San Jose scale is a relatively easy pest to control, but a dangerous one to leave uncontrolled. After a few years of infestation, limbs and even entire trees can be killed if heavily attacked, and high percentages of the fruit can be infested. Large trees are most often associated with scale problems, because of the suitable habitat they provide and the difficulty of obtaining good spray coverage. However, young trees can also develop a scale problem surprisingly quickly. San Jose scale is a pest that is easily prevented, but hard to control if the populations have built up too much.
Typically, oil sprays have been applied to control the scales in the delayed dormant period. If the population was high the year before, you can also target the first crawler generation. Oil gives good control of scale crawlers and combined with biological control it can provide good control if spray coverage is good. Oil applied for codling moth or mites does control scale if used at a time when scale crawlers are present. In older trees in low density orchards, populations can build up in the top of the tree when coverage is poor, and escape notice until they are extremely high and have spread to other areas of the tree.
Sprays in the second crawler generation have not normally been recommended, mostly because if the crawlers move to the fruit, they will cause cosmetic damage (they cause red rings around the mature scales) or misshapen fruit. In addition, sprays in the past might have been timed incorrectly because the phenology models in the second crawler generation were incorrect. The new model on WSU-DAS corrects the second-generation curve, and so sprays in the second generation are possible, but because of the threat of fruit damage, should be considered only for rescue treatments, not for the normal spray program.
This timing is fundamental to long-term prevention, and trying to get ahead of an existing problem. The optimal timing for using conventional pesticides the first generation is 730 DD if you are applying only a single spray. If your population is very high, apply a pesticide at 630 DD and re-apply after the residual is over (see retreatment intervals on the label). If using oil, two applications of oil with the first one applied at 885 DD and the second spray 220 DD later gives a very good suppression of the population, particularly if a treatment in delayed dormant was applied. As mentioned above, oil (at 1%) applied for codling moth or mites will also provide some suppression and should be taken into account in your SJS program. Check WSU-DAS for materials that may be used in addition to oil for best SJS control.
San Jose scale on apple twig (E. Beers)
Prepared by Matthew Jones, Ute Chambers, and Vince Jones , May 2019