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