Written by Gurreet Brar, UCCE Farm Advisor (Nut Crops), Fresno & Madera Counties
This summer we observed many cases of lower branch dieback in almond trees (like the one shown in picture). In many cases these symptoms were found to be associated with high scale populations. Diagnosis of such branches revealed that San Jose Scale and Walnut Scale caused lesions beneath the bark resulting in death of the tissue. These scales suck plant juices from the inner bark by inserting their mouthparts into twigs and injecting a toxin. When the populations become high enough, the numerous lesions may coalesce and cause the whole branch to die. Uncontrolled populations can kill branches within 1-3 years. San Jose Scale can be found on most, if not all, almond varieties. Walnut scale seems to like Monterey variety more than the others.
What it looks like? San Jose Scale is small (<1/16th” in diameter) and may have a white, black, or brown cap. Walnut Scale is grayish and is often found in daisy-shaped groups formed by scale coverings. When you lift off the covering from a female scale, the yellowish body is revealed underneath.
How to manage it? Normally, natural enemies keep the scale populations below economic damage threshold. However, a surge in scale population is usually associated with disruption in their biological control agents due to a shift in pesticide use trends. There can be up to 3-4 generations per year. Delayed dormant oil sprays can suppress scale populations and are less disruptive to natural enemies. Spring time applications of growth regulators are also effective while late summer to fall applications are not as effective.
Dormant spur sampling is important. A good management practice is to scout for the scale by doing dormant season spur sampling. If the dormant sampling results warrant a treatment, you can treat in the delayed dormant period according to UC IPM guidelines. During spur sampling, watch for any evidence of parasitization. Parasitized scales will have characteristic emergence holes in the body of dead mature scales. If a high degree of parasitization is observed, you may not need to treat during dormant period, as the parasites will keep the populations under check. In this case, delay the treatment until late spring when the crawlers emerge.
More information on Scale can be found at the UCIPM website. It has up-to-date treatments, describes scale monitoring during dormant and spring, and provides treatment thresholds.