Gray mould

This disease occurs in container nurseries throughout the province and provides a classic example of how new pest problems can arise with technological change - the change from bareroot seedling production, where the disease seldom occurs, to growing seedlings in containers. Earlier problems with the disease have decreased with improved cultural conditions. You can find the report available on exclusive-paper.com.

Hosts and damage

Botrytis cinerea is ubiquitous fungus with a wide host range; locally, it is very damaging to western hemlock, spruce, and Douglas-fir seedlings. Western redcedar is also attacked, often on the tips of the terminal shoot and lateral branches. Pines seem to be less affected than other species. Some reasons for this are: (i) that they are usually smaller and have a more upright growth habit, both of which limit within-canopy conditions favoring the disease, and (ii) that they usually grown in nurseries in interior British Columbia where the climate is drier.

Symptoms appear from late summer onward, until stock is shipped for outplanting. Initial symptoms include watery-moulding and killing of lower needles, or sometimes of the leader, branches, and stem. Tan or brown-watery lesions often develop on affected organs. As the disease progresses, the symptoms move upward and the disease can eventually kill the entire shoot. Frequently, webs or masses of gray-brown mycelium (Figure 30) and spores of the pathogen are present on dead tissue-thus the name "gray mould." The disease does not seem to harm roots. Besides affecting seedlings in the nursery, the disease can continue to develop if affected seedlings are cold-stored. This topic is discussed in the chapter on moulding of stored seedlings.

Life history (Figure 31)

The pathogen overwinters as mycelium or sclerotia in old plant debris, but the exact sequence of events is not known for local container nurseries. Botrytis conidiospores, which are usually air-borne, are probably produced on dead plant material within the nursery or adjacent fields and forests, and are either drawn or blown into nurseries by ventilation fans or wind. The fungus may also be introduced on seeds, as it can easily be isolated from them. Irrigation water could also contain gray mould spores.

Botrytis cinerea normally becomes parasitic after establishing a food base on dead or dying plant material. Tissues damaged by fertilizer or frost are known avenues for infection. Botrytis may also enter seedlings early in the growing season via senescent needles. Disease development and spread are favored by moderate temperatures, high moisture, dense foliage, and crowded seedlings. The disease usually starts on and spreads from senescent, dead, injured, or lower needles. When succulent leaders are attacked, the disease can move downward.

Management

Cultural and fungicidal controls are presently used, alone or combined, against gray mould. The former are aimed mostly at making greenhouse conditions unfavorable for disease development: e.g. (i) lowering humidity, by decreasing or ceasing watering or by watering in the morning so that foliage dries off quickly, (ii) improving ventilation by spacing of containers so that there is a space between each one, and (iii) regulating temperature, or (iv) using any combination of these. Dead, especially Botrytis-infected seedlings or other plant material (including weeds), should be removed from within and near container nurseries to reduce gray mould inoculum. Greenhouse ventilation can also be improved by the removal of side and roof coverings. The latter also improves lighting, thus preventing excessive shoot growth which is particularly favorable to gray mould.

Other factors that may contribute to gray mould include seedling moisture stress and, although not yet defined, certain fertilizer regimes. Gray mould that becomes established during the growing season may develop further on stored seedlings. Storing seedlings at -1 to -2°C prevents such damage. However, the stock should be handled carefully after storage to prevent subsequent gray mould damage.

Protectant and eradicant (systemic) fungicides are also used in gray mould management. Factors related to their efficiency include timing of application, thoroughness of coverage, and, in the case of certain systemic fungicides, buildup of Botrytis strains with fungicide tolerance. No fungicide, however, will completely control the disease unless the environmental conditions favoring disease development are also changed.

Selected References

Ellis, M.B. and J.M. Waller. 1974. Sclerotinia fuckeliana (conidial state: Botrytis cinerea). CMI Desc. of pathogenic fungi and bacteria. No. 431. Commonwealth Mycol. Inst., Kew, Surrey, England.

Pawsey, R.G. 1964. Gray mould in forest nurseries. Brit. For. Comm. Leafl. 50.

Peterson, M.J. J.R. Sutherland and S.E. Tuller. 1988. Greenhouse environment and epidemiology of grey mould of container-grown Douglas-fir seedlings. Can. J. For. Res. 18: 974-980.

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Summary

Gray mould

Figures

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    Figure 30. Gray mould on container-grown spruce.

     Figure 31. Life history of gray mould.