Persistent spring rains the last couple of seasons have brought an increase in late-spring and summer diseases, such as scab, rust and Alternaria leaf spot. These are associated with orchards with dense canopies in combination with micro-sprinkler irrigation.
Almond Board–funded research by UC plant pathologist Dr. Jim Adaskaveg reveals that delayed-dormant sprays of copper and oil reduce and delay the initial sporulation of scab from overwintering twig lesions during springtime.
Recent work demonstrates that a delayed-dormant application with a combination of chlorothalonil and oil is better than copper and oil. Efforts are under way to obtain a supplemental label for application of chlorothalonil at a delayed-dormant timing. Check with your crop protection professional for label status.
Dormant chlorothalonil with oil also provides activity against rust and shot hole inoculum. This may be important for growers who were unable to apply zinc sulfate last fall to hasten leaf drop and reduce inoculum production of the pathogens causing these diseases.
Delayed-dormant applications are the first step in an effective resistance management program in which scab resistance to the strobilurin (QoI) fungicides (FRAC group 11) has emerged. Orchards with scab history should be treated in-season with a rotation of either single-site fungicides, pre-mixtures of different mode-of-action fungicide groups, or with multi-site fungicides such as chlorothalonil (FRAC group M5).