Microbial mixtures for biological control of Fusarium diseases of tree seedlings (1997)

Alternatives to soil fumigation with methyl bromide are needed for controlling Fusarium diseases in tree nurseries. Studies are under way for developing microbial mixtures (bacteria & ectomycorrhizal fungi) that control Fusarium diseases of eastern white pine (Pinus strobus L.) seedlings. Greenhouse studies in containerized production have shown that the application of rhizosphere bacteria to conifer seed, coupled with ectomycorrhizal fungi application at sowing, can protect seedlings against Fusarium root rot. Not only do seedlings have a reduced incidence and severity of root rot, they also have greater levels of ectomycorrhizal roots. When applied to seeds for field (bareroot) production, the bacterial strains are associated with increased stand numbers. INTRODUCTION Mortality of field-grown (bareroot) Pinus strobus L. (eastern white pine) seedlings from Fusarium root rot (17) causes serious economic losses in the Lake States region (12,15,16). Infected seedlings suffer reduced vigor and growth, if not killed by root rot. In one survey, up to 75 % of inspected seedlings had root rot (13). Moreover, 68 % of the seedlings intended for sale from one nursery were culled due to root rot (14). In addition to causing root rot, Fusarium spp. are known to incite damping-off of P. strobus (6,7). Containerized seedlings are also susceptible to Fusarium diseases. Current control measures include soil fumigation with methyl bromide-chloropicrin (bareroot production) and biweekly applications of fungicides (containerized production), but root rot of white pine is still a serious problem in nurseries. Disease problems will undoubtedly increase when methyl bromide is no longer available for soil fumigation(19). Alternatives are needed for controlling Fusarium diseases in tree nurseries and use of biological control agents is an important method of alternative pest control. Use of microorganisms as biological control agents has been studied with agronomic and horticultural plant diseases but limited information is available for diseases of conifer seedlings. Microorganisms antagonistic to Fusarium spp. can be applied to conifer seed. Upon successful colonization of the rhizosphere, conifer seedling roots can be protected Fusarium against root rot. Biological control microbes may also protect germinates from damping-off (1). Successful use of ectomycorrhizal fungi for suppressing pathogenic Fusarium spp. has been reported with Laccaria laccata on Pinus banksiana (1) and Pseudotsuga menziesii (18); as well as Paxillus involutus on Pinus resinosa (4,5). Abundant ectomycorrhizal root formation enables nursery managers to significantly decrease fertilization rates; generating savings above those from minimized seedling losses due to death and culling. In addition, biological control agents may preclude other soilborne pathogens from causing disease. Biological control microbes will probably benefit other Lake States conifer species, as well as conifer seedlings grown in other geographic areas. By introducing rhizosphere colonists that are beneficial to conifer seedlings and antagonistic to soilborne pathogens, growth and spread of indigenous or exotic pathogens may be minimized through protection of susceptible host tissue. TESTING IN CONTAINERIZED PRODUCTION Representative isolates of F. oxysporum, F. oxysporum var. redolens (W. L. Gordon), F proliferatum (T. Matsushima) Nirenberg, and F solani (Mart.) Sacc. were collected from necrotic P. strobus roots or nursery soil, purified by the single-spore method, and stored on silica gel at 5 C (20). Inoculum was increased by transferring 5-mm agar plugs from carnation leaf agar (10) cultures to sterile cornmeal-sand medium (97 g sand, 3 g cornmeal, 40 ml distilled water). Each pathogenic Fusarium isolate was added to a growing medium (Fafard #2) at a rate of 0.005 g/cc soil. Bacteria (Table 1) were isolated from white pine rhizosphere soil (11), stored in sterile water at 24 C, and increased in oatmeal broth (2). The mycorrhizal fungi, Hebeloma sp. and Laccaria sp., were stored as outlined in Doudrick and Anderson (3) and grown in modified Melin-Norkrans' nutrient solution (9). White pine seeds (lot A05884, courtesy of G. Dinkel, USDA Forest Service) were surface-disinfested by agitation in 3 % H2O2 for 2 hr, rinsed four times in sterile water, wrapped in moist cheesecloth, and placed in cold storage (5 C) for eight wk. After stratification, seeds were soaked in bacterial cultures for 60 min then airdried. Table 1. Bacteria used as biological control agents for suppression of Fusarium root rot in containerized or field production Acquisition # Identification via Fatty Acid Analyses BCT5a Streptomyces violaceusniger subsp. violaceusniger BCT19b Streptomyces rochei subsp. rochei BCB175 Bacillus megaterium BCB176 Streptomyces lavendulae BC19 Methylobacterium mesophilicum BC20 Rhodococcus erythropolis/Kocuria varians/Pseudomonas diminuta