Symptoms and Signs
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.
Comments on 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 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.
|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.|
|(Fosphite)||1–3 qt/100 gal water||4||0|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33)|
|COMMENTS: Most effective as a foliar spray; apply at 2- to 4-week intervals after trees become established. Early fall treatments, before leaves begin to senesce, and trees that are actively translocating from leaves to roots, are most effective. If treatments are applied earlier in season, best applied after foliage has fully emerged. Do not apply more than six times per crop cycle or year. Do not apply with copper-based fungicides or fertilizers. Do not apply to trees that are heat or moisture stressed. Thought to provide protection by direct toxicity, blocking the phosphorus starvation response, or by systemic acquired resistance (increasing ability of the plant to produce its own defense chemicals).|
|(Ridomil Gold SL)||See comments||48||0|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4)|
|COMMENTS: Application rate varies with method of application and size of tree. Applied when trees are actively transpiring, up to three times per season (e.g., March, June or July, late August or September). Do not apply to trees within 90 days of planting. The effectiveness of this pesticide may not warrant its use and expense.|
|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.|
|‡||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 these two intervals is the minimum time that must elapse before harvest may occur.|