Submitted to: American Association for the Advancement of Science Meeting
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2003
Publication Date: 1/30/2004
Citation: Cohen, M.F., Mazzola, M., Yamamoto, E., Arita, N., Yamasaki, H. 2004. Fungal root disease and nitric oxide levels in rapeseed meal-amended soils. Proceedings AAAS annual meeting. p. A105.
Technical Abstract: Organic soil amendments have long been known to suppress certain crop diseases. The impending phase-out of the popular broad spectrum fumigant methyl bromide has brought about renewed interest in the mechanisms of amendment-induced disease suppressiveness. We have consistenly observed substantial declines in infection of apple roots by the fungal pathogen Rhizoctonia solani following incorporation of <1% rapeseed meal (RSM) into orchard soil. Lower infection frequencies were found to correlate with higher yields of fruit - 18 months after planting, yields from Gala/M26 apple were 1.32 kg tree-1 in 0.3% RSM-treated rows compared to 0.0, 0.11 and 0.37 kg tree-1 from untreated, solarized and Telone C17-fumigated rows, respectively. Like many amendments, RSM altered the soil microbial community structure and stimulated a temporary >10-fold increase in the production of nitrogen oxides (NOx) coincident with a period in which the amended soil was toxic to seedlings; the rise in NOx could be blocked by nitrapyrin or dicyandiamide, inhibitors of bacterial nitrification. Unlike some other amendments such as soybean meal, RSM amendment did not increase numbers of fluorescent populations of native Streptomyces spp. was observed, resulting in colonization of apple roots at 4- to 5-fold higher frequencies in RSM-amended soil compared to non-amended soil. RSM amendment did not inhibit R. solani saprophytic growth in soil and although many Streptomyces spp. are known to release fungal inhibitors, in vitro inhibition of R. solani mycelial growth by our isolates was rare (<20%) and relatively weak when it did occur. The majority (>95%) of apple root-colonizing Streptomyces strains produced nitric oxide (NO) via NO synthase. NO production was much more pronounced during exponential growth, occurred along hyphae as visualized in confocal micrographs of mycelia stained with the NO-specific dye DAF-FM DA and was not inhibited by DCD. Based on known roles of NO in plant defense mechanisms and the long-term nature of Streptomyces root colonization we hypothesize functions for NO in RSM-mediated disease suppression.