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).