Almond leaf rust (Tranzschelia discolor f. sp. dulcis)can cause defoliation of almond trees during the late season. Rust is favored by high humidity and is worse in years in which late rains occur. Appearing as small yellow lesions on the upper surface of leaves with brownish/red pustules on the bottom (Pictures below), rust will first appear in late spring or early summer. The disease does not appear to affect the fruit. Spread through the orchard is through air movement, which blows spores from an infected leaf to an uninfected leaf. Infected leaves will eventually fall off of the tree affecting crop and tree health. The disease overwinters on leaf material.
Prevention: Orchards favoring high humidity often have rust problems. Encouraging air movement by planting on wider spacings (22′ between rows), hedging, or selective pruning may help reduce canopy humidity. Microsprinklers and solid set sprinklers may increase canopy humidity since evaporation of sprayed water may occur. Sanitation (leaf mowing, breakdown) should be employed to reduce overwintering inoculum.
Treatment: In orchards that have a history of rust, a two spray fungicide program should be used to reduce disease and clean up the orchard. The first application should be applied 5 weeks after petal fall and followed up with a 2nd application at 10 weeks post petal fall. DMI (FRAC Group 3) or strobilurins (FRAC Group 11) provide good to excellent control. Broad spectrum fungicides such as sulfur and topsin provide a good, cheap control and also allow an option for fungicide rotation. More information on fungicide efficacy can be found here.
Applications of zinc sulfate (20-40 lbs/acre) applied in late October/early November should be made to help reduce overwintering populations of rust. The zinc will hasten leaf fall, and prevent the rust inoculum from increasing. In orchards of severe infestation, applying a low rate of nitrogen to the surface leaf debris will help speed up the natural degradation of the leaves. A similar treatment is used to reduce overwintering populations of apple scab in Michigan.