Forest Pests: Insects, Diseases & Other Damage Agents

Gray Mold

Michael D. Srago - Regeneration Specialist, Region 5, USDA Forest Service, San Francisco, CA, and
Arthur H. McCain - Plant Pathologist, Department of Plant Pathology, University of California, Berkley, CA.

Cordell C.E., Anderson R.L., Hoffard W.H., Landis T.D., Smith R.S. Jr., Toko H.V., 1989. Forest Nursery Pests. USDA Forest Service, Agriculture Handbook No. 680, 184 pp.

Hosts

Gray mold, caused by the fungus Botrytis cinerea (sexual state: Botryotinia fuckeliana), has an extremely large number of hosts. In fact, nearly all forest tree seedlings are susceptible to some degree; but redwood, giant sequoia, and Monterey and Italian cypress are especially susceptible.

Distribution

Gray mold is found in all the temperate regions of the world.

Damage

Gray mold occurs sporadically in nurseries and greenhouses. In less susceptible species, losses are often confined to pockets of seedlings, but the disease can cause heavy mortality in highly susceptible species such as giant sequoia.

The disease also can originate or continue in storage facilities and transit, especially if temperatures are above 32 ^oF.

Diagnosis The fungus commonly infects young branches, particularly the lower shaded branches. Once established on a branch, it moves downward into the stem, killing the tissues it infects. In succulent 1-year-old stem tissue, look for black sunken cankers, usually low on the stem, which eventually girdle the stem (fig. 9-1). The portion of seedling above the girdled area dies (fig. 9-2). During periods of high humidity, the fungus can be seen as a thin, gray web of mycelium on infected plant parts (fig. 9-3). Tufts of dark conidiophores grow from this mycelium and from infected tissue. The conidiophores produce clusters of spores. If they are not present on infected tissues, the spores will develop when the seedling is placed in a moist chamber. Spores are dark, ellipsoid to oval, and 6-18 x 4-11 microns in size. The fungus also produces small, black sclerotia, which provide a means by which the fungus can overwinter. The sexual state of the fungus, B. fuckeliana, is also produced on the sclerotia; however, it is rarely seen in nurseries. Biology Airborne spores are the primary means by which B. cinerea spreads. Botrytis can grow within a range of temperatures from about 28 to 90 oF. However, it grows very slowly at the extremes and does best at temperatures from 70 to 77 oF. It is more active below 70 oF than it is above 77 oF. The fungus will survive if seedlings are refrigerated at temperatures near 32 oF. Refrigeration at this temperature will retard, but not completely stop, the development of the fungus.	Figure 9-1. - Stem canker (arrow) on redwood seedling resulting from branch infection by B. cinerea.
	Figure 9-2. - Tops of redwood seedlings killed by B. cinerea.
	Figure 9-3. - Web of mycelium of B. cinerea covering infected redwood seedlings (left).

Moisture is often a more limiting factor in disease development than temperature: the spores of the fungus require free water to germinate.

The disease is favored by cultural practices, such as high planting densities or lath shading, that limit air movement and raise the humidity around the seedling.

Control

Cultural - Modify the environment to increase aeration and decrease humidity around the seedlings; this will reduce spore germination and growth of the fungus. Beneficial cultural practices include reducing seedling density, improving air circulation, and irrigating less frequently or in the early morning.

Chemical - Numerous fungicides- including benomyl, chlorothalonil, and DCNA or dicloran-are effective. However, Botrytis has developed strains resistant to some fungicides, especially the systemic benzimidazole fungicides such as benomyl. Use several different fungicides, preferably with different modes of action, in rotation.

Standard spray programs for Botrytis in California and the Pacific Northwest involve foliar fungicides applied at 1- to 2-week intervals in greenhouses and at 2-to 4-week intervals in bareroot nurseries.

Selected References

Coley. JR.; Verhoeff, D.; Jarvis, W.R. 1980. The biology of Botrytis. London, England: Academic Press. 318 p.

Ellis, M.B. 1971. Dematiacious hyphomycetes. Kew, England: Commonwealth Mycological Institute. 608 p.

Halber, Max. 1963. Botryris sp. on Douglas. fir seedlings. Plant Disease Reporter. 47: 556.

Smith, Richard S.; McCain, A.H.; Srago. M.D. 1973. Control of Botrytis storage rot of giant sequoia seedlings. Plant Disease Reporter. 57: 67-69.