Brassica Soil Amendments for Soil Borne Pathogen Control in Douglas-Fir Seedlings.

Fusarium root rot is one of the major soil borne diseases of conifers. So far, soil fumigation with methyl bromide has been the most effective control method. Because of safety and environmental concerns, methyl bromide use is increasingly restricted. However, the conifer seedling industry continues to use methyl bromide under a critical use exemption due to a lack of effective alternatives. A field study was conducted to examine the effects of Brassica seed meals and green manures on management of selected soil borne pathogens. The study consists of five treatments: Brassica juncea seedmeal (BjSM), Brassica carinata seedmeal (BcSM), Brassica juncea green manure (BjGM), methyl bromide/chloropicrin fumigation (MBC) (positive control) and no treatment (negative control) with four replications in randomized complete block design (RCBD). The treatments were incorporated into soil in fall (September 2011) at 2 tons acre^-1 and Douglas-fir (Pseudotsuga menziesii) seedlings were transplanted into plots in spring (May 2012). Pathogen populations, nitrogen mineralization rate, and dehydrogenase enzyme activity in soil were assessed. The baseline pathogen level was not significantly different among treatments. At transplant time, Fusarium spp. were significantly lower in BjGM (146 CFU g^-1) than MBC (290 CFU g^-1), and control (357 CFU g^-1); similarly Trichoderma were significantly higher in MBC (5716 CFU g^-1) followed by BjGM (3031 CFU g^-1), and control (1763 CFU g^-1). Dehydrogenase enzyme activity was highest in BjGM (0.81 µg TPF g^-1 hr^-1) followed by BjSM (0.62 µg TPF g^-1 hr^-1). Similarly, the mineralizable nitrogen was higher in BjGM (5.8 and 6.8 µg NH4 g^-1 during 7- and 28-days incubation respectively) followed by BjSM (4.0 and 5.8 µg NH4 g^-1). These preliminary results suggest that B. juncea green manure has a suppressive effect on soil borne pathogens, and both green manure and seed meals have positive impacts on soil quality.