Functional and Anatomical Disruption of Apple Roots by the Woolly Apple Aphid (Homoptera: Aphididae)

Root galls on apple trees (M. 7A rootstock) created by woolly apple aphid, Erlosoma lanigerum (Hausmann), feeding were collected in the field. Root galls, ungalled roots, and ungalled sections of galled roots were analyzed for water conductivity and nitrogen concentration. Water conductivity was significantly reduced through root gall tissue. Root galls had a significantly higher concentration of nitrogen than ungalled roots. Roots of apple trees in the green house were treated with the plant hormones indole-3-acetic acid and 6-benzyl-aminopurine to induce gall formation. Woolly apple aphid galls were characterized by a proliferation of anomalous nonfunctional xylem. Growth anomalies On roots treated with 6-benzyl-aminopurine had typical xylem with a proliferation of phloem tissue. Very little internal or external deformation of roots was observed after treatment with indole-3-acetic acid. Disruption of root xylem, resulting in resistance to water conduction, is one mechanism by which woolly apple aphids reduce the growth of apple trees.