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 in 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.
Management
The most effective ways to manage Phytophthora root and crown rot are to select a good planting site, select an appropriate rootstock, and properly manage irrigation water. Avoid over-irrigating in spring and fall when soil temperatures are most conducive to disease development and water use by the tree is low. Planting on raised berms can also help with disease management.
Chemical Control
Fungicides are available to treat soil around newly planted trees. If there is a history of Phytophthora root rot in the orchards and problems are anticipated, treatments may be warranted.
| Common name | Amount to use | 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: NONBEARING TREES ONLY. Foliar spray, 60-day interval. Do not apply more than 5 lbs Aliette/acre per application. | ||||
| B. | PHOSPHORUS ACID | |||
| (Fosphite) | 1–3 qt/acre | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33) | ||||
| COMMENTS: Allow 10 days before applying a copper-based compound following an application of Fosphite. Allow 20 days before applying Fosphite following a treatment with a copper product. Do not apply with copper-based fungicides or fertilizers. | ||||
| C. | MEFENOXAM | |||
| (Ridomil Gold) | Varies with method of application and size of tree | 0 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4) | ||||
| COMMENTS: Applications made in early spring and fall. | ||||
| ‡ | 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 (for more information, see http://www.frac.info/). 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 a fungicide with a different mode of action Group number. |
| NA | Not applicable. |