Sooty molds of trees and shrubs commonly occur throughout Virginia on conifers, hardwoods, and other plant species. These fungi are named for the black spots or crusts of growth they form on foliage or stems. Sooty molds rarely cause problems in the forest, but in some cases they can interfere with proper growth of Christmas trees, nursery stock, or landscape plants. In most cases, however, the unsightly, black fungal growth is mainly a cosmetic problem.
Although a few sooty mold fungi grow on plant substances exuded by the leaves, most grow on the excrement of certain sucking insects. The most common insects associated with sooty mold infestations are aphids, scales, mealybugs, and whiteflies. These insects feed on the sugar-rich contents of the phloem of plants and excrete a substance called "honeydew" that is very high in sugar. Sooty molds live entirely on the excretions of the insects, and do not penetrate leaf or bark tissues. No direct injury to the plant results from the presence of these fungi. However, sooty molds may be indirectly detrimental to plants by excluding sunlight and interfering with photosynthesis in the leaves.
The presence of a black, soot-like fungus, frequently appearing as a thin crust over the surface of leaves, is the best indicator of this problem (Fig. 1). However, some species of sooty molds grow as a thick, spongy mass that encases the needles of conifers or the twigs of deciduous trees (Figs. 2 and 3). Insect activity may or may not be apparent. Sooty molds may persist long after the insects themselves have disappeared. Although sooty molds do not directly infect plants, severely affected plants may be yellowed and suffer defoliation from the combined effects of insect feeding and the reduction in photosynthesis that results from blockage of sunlight by the fungus. Although sooty mold usually does not cause dieback or mortality, the insect feeding that preceded the sooty mold infestation may be severe enough to weaken or kill portions of infested plants. Pruning out dead and dying branches helps prevent infection by secondary pathogens.
Crawler treatment dates for common scale pests
|Pest||Crawler and Treatment Dates|
|azalea bark scale||June 5-30; treat June 10 and 20.|
|cottony camellia scale||June 1-10; treat June 10-20.|
|magnolia scale||Late August-late September; treat September 1-20.|
|pine tortoise scale||June 10-July 5; treat June 20-25.|
|tuliptree scale||Late August-late September; treat September 1-20.|
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Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences; Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
May 1, 2009