Symptoms and Signs
Rust appears as small, yellow spots on the upper surface of leaves. On the lower surface of the leaf these spots take on a rusty red appearance when the rust-colored spores produced in the lesions erupt through the surface. These spores are spread by air movement and infect other leaves to continue the disease cycle. Young twigs may be infected, but twig lesions are seldom seen on almond.
Comments on the Disease
Rust occurs sporadically throughout almond-growing areas in California. The development of rust is favored by humid conditions (e.g., orchards near rivers or streams), and the disease becomes worse when rain occurs in late spring and summer. Excessive levels of nitrogen are also known to increase the tree's susceptibility.
Trees can be defoliated quickly when rust becomes severe. The disease causes leaves to fall prematurely and will weaken trees, reducing the following year's bloom if not controlled. Rust is often observed in second- and third-leaf nonbearing orchards where fungicides have not been applied.
The rust fungus survives from one season to the next in infected leaves and possibly also in infected twigs.
Management
In orchards with a history of rust, apply sulfur 5 weeks after petal fall and follow 4 to 5 weeks later in late spring and summer with a quinone outside inhibitor fungicide (QoI; FRAC group number 11) or demethylation inhibitor (DMI; FRAC group number 3) to control leaf infections. Two or three applications may be needed in orchards that have had severe rust problems. To be effective, fungicide must be applied before rust symptoms are visible.
When zinc sulfate (20–40 lb/acre) is applied in late October to early November to hasten leaf fall, rust inoculum is prevented from increasing. Otherwise, the inoculum may build up, overwinter on the trees, and infect leaves the following spring. In southern growing regions, leaves of some cultivars such as Sonora may remain attached over the winter and provide inoculum for new infections as leaves emerge the following spring.
| 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. | METCONAZOLE | |||
| (Quash) | 3.5 oz | 12 | 25 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3) | ||||
| COMMENTS: Do not make more than two successive applications, and no more than four per season, to limit the development of resistance. | ||||
| B. | TEBUCONAZOLE | |||
| (Toledo, Tebucon) | 4–8 oz | 12 | 35 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3). | ||||
| COMMENTS: Do not make more than two successive applications and no more than four applications per season to limit the development of resistance. | ||||
| C. | AZOXYSTROBIN/PROPICONAZOLE | |||
| (Quilt Xcel) | 17.5-26.0 fl oz | 12 | 60 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and demethylation (sterol) inhibitor (3) | ||||
| COMMENTS: Do not make more than two successive applications, and no more than four per season, to limit the development of resistance. | ||||
| D. | PROPICONAZOLE | |||
| (Tilt, Bumper) | 8 fl oz | 12 | 60 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation (sterol) inhibitor (3) | ||||
| COMMENTS: Do not make more than two successive applications and no more than four per season to limit the development of resistance. | ||||
| E. | PYRACLOSTROBIN/FLUXAPYROXAD | |||
| (Merivon) | 6.5 fl oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): quinone outside inhibitor (11) and succinate dehydrogenase inhibitor (7) | ||||
| COMMENTS: Do not make more than three applications per season of QoIs (strobilurins) or SDHIs to limit the potential for the development of resistance. | ||||
| F. | PYRACLOSTROBIN/BOSCALID | |||
| (Pristine) | 10.5–14.5 oz | 12 | 25 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and succinate dehydrogenase inhibitor (7) | ||||
| COMMENTS: Chemical class: carboxyanilide/strobilurin. Do not make more than four applications per season of QoIs or SDHIs and no more than two sequential applications before rotation to a different mode of action fungicide to limit the potential for the development of resistance. | ||||
| G. | AZOXYSTROBIN | |||
| (Abound) | 12.0–15.5 oz | 4 | 28 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: Do not apply more than two sequential sprays before alternating with a fungicide that has a different mode of action. Do not apply more than four applications of strobilurin fungicides per year or apply more than 92.3 fl oz/acre per season. | ||||
| H. | AZOXYSTROBIN/DIFENOCONAZOLE | |||
| (Quadris Top) | 14 oz | 12 | 28 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and demethylation (sterol) inhibitor (3) | ||||
| COMMENTS: Do not make more than four applications per season or more than two sequential sprays of QoIs or SDHIs to limit the potential for the development of resistance. | ||||
| I. | FLUOPYRAM/TRIFLOXYSTROBIN | |||
| (Luna Sensation) | 5.0-7.6 fl oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): succinate dehydrogenase inhibitor (7), Quinone outside inhibitor (11) | ||||
| COMMENTS: Do not make more than two successive applications, and no more than four per season, to limit the development of resistance. | ||||
| J. | TRIFLOXYSTROBIN | |||
| (Gem 500SC) | 3.8 oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: Chemical class: strobilurin. Do not apply within 14 days of harvest or after hullsplit. Do not make more than four applications per season and no more than two sequential sprays of QoI or SDHI fungicides to limit the potential for the development of resistance. | ||||
| K. | WETTABLE SULFUR# | |||
| Label rates | See label | 0 | ||
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | ||||
| COMMENTS: To be effective, sulfur treatments must be applied before rust symptoms appear, which can be anytime from late spring through fall. Do not apply within 3 weeks of an oil application. Check with your certifier to determine which products are organically acceptable. | ||||
| L | SULFUR DUST# | |||
| Label rates | See label | 0 | ||
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | ||||
| COMMENTS: To be effective, sulfur treatments must be applied before rust symptoms appear, which can be anytime from late spring through fall. Do not apply within 3 weeks of an oil application. Check with your certifier to determine which products are organically acceptable. | ||||
| ‡ | 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. |
| # | 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 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. |