Some branches on my pyracantha have areas which are rough and swollen. These areas are covered with a white, cottony substance. Is this a pest or a disease and what can I do to control it?

The symptoms you describe are probably caused by a pest known as the woolly apple aphid.  The insect feeds on the leaves, stems, and roots of pyracantha, apple, cotoneaster, and hawthorn. It removes sap from plants and secretes a toxin which causes woody, gall-like swellings on branches and roots. The aphid secretes honeydew, a clear, sticky liquid that collects on plant parts and promotes the growth of sooty mold. This blackens foliage but does not harm the plant.

The woolly apple aphid is a small, reddish insect less than 1/8 inch long.  The insect’s body is covered with white cottony material and it moves slowly.  Aphids spend winter on roots and branches and produce little visible cottony material at this time. Their activity increases in spring.  During summer and fall successive generations migrate simultaneously from roots to branches and vice versa.

Certain tiny parasitic wasps that occur in landscapes are important natural enemies.  Plants should not be sprayed with insecticides that leave a toxic residue on foliage since this will harm the beneficial wasps and allow aphid populations to increase.  Dormant oil sprays can be applied to kill aphid colonies on branches. Aphids are not easily washed off with soapy water solution. Aphids feeding on roots are difficult to control.   A sticky barrier such as Tanglefoot can be applied on a protective collar around the trunk and branches to prevent aphid migration. 

For more information see UC Pest Note 7404 Aphids: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7404.html
 
and

UC Pest Note 74108 Sooty Mold: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74108.html

 

By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, October 2012

 

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