sensitive to ammonia and salts.

Rotate crops

Replanting infested soil to resistant crops for at least several years reduces Phytophthora root rot propagules in soil. The fungus has a wide host range, but plants such as cherimoya, citrus, and persimmon are highly resistant to the Phytophthora sp. causing Phytophthora root rot in avocado.

Certain phosphonate fungistats (phosphorous acid and phosphonate compounds) can markedly improve trees' ability to tolerate, resist, or recover from infection by Phytophthora cinnamomi. Good control requires using materials in combination with other recommended practices, such as careful irrigation practices and applying wood chip mulch. Phosphonates cannot eradicate Phytophthora from the grove and Phytophthora root rot requires ongoing management throughout the life of the trees.

Fumigation is not recommended even if the the maximum rate of fumigant is applied. Often P. cinnamomi re-invades fumigated soil and the Phytophthora root rot becomes worse than before because the soil microbial community and competing microorganisms have been reduced by the fumigation.

Application methods

Varying with the product label, phosphonate (phosphite; FRAC Group 33) may be sprayed onto bark or foliage, injected into soil with irrigation water (chemigation), or injected into trunk vascular tissue. If permitted on the product label, proper trunk injection is generally the most effective application method when treating severely diseased trees. Proper application timing is critical. Phosphites can move both up and down within plants. To induce phosphites to move to roots, apply phosphites prior to initiation of new root growth. This effective application time is when about three-fourths of leaf flush is complete or just as new leaves harden, usually in late spring (May) and summer (August). Optimal application dates vary according to local conditions. If applied during early flush or when many new leaves are flushing, most of the phosphite will move to leaves and provide little Phytophthora control. If injected when new leaves are hardening, phosphites will move upward in the xylem stream, then move downward in the phloem where they can encourage healthy new root growth.

Inject trunks using proper equipment, such as spring powered or gas powered (CO₂) injectors. Drill relatively small diameter holes to the depth of the drill bit, at a slightly downward and sidewise angle so that more of the phosphonate material is deposited in the outer wood. Larger holes do not heal properly and continuous weeping and bacterial infection in the holes often occurs. Drill holes into smooth sections of the trunk or main limbs, avoiding knots and side branches. Where feasible, locate holes above any trunk area that is wetted by mini-sprinklers to facilitate injection wound closure.

Application (spraying) directly onto bark is usually not effective for managing Phytophthora root rot. Bark application may be more effective in managing the trunk canker fungus Phytophthora mengei. Application through the irrigation system is more effective in slowing down the spread of Phytophthora root rot disease than it is in controlling disease in already infected trees.

• Photos of disease signs and symptoms