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Market Diseases of Apples, Pears, and Quinces
Scale Insect Injuries |
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Aspidiotus spp.
San Jose scale (Aspidiotus perniciosus Comstock) is the most important of the scale insect injuries discussed. San Jose scale may occur wherever apples are grown. East of the Rocky Mountains two other scale insects, Forbes scale (Aspidiotus forbesi Johnson) and Putnam scale (Aspidiotus ancylus Putnam), cause injuries on apples identical to those caused by San Jose scale. Although Putnam scale has a wide host range, it causes little damage on apples. Forbes scale is quite common on poorly sprayed apples in the eastern United States, but the damage is not as widespread or as injurious as that from the San Jose scale. Years ago scale insects were serious orchard pests, but presently they are not a problem in properly sprayed orchards. San Jose scale and, in the East, Forbes scale reappear and seriously blemish apples in orchards if oil sprays are not used each year. Typical scale injury appears on apple fruits as small red spots about 1/8 inch in diameter. At the center of each of these spots is a light-colored area (top photo), marking the place occupied by the tiny insect before it was rubbed off in the handling of the fruits during or after harvest. In depressed areas, such as the calyx or stem ends of the fruits, the scales may still be attached. The adult scale is less than 1/16 inch in diameter, gray to grayish brown with a small dirty yellow nipple-like center surrounded by a depressed ring. Occasionally a small black scale, the stage that lives through the winter, is found. An unusual symptom of scale injury was observed in Maryland on Grimes Golden apples. Instead of red spots, the affected areas appeared bleached (second photo). In some instances the scale insect was present and was identified by an entomologist as San Jose scale. The three species of scale insects have certain distinguishing characteristics, but they can be identified only by a specialist. Red spots on apples may also be caused by early stages of fungus infection, green aphis injury, spray injury, or other causes, but none are typical of scale insect injury and they should be readily distinguishable. For control of scale insects consult the State agricultural extension service or the experiment station. |
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