Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties

Timing may not be everything, but it’s really important in disease control in almonds. Fungicides work best when applied ahead of infection. Post infection fungicide spray results usually range from so-so to no good.

Dr. Jim Adaskaveg (UC Riverside) has identified critical spray timings and control factors in foliar and nut diseases of almonds. Here’s a quick review of his findings.

Disease is triggered when all the following conditions occur in the orchard 1) the disease pathogen is present, 2) vulnerable plant tissue is present, and 3) the environmental conditions for infection are “right”. When one factor is missing, infection doesn’t occur. Knowing the status of these three factors in each orchard is key to cost effective disease management. Unneeded spraying wastes money. Early defoliation due to an uncontrolled disease outbreak could be really costly in lack of flowers (and yield) next year.

Vulnerable tissue:  Blossom brown rot and jacket rot infect flower tissue. Since bloom is the critical time of the year in orchards, Dr. Adaskaveg recommends at least one bloom spray against brown rot. The pathogen risk is difficult to know with any certainty (there is no way to test spore load in an orchard) and the stakes are sky high. Dew can last long enough to cause major infections in susceptible varieties. Selecting a fungicide with jacket rot activity is recommended when spraying for brown rot at full bloom.

Presence of the pathogen: In general, rust, shothole, and/or scab can be controlled with a fungicide spray after the earliest signs of the disease appear in the orchard. For example, a fungicide spray is recommended if rain is forecast and one leaf in 100 in an orchard has a rust spot, or scab and/or shothole spores are seen on twig lesions (scab) or in leaf lesions (shot hole). In orchards with a history of scab and/or rust, preventative sprays at 2 and 5 weeks after petal fall (scab) or 5 and 10 weeks after petal fall (rust) maybe needed for the best results.

Environmental conditions: The risk of some diseases increases substantially when certain weather conditions occur. If spring temperatures are warm (above 63^oF) and leaves are wet for extended time, anthracnose infections can occur, producing leaf and nut loss as the season progresses. If bloom weather is warm, anthracnose infection is also a risk then. Alternaria infections cause significant leaf loss and reduce return bloom if defoliation occurs before or during harvest. Alternaria infections commonly occur when high orchard relative humidity results in long hours of leaf wetness overnight. Warmer temperatures (average temp more than 60^oF) increase disease risk. Models predicting anthracnose and Alternaria risk have been developed and are available for growers and PCAs.

Disease control: Critical spray timing is important. Additional important factors in effective disease control include proper fungicide selection (efficacy, resistance management, etc.), and careful application (slow ground speed, complete spray coverage of the trees, every row spraying). Also, because multiple diseases can occur at the same time, making sure that the fungicide(s) in the tank work on all potential diseases is important. Dr. Adaskaveg and colleagues have a great, free, publication on disease management in deciduous tree crops. Click HERE to view/download it.

In conclusion. several steps must all be done correctly for fungicide sprays to be most effective. Missing one step can severely limit the outcome (and cost the grower a lot of money). That is especially true in a wet spring like this year (2024). Working together, growers and PCAs can keep almond orchards healthy and productive.