David Doll0 commentsSeptember 14, 2016Diseases

Defoliating for Rust Management…Good Idea?

Written by David Doll (UCCE Merced County) and Brent Holtz (UCCE San Joaquin County) The foliar disease rust caused by the fungus Tranzschelia discolor has been a major problem this year. This disease is characterized by rusty-brown- or black-colored spores that form on the lower side and angular chlorotic (yellow) lesions on the upper side of the leaf. Rust can spread easily from orchard to orchard with wind, and minimal periods of leaf wetness are required for the pathogen to infect. This past year, long periods of conducive conditions, which include warmer temperatures, late spring rains, and increased canopy humidity were experienced, making rust management a challenge. Rust is easily controlled by properly timed fungicides. Several modes of action provide protection. Those with the highest activity are FRAC Groups 3, 11, and 19 or mixtures of these groups. Also several broad-spectrum fungicides such as micronized sulfur (M2, microthiol) and chlorothalonil (M5, Bravo) have very good efficacy with short- and long-residual activity, respectively. Since rust can infect from spring to summer, sprays need to be timed accordingly. Spring sprays (2-5 weeks after petal fall) based on monitoring for rust symptoms is often used to initiate rust control practices at the beginning of a potential epidemic. One fungicide application, however, may not provide effective control if favorable conditions persist and later sprays may be needed. Typically, spring-time disease management programs for scab and Alternaria leaf spot will also provide management for rust. High populations of rust can prematurely defoliate trees. As the fungus spreads, it ruptures leaf tissues, reducing the photosynthetic potential of the leaf, and eventually causes leaves to fall. If too many leaves fall from the tree, the tree will re-leaf, reducing the cropping potential for next year. Although this re-leafing is concerning, it is better for the tree to re-leaf

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David Doll0 commentsOctober 12, 2010Diseases

Tree Defoliation due to Rust or Scab

Due to the wet year, foliar diseases have been problematic. In many cases, growers have observed late season defoliation due to rust (Tranzschelia discolor f. sp. dulcis) and scab (Cladosporium carpophilum). These two foliar fungi initially infect the tree in mid to late spring, reproducing within the canopy of the orchard, killing leaves by late summer/early fall. The fungi must be controlled before the infection occurs to prevent late season defoliation. To help prevent an increase of rust and scab for the coming growing season, the following practices are recommended: Rust: Rust symptoms on almond leaves. Note the brown spores on the underside of the leaf. Orchards that have defoliation due to rust should have zinc sulfate (20-40 lbs/acre) applied in late October/early November. This practice will hasten leaf fall. Once the leaves are in contact with the soil, the leaves will begin to break down, reducing the overwintering inoculum. The rust fungus is not a soil inhabiting fungi and will not survive without leaf tissue. It is also important to remove any leaves from the crotch of the tree. These leaves serve as an overwintering reservoir and infect leaves the following spring. Fungicide sprays should be considered to help control another outbreak the following year. Orchards with a history of rust should apply sulfur or maneb 5 weeks after petal fall and follow 4 to 5 weeks later in late spring and summer with a FRAC Group number 11 fungicide (strobilurin) to control leaf infections. To be effective, fungicides must be applied before rust symptoms are visible. Scab: Scab lesions on almond twigs. These lesions are fruiting bodies that can re-infect the tree the following spring. Orchards experiencing defoliation due to scab should have a dormant application of either copper/oil or liquid-lime sulfur. These applications will help reduce and/or delay the

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David Doll0 commentsApril 5, 2010Diseases

Almond Leaf Rust – Treat now to prevent late season defoliation

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

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