Symptoms and Signs
The most common symptoms of rust are bright yellow, angular spots on leaves, with red-brown spore masses on lower sides of leaf, and premature defoliation.
Comments on the Disease
The rust pathogen depends on moisture for infection. Twig lesions do not occur on prune trees.
Management
If rust is a problem in an orchard, evaluate the orchard to develop a treatment program. Treating at bloom won't be effective because it is too far in advance of leaf symptoms.
Begin monitoring orchards by May 1; in the San Joaquin Valley monitor at least every other week and in the Sacramento Valley monitor every week until July 15. Examine 40 trees randomly each time. Examine each tree for the presence of leaves with prune rust symptoms; be sure to look at low hanging branches. In addition, be sure to monitor nonbearing replants in the orchard, trees with vigorous growth, and known rust hot spots. If there are no trees with rust present, continue to monitor weekly or every other week.
Treat at the first sign of rust in the orchard. Additional treatments may be necessary, especially if a treatment was required early in the season. After a treatment is applied, continue to monitor weekly or twice monthly. If the number of trees with rust increases from the last reading, a second treatment is recommended if there is significant time remaining to harvest. No additional treatments or monitoring are necessary within 4 weeks of harvest.
| 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. | AZOXYSTROBIN | |||
| (Abound) | 12.3–15.4 fl oz | 4 | 0 | |
| MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: A strobilurin fungicide. Do not apply more than three sequential applications for rust control before alternating with a fungicide that has a different mode of action. | ||||
| Note: Minimize the use of sulfur because it kills predatory mites, leading to an increase in mite populations. | ||||
| B. | WETTABLE SULFUR# | 20 lb | 24 | 0 |
| MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | ||||
| COMMENTS: Controls rust for only 2 weeks. | ||||
| C. | SULFUR DUST# | 50 lb | 24 | 0 |
| MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | ||||
| COMMENTS: Controls rust for only 2 weeks. | ||||
| # | Acceptable for use on organically grown produce. |
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with different group numbers are suitable to alternate in a resistance management program. In California, make no more than one application of a fungicide with a mode-of-action group number associated with high resistance risk before rotating to a fungicide with a different mode-of-action group number; for other fungicides, 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 two intervals is the minimum time that must elapse before harvest. |