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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.
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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.
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| Figure 1. |
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| 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):
- Keep seedlings healthy and vigorous and avoid injuring the foliage.
Fertilizer-burned or frost- damaged foliage is particularly susceptible to
Botrytis infection.
- 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.
- 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.
- 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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