Phytophthora Root and Crown Rot

Symptom expression of Phytophthora root and crown rot depends upon how much of the root or crown tissues are affected and how quickly they are destroyed. Generally, crown rots advance rapidly and trees collapse and die soon after the first warm weather of spring. Chronic infections, usually of the roots, cause reduction in growth and early senescence and leaf fall. These trees may have decreased yield and vigor for several years before succumbing to the disease.

Periods of 18 to 24 hours or more of water-saturated soil favor Phytophthora infections. Conversely, good soil water drainage reduces the risk of root and crown rot. Rootstocks vary in susceptibility to the different Phytophthora species; none are resistant to all pathogenic species of Phytophthora. Thus, the success of a rootstock may depend in part upon the species of Phytophthora present in the orchard. In general, Paradox rootstock is more tolerant of several Phytophthora species than is Northern California black walnut or English walnut.

The most effective management strategies for Phytophthora on walnut include careful management of soil water, prudent rootstock selection, and good general sanitation practices.

• Plant on berms.
• Avoid soil compaction.
• Do not allow irrigation water to run or stand for more than 24 hours; limiting run times to under 18 hours will limit spore production. Pulse irrigation at short durations is best.
• If using sprinklers, do not allow water to splash on trees (use water guards).
• Use practices that promote good water infiltration and penetration.

Rootstocks differ (PDF) in their ability to tolerate different Phytophthora species.

• Use clonal Paradox RX1 rootstock, which has high resistance to P. cinnamomi and moderate to high resistance to P. citricola, in soils infested with Phytophthora or conditions favorable to Phytophthora development (heavy soil or restricted root zone).
• Seedling Paradox has variable resistance to some Phytophthora species.
• Northern California Black rootstock and English rootstock are both susceptible to Phytophthora.

Eradicating Phytophthora from orchard soil is generally not possible. Fumigating the soil after you have removed a diseased tree typically results in a beneficial, though incomplete, reduction in the population of Phytophthora in the soil. The population can rebuild quickly under conditions conducive to disease.

Phosphonate fungicides (FRAC Group 33) such as phosphorous acid (KPO₃) may be used to manage the disease in an integrated management program. The exact mode of action is not known, but direct toxicity to the pathogen occurs as inhibition of mycelial growth and sporulation. Applications are typically made one to two times a growing season, such as in spring and early fall as preventative treatments. Additional applications may be needed as a curative treatment. The fungicide is systemic following phosphate (PO₄) pathways in the plant.

• Phosphite (PO₃) residues persist for months after treatment. Because the fungicide is systemic, residues can be found in the harvested crop.
• In the United States the fungicide is a biofungicide and as such is exempt from tolerance. However, some export countries may have maximum residue limits (MRLs) that prevent marketing crops from treated orchards that exceed tolerances.
• Resistance has been detected in some Phytophthora spp. on other crops but not in walnut. Rotation with other fungicides is recommended.

Mefenoxam (FRAC Group 4) is registered for walnut and can be applied as a broadcast, band, or irrigation application to soil around the tree to cover the root zone. Apply 90 days after planting or in the spring before root growth in established orchards. Additional applications may be made in 2- to 3-month intervals but no more than three applications per year. Use before symptoms develop. Applications made to trees with moderate to severe symptoms may not be effective. Resistance to mefenoxam has been reported in some Phytophthora spp.