Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 3, 2012
Publication Date: February 10, 2013
Citation: Crane, J.M., Gibson, D.M., Vaughan, R.H., Bergstrom, G.C. 2013. Iturin levels on wheat spikes linked to biological control of fusarium head blight by Bacillus amyloliquefaciens. Phytopathology. 103:146-155. Interpretive Summary: Managing fungal diseases of plants with the gram-positive bacterium Bacillus subtilis or Bacillus amyloliquefaciens could be of great benefit due to their low environmental impact, and to help reduce growers’ dependence on chemicals, thereby slowing the development of fungicide resistance in pathogen populations. Several Bacillus formulations have shown consistent suppression of aerial diseases under controlled environments, but have shown less consistent control under field conditions. In this study, we show that successful control of Fusarium wheat head blight and reduction of the fungal toxin deoxynivalenol (common name vomitoxin) relies on adequate levels of a key antifungal compound, iturin, and that insufficient levels of this metabolite on wheat spikes in the field are responsible for the lack of disease control in this environment. We also show that iturins decline rapidly on wheat spikes in the field and greenhouse despite the presence of significant Bacillus populations, indicating that Bacillus cells are not adequately replenishing lost iturins on wheat surfaces. Our data suggest that enhancing Bacillus control of foliar diseases should focus on increasing the level and longevity of iturins on plant surfaces, particularly when combating diseases with long infection periods.
Technical Abstract: The TrigoCor strain of Bacillus amyloliquefaciens provides consistent control against Fusarium head blight of wheat in controlled settings but there is a lack of disease and deoxynivalenol suppression in field settings. Since production of antifungal compounds is thought to be the main mode of action of TrigoCor control, we quantified levels of a key family of antifungal metabolites, iturins, as well as monitored Bacillus populations on wheat spikes over 14 days post-application in both the greenhouse and the field. We found that initial iturin levels on spikes in the greenhouse were three times greater than on spikes in the field, but that by 3 days post-application, iturin levels were equivalent and very low in both settings. We also determined that iturins declined rapidly over a 3 day post-application period on wheat spikes in both environments, despite the presence of significant Bacillus populations. Greenhouse trials and antibiosis tests indicated that the lower iturin levels on wheat spikes in the field could be a major factor limiting disease control in field settings. Future efforts to improve Bacillus disease control on wheat spikes and in the phyllosphere of various plants should focus on maintaining higher levels of iturins over critical infection periods.