UC IPM: UC Management Guidelines for Rust on Almond

UC Pest Management Guidelines

Almond

Rust

Pathogen: Tranzchelia discolor

(Reviewed 3/09, updated 3/09)

In this Guideline:

Symptoms
Comments on the disease
Management

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.

Rust occurs sporadically throughout almond-growing areas in California. It is more likely to become serious in orchards near rivers or streams or other locations where humidity is relatively high in spring and summer. Excessive levels of nitrogen are also known to increase the tree's susceptibility. 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 development of rust is favored by humid conditions, and the disease becomes worse when rain occurs in late spring and summer. Trees can be defoliated quickly when rust becomes severe. The rust fungus survives from one season to the next in infected leaves and possibly also in infected twigs.

In orchards with a history of rust, apply sulfur or maneb 5 weeks after petal fall and follow 4 to 5 weeks later in late spring and summer with a QoI fungicide (FRAC Group number 11) 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.

Common name

Amount/Acre

R.E.I.+

P.H.I.+

(trade name)

(hours)

(days)

When choosing a pesticide, consider the general properties of the fungicide as well as information relating to fungicide resistance and environmental impact.

PROPICONAZOLE

(Orbit)

8 fl oz

MODE OF ACTION GROUP NAME (NUMBER¹): Demethylation inhibitor (3)

PYRACLOSTROBIN /BOSCALID

(Pristine)

10.5–14.5 oz

MODE OF ACTION GROUP NAME (NUMBER¹): Quinone outside inhibitor (11) and Carboxamide (7)

COMMENTS: Chemical class: carboxyanilide/strobilurin. See label for current preharvest interval. Do not make more than 4 applications per season of strobilurins or carboxyanilides to limit the potential for the development of resistance.

AZOXYSTROBIN

(Abound)

12.3–15.4 oz

MODE OF ACTION GROUP NAME (NUMBER¹): Quinone outside inhibitor (11)

COMMENTS: Do not apply more than 3 sequential sprays before alternating with a fungicide that has a different mode of action. Do not apply more than 4 applications of strobilurin fungicides/year or apply more than 2.88 qt/product/acre/season.

TRIFLOXYSTROBIN

(Gem)

3–4 oz

60 – see comments

MODE OF ACTION GROUP NAME (NUMBER¹): Quinone outside inhibitor (11)

COMMENTS: Chemical class: strobilurin. Do not apply within 60 days of harvest or after hull split. Do not exceed more than 3 applications of all strobilurins per season to limit the potential for the development of resistance. Do not apply more than 12 oz/acre/season.

MANEB

(various)

Label rates

145

MODE OF ACTION GROUP NAME (NUMBER¹): Multi-site contact (M3)

COMMENTS: Do not apply more than 32 lb of product/acre/season.

WETTABLE SULFUR#

Label rates

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.

SULFUR DUST#

50 lb/acre

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.

Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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.

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 fungicide with a different mode of action Group number.

UC IPM Pest Management Guidelines: Almond
UC ANR Publication 3431
Diseases
W. D. Gubler, Plant Pathology, UC Davis
J. E. Adaskaveg, Plant Pathology, UC Riverside
Roger Duncan, UC Cooperative Extension, Stanislaus County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center
B. A. Holtz, UC Cooperative Extension, Madera County
Acknowledgment for contributions to Diseases:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See How to manage pests, or in the U.S., contact your local Cooperative Extension office for assistance. /PMG/r3100711.html revised: March 24, 2009. Contact webmaster.

How to Manage Pests

UC Pest Management Guidelines

Almond

Rust

Pathogen: Tranzchelia discolor

SYMPTOMS

COMMENTS ON THE DISEASE

MANAGEMENT

PUBLICATION