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 thorough spray coverage. However, young trees can also develop a scale problem surprisingly quickly.
San Jose scale is a pest that is better prevented than cured. Use of oil in the delayed dormant period is one of the best and least expensive ways to prevent this pest from becoming a problem. This timing is fundamental to long-term prevention, and trying to get ahead of an existing problem. The use of an organophosphate with the oil provides some additional control, but the oil is thought to do most of the work.
In addition to a delayed dormant program, scale may be controlled during either (or both) of the two crawler generations. Crawler control is more problematic, and while it helps prevent fruit infestation in the current season, it will not provide complete control. The long period of crawler movement (6 weeks each) makes it very difficult to cover the entire generation. Materials such as Esteem provide scale control when applied at delayed dormant; however, Esteem at petal fall for codling moth control will be too early for scale crawlers. Timing for first generation crawlers starts at 770-790 degree days. Timing for the second generation starts at 1975 degree days. In both generations, multiple sprays are needed to completely cover the generation, and not all materials allow that many applications per season. If timing of San Jose scale crawlers and codling moth egg hatch overlap, then using a codling moth larvicide with some scale suppression (Assail or an organophosphate) will help control crawlers post-bloom, in addition to Esteem. But if you have some level of fruit infestation in the current season, gear up for a full-scale assault (pun intended) next spring.(Elizabeth Beers, WSU-TFREC)