Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 7, 1997
Publication Date: N/A
Interpretive Summary: Studies of the epidemiology of root diseases suggests that a given amount of disease control requires a longer crop rotation with no-till than with conventional tillage. However, the trend with dryland cereals worldwide is towards shorter or no crop rotation while simultaneously adopting no- till. At least three root diseases caused by three different kinds of soil-inhabiting fungi are major problems for no-till wheat and barley, namely, take-all, rhizoctonia root rot, and pythium root rot. A search was made for spore-forming rhizosphere bacteria (rhizobacteria) with ability to protect wheat roots against these diseases. Sporeforming bacteria are more resistant to drying and other adverse environments. Young wheat plants from eastern Washington fields cropped continuously to wheat for up to 25 years were used as a source of rhizobacteria most likely to be adapted to the roots of wheat. Roots were washed thoroughly and then macerates were treated at 80 C for 10 minutes before culturing to kill all but those strains with ability to produce spores. Out of some 2000 strains of sporeforming bacteria cultured and tested, one strain, newly reported as Bacillus sp. L324-92, was inhibitory to all isolates of the three kinds of wheat root pathogens in the laboratory, suppressed all three root diseases in the greenhouse, and increased yields of wheat planted directly into the stubble of wheat in the field. Unlike most Bacillus species, which cannot grow at temperatures below 10 C, strain L324-92 can grow at temperatures down to 4 C. The discovery of L324-92 represents a major breakthrough for biological control of root diseases of no-till wheat and provides the first evidence for the simultaneous control of all three root diseases by a single microorganism.
Technical Abstract: Strain L324-92 is a novel Bacillus sp. with biological activity against three root diseases of wheat, namely, take-all caused by Gaeumannomyces graminis var. tritici, Rhizoctonia root rot caused by Rhizoctonia solani AG8, and Pythium root rot caused mainly by Pythium irregulare and P. ultimum, exhibits broad-spectrum inhibitory activity, and grows at temperatures from 4 to 40 C. Strain L324-92 was selected from among approximately 2,000 rhizosphere/ rhizoplane isolates of Bacillus species isolated from roots of wheat collected from two eastern Washington wheat fields that had long histories of wheat. Roots were washed, macerated, and the macerates heat-treated (80 C for 30 min), and dilution-plated on 1/10 strength tryptic soy agar. Strain L324-92 inhibited all isolates of G. g. tritici, Rhizoctonia species and anastamosis groups, and Pythium spp. tested on agar at 15 C; provided significant suppression of all three root diseases at 15 C in growth chamber assays; and controlled either Rhizoctonia root rot, take-all, or both and increased yields in field tests where one or more of the three root diseases of wheat were yield-limiting factors. The ability of L324-92 to grow at 4 C probably contributes to its biocontrol activity on direct drilled winter and spring wheat because, under Inland Northwest conditions, leaving harvest residues of the previous crop on the soil surface keeps soils cooler compared to when soils are tilled. These results suggest that Bacillus species with desired traits for biological control of wheat root diseases are present within the community of wheat rhizosphere microorganisms and can be recovered by protocols developed earlier for isolation of fluorescent Pseudomonas species effective against take-all.