Sharp eyespot of wheat is caused by the fungus Rhizoctonia cerealis and is an important soil-Borne disease of wheat worldwide. At present, no commercially cultivated disease resistant varieties are available to efficiently protect wheat against infection by this pathogen. Control of the plant disease sharp eyespot currently relies on application of pesticides such as Triadimefon and Validamycin; however, pesticides are expensive and pose serious health and environmental hazards. Persistent application of pesticides can cause selection of pesticide resistant pathogenic fungi. Biological control is an environmentally sound and effective means of reducing or mitigating plant diseases and the effects of plant pathogens through the use of natural microorganisms. In the rhizosphere (on the plant root or in close vicinity to the root) bacteria are abundant, most often organized in microcolonies. Bacteria in the rhizosphere, known as rhizobacteria, not only benefit from the nutrients secreted by the plant root but also beneficially influence the plant in a direct or indirect way, resulting in stimulation of plant growth. Among the beneficial microorganisms isolated from the rhizosphere, Bacillusspp. offer several advantages over fluorescent pseudomonads and other Gram-negative bacteria as seed inoculants for protection against root pathogens. Bacillus spp. have a long shelf life because of their ability to form endospores and their unique mechanisms of action as they can produce broad-spectrum antibiotics and compete for ecological niches against plant pathogenic fungi.
Bacillus cereus B3-7, a biological control strain isolated from the wheat rhizosphere, shows potential for control of sharp eyespot and for efficient yield increase in wheat. To elucidate the ecological adaptability of B3-7 and its efficacy for biological control of sharp eyespot of wheat under field conditions, the green fluorescent protein (gfp) gene was inserted into the B3-7 genome and a GFP-tagged B3-7 strain was constructed. Colony shape, growth rate, biofilm formation and colonization dynamics on wheat roots were analyzed with B3-7 and the GFP-tagged strain in our laboratory. No significant differences were obtained for any of the characteristics tested between the two strains. Colonization ability of the B3-7 GFP-tagged strain on wheat roots was assessed under field conditions. The results indicated that the GFP-tagged strains could persistently colonize the roots of wheat. The number of colonizing bacteria was highest during the tillering period at 10⁵ CFU/g roots. The colonization number decreased to 10⁴ CFU/g roots in the heading period, and persisted through the milking period. Efficacy of B3-7 and the GFP-tagged strain as biological control agents against sharp eyespot of wheat was also evaluated under field conditions. The results revealed that the two strains could reduce disease severity and effectively increase the yield of wheat. The disease control efficacy of the two strains for sharp eyespot of wheat reached 60% to 62%, 34% to 39%, and 34% to 38% during the tillering, heading, and milking periods, respectively. The yield of wheat treated with the biological control strain increased 13% to 15% compared with that of the untreated control. The results indicated that Bacillus cereus B3-7 has the capacity for environmental adaptability and the potential to act as a biological control agent against sharp eyespot of wheat under field conditions.