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Diseases in Container Tree Nurseries

Excerpt from: Landis, T.T. 1989. Disease and pest management. Pp. 1-99. In T.D. Landis, R.W. Tinus, S.E. McDonald, and J.P. Barnett (eds). The Container Tree Nursery Manual. Volume 5. U.S. Department of Agriculture Agric. Handbook. 674.

Grey Mold

The fungus that causes grey mold (Botrytis cinerea) is the most damaging pest of ornamental container plants (Nelson 1978) as well as the most damaging pest of conifer seedlings in container tree nurseries (James 1984).  The importance of B. cinerea was confirmed by the Container Nursery Survey, in which growers from the United States and Canada ranked it as the most serious pest, with 39% of the total responses.  Mittal and others (1978) have reviewed grey mold disease.

Hosts.  Grey mold attacks most species of container seedlings but certain species are particularly susceptible: redwood and giant sequoia (Peterson 1974); western larch, lodgepole pine, Engelmann spruce, and ponderosa pine (James and Woo 1984); western hemlock and Douglas-fir (Sutherland and Van Eerden 1980); Scotch pine, and blue spruce (Gilman and James 1980); mountain hemlock, noble fir, and Alaska-cedar (Matthews 1983).   Grey mold is also a serious problem in eastern container nurseries where most species are affected.

Symptoms/Damage.  As the name implies, this disease can be identified by the grey, cottony mycelia and spore masses on the surface of affected shoot tissue, especially on senescent needles of the lower shoot.  Examination of the fungus with a hand lens will reveal stalked, spore-producing structures that produce a fuzzy appearance (fig. 1).  As the disease progresses, infected shoot tissue becomes watersoaked and brown lesions often develop. The fungus may spread to the main stem, where cankers eventually girdle and kill the shoot (fig. 2) (Sutherland and Van Eerden 1980).  Because the fungus is an aggressive saprophyte, symptoms usually appear first on the shaded, senescent foliage at the base of the shoot.  The disease is most common in the fall when the seedling canopy becomes closed, natural light levels are lower and moisture often condenses on seedling foliage (Sutherland and others 1982).   Under favorable conditions, B. cinerea can spread rapidly from one seedling to another and disease "pockets" can develop within the seedling crowns.   Peterson (1974) estimates that losses to this disease have exceeded 20% in some greenhouses.  Mittal and others (1978) reported that 40% of a container pine crop (52,000 seedlings) was killed by grey mold.

Disease development.  B. cinerea spores can be introduced into a container nursery in the air, on seeds, or in irrigation water (Sutherland and Van Eerden 1980).  Peterson and others (1988) state that the likelihood of spore carrying over between crops is low; instead, Botrytis spores originate outside the greenhouse and are drawn in by cooling fans.  They also speculate that Botrytis infections may develop in July or August, several months earlier than widely believed.  The fungus usually invades weak or damaged foliage first and then spreads to adjacent healthy tissue.   James (1984) reviewed the literature on B. cinerea and lists free surface moisture, high humidity, and cool temperatures as conducive to infection.   When environmental conditions become unfavorable, resting structures called sclerotia form and can persist in soil or plant debris; these sclerotia produce spores when favorable conditions return.

Disease management.  Reducing grey mold damage requires a combination of both cultural and chemical control methods.  Container nursery managers should strive to make the environment less favorable for Botrytis growth and apply protective fungicides to limit initial infections (Sutherland and others 1982).  The type of greenhouse may actually influence development of grey mold: fiberglass houses produced an environment that was over 14 times more favorable for grey mold than the environment in plastic-covered greenhouses (Peterson and others 1988).  Apparently, the fiberglass houses produce taller, more succulent seedlings that are prime hosts for Botrytis infection.

Figure 1.
Figure 2.

Cultural.  Several cultural practices can be used to reduce the incidence and extent of grey mold infections (Sutherland and others 1982, Cooley 1981):

  1. Keep seedlings healthy and vigorous and avoid injuring the foliage.  Fertilizer-burned or frost- damaged foliage is particularly susceptible to Botrytis infection.
  2. Avoid overly dense seedling growing levels by selecting a container that allows adequate  spacing for seedling development.  Containers can also be placed  at a wider spacing to allow  better air circulation during periods when seedlings are especially vulnerable.
  3. Reduce the time that seedling foliage is wet by encouraging air circulation, irrigating early in  the day, using surfactants in the irrigation water, providing underbench heating, or force-drying  foliage with fans.
  4. Follow a strict sanitation policy that includes removal and destruction of all plant debris, prompt  roguing of infected seedlings, and sterilization of containers and growing area surfaces between  crops.

Chemical.  All fungicides registered for the control of grey mold are protectants that must be applied before infection takes place.  There are a number of chemicals registered for controlling B. cinerea on ornamental plants, but not all of these are registered for tree seedlings.  New chemicals are also continually being developed, so growers should monitor trade publications and check with a certified pesticide specialist for up-to-date information.

The timing of protective fungicide applications is important.  These chemicals must cover susceptible plant tissue before Botrytis spores germinate and penetrate the foliage.  Because Botrytis infections are most common in the fall, fungicide applications should begin in late summer.  Peterson and others (1988) found that environmental conditions favoring spore germination were most prevalent in July and August in British Columbia tree nurseries, earlier than previously thought.  Because of the ingrowth of new foliage and the rinsing effect of irrigation, protectant fungicides should be applied at regular intervals (1 to 2 weeks) during the susceptible period.

Botrytis spp. may develop tolerance to fungicides that are used repeatedly (Cooley 1981; James and Woo 1984; Gillman and James 1980), and so fungicides should be used in rotation during the growing season.

Regardless of the effectiveness of pesticides, chemical control of grey mold is virtually impossible without a corresponding and coordinated program of cultural control practices.



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