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.
Fusarium Root Rot
Fusarium root rot is one of the most common diseases of conifer
seedlings in the world and is widespread in North American nurseries (Bloomberg
1981, Smith 1975). Fusarium wilt affects many different horticultural
plants and is the most important pathological problem of plants grown in
artificial growing media (Couteaudier and Alabouvette 1981). Because this
fungus prefers warmer temperatures, heated container nurseries are ideal for
build-up of this disease.
Hosts. Most conifer seedlings, including spruces,
true firs, pines and larch are susceptible to fusarium root rot, but the disease
is apparently most serious on Douglas-fir.
Symptoms/Damage. The foliar symptoms of fusarium
root rot are variable: newly infected seedlings typically have scattered
chlorotic or curled needles (fig. 1) followed by tip dieback, wilt symptoms, and
stunting as the disease progresses. The seedling foliage often turn a
reddish brown just before the seedling dies. Diseased root systems show
lack of fine root development and extensive cortical decay so that the epidermis
is easily stripped away from the core tissues (fig. 2). One of the most
diagnostic signs of this disease is the production of fruiting structures
(sporodochia) on the seedling stem (fig. 3), where yellow-orange spore masses are
exuded (James 1985a, Landis 1976). These spores are typically multicellular
and sickle-shaped and can be used to positively identify the fungus.
Disease development. Apparently, several different
species of Fusarium can cause root rot of container tree seedlings,
including F. oxysporum (Graham and Linderman 1983), F. solani
(James 1983, Landis 1976), and F. avenaceum (James 1985a). One of
the main sources of Fusarium inocula in container nurseries is the seed,
although the fungus has also been isolated from growing media, used containers,
weeds, and irrigation water. Pawuk (1981) isolated only low levels of
Fusarium spp. from air and water samples around greenhouses but recovered
the fungus from four species of southern pine seed with infection intensities of
54 to 91%. Graham and Linderman (1983) isolated F. oxysporum
from Douglas-fir seed and James (1983) recovered F. oxysporum and F.
solani from the exterior and interior of conifer seed. Seed from
squirrel-cache-collected cones often are infested with Fusarium
spp. and other potentially pathogenic fungi (James 1986). This widespread
fungal pathogen has also recently been isolated from reusable containers that
had already been sterilized.
Although initial infections are
usually random, secondary spread is probably due to
spores splashed from diseased seed or seedlings
during irrigation (Graham and Linderman 1983), and
disease pockets apparently develop as a result of
this secondary spread. Bloomberg (1981)
discusses fusarium diseases in detail and reports
that high temperatures [25 to 35 oC (77
to 93 oF)] stimulate fungal growth and
that high nitrogen fertilization appears to increase disease losses.
Normal-appearing seedlings are often infected with the fungus but do not develop foliar
symptoms. Fusarium is a common rhizosphere inhabitant, and the disease only
develops when the seedling becomes stressed, by drought or heat for example. The
cultural practice of moisture-stressing seedlings to harden them may thus actually enhance
disease development (James 1986).
Chemical. Chemical control methods can be divided into
sanitation of containers and surfaces in the growing area, seed treatments, growing medium
treatments, and fungicidal drenches. Because Fusarium spp. can be introduced
into container nurseries on seed, growers might consider seed treatments prior to sowing.
Growing media should be assayed to make sure that they are pathogen-free, and
containers should be carefully cleaned and treated with heat or chemical sterilants.
Fungicidal drenches are commonly used to control fusarium root rot, but these treatments
function primarily to limit the spread, rather than cure the disease.
Cultural. Growers can reduce the impact of fusarium root rot by
using a growing medium that stimulated healthy root growth and discourages pathogens
(Couteaudier and Alabouvette 1981) and by promptly roguing diseased seedings to prevent