Symptoms and Signs
Symptom expression 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. Leaves of such trees wilt, dry, and remain attached to the tree. Chronic infections, usually of the roots, cause reduction in growth and early senescence and leaf fall. These trees may be unthrifty for several years before succumbing to the disease. Phytophthora infections typically kill young trees because their root systems and crown areas are small compared to those of mature trees.
Comments on the Disease
Periods of 24 hours or more of saturated soil favor Phytophthora infections. Conversely, good soil drainage and more frequent but shorter irrigations reduce the risk of root and crown rot. Rootstocks vary in susceptibility to the different Phytophthora species; none are resistant to all pathogenic species of the fungus. Thus, the success of a rootstock may depend in part upon the species of Phytophthora present in the orchard. Mazzard and Colt rootstocks are more resistant than is Mahaleb.
Management
Avoid locations with a history of Phytophthora root and crown rot, especially when planting susceptible rootstocks. Plant new orchards on berms to improve drainage at the crown area and design the irrigation system so that the trunk and crown of the tree is not wet by sprinklers. You can use various methods to reduce the soil population of the pathogen, but you cannot eliminate it. Be sure to verify that Phytophthora is the causal agent before treating a new planting with fungicides, because a number of factors or pest problems can cause poor growth and death of trees.
| 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. | FOSETYL-AL | |||
| (Aliette WDG) | 5 lb/100 gal | 12 | NA | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33) | ||||
| COMMENTS: For use on nonbearing trees only. Apply as a foliar spray, at 60-day intervals. | ||||
| B. | MEFENOXAM | |||
| (Ridomil Gold SL) | Varies with method of application and size of tree | 48 | NA | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4) | ||||
| COMMENTS: Applications made in early spring and fall. Do not apply to trees within 45 days of planting. | ||||
| C. | PHOSPHOROUS ACID | |||
| (Fosphite) | See label | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33) | ||||
| COMMENTS: For use as a foliar or soil treatment. | ||||
| ‡ | 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. |
| NA | Not applicable. |