What to do for bad infestation of brown/black scale on Purple Sandcherry?
Scales are small (less than 1/8 inch long), and with close examination you can see the white, longitudinal ridges of the males and the dark oystershell shaped females. The scales spend almost all of their life feeding on the same spot while being protected by their hard covering. A scale cover will remain on the plant even after the insect has matured or died.
If a scale problem is found early, minor scale can be manually removed with a soft brush or toothbrush. By rubbing the scales off of the plant, their mouthparts are injured and they are unable to re-infest the plant.If your problem is further along, you can try suffocating the scale with horticultural oil, available at any nursery or plant store. Ideally, an oil spray should be applied during the dormant season (winter to early spring) to kill overwintering scales. Repeated applications may be necessary with heavy infestations and should be targeted when the crawlers are most active. It is important to note that repeated spraying with the oil can burn the leaves. make sure to follow directions on the package.
Good sanitation in the fall is indeed important to keep insect populations under control.
Next year, in early spring, before the leaves bud, horticultural oil can be used again. By following the same directions, and being aware that the day and night temperatures must be above freezing, and no rain or high winds are in the forecast, spraying the overwintering nymphs will smother them. As there are no leaves yet, the risk of burn is minimal.
Finally, if the infestation is large or the tree is too big that you cannot do the job yourself, it may be the time to call in the professional or remove the diseased tree all together. Do not put diseased branches and leaves in your compost instead bag up the plant material and place it in bags for collection by the City. Garden waste collected by the City are subjected to higher temperatures in the composting process, than homeowners can ever manage, which should destroy the scale and their nymphs.