Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/1999
Publication Date: N/A
Interpretive Summary: Bacterial leaf streak of wheat occurs in regions where weather is often warm and humid during the growing season. In severe epidemics, yield losses have been as high as 40%. Resistance in current wheat varieties is only partially effective, and no chemical control has been developed. Biological control of the disease should be feasible, because the pathogenic bacteria must multiply on the leaf surface until they exceed a threshold population size before they infect the leaf interior. We collected 44 non-pathogenic bacteria that inhabit leaf surfaces and tested their ability to inhibit infection. First we sprayed the non-pathogenic bacteria on wheat leaves, and 2 days later we inoculated the leaves with the pathogen. Several of the non-pathogenic bacteria reduced disease development. Their biocontrol success was directly related to how much they suppressed pathogen multiplication on the leaf surfaces in the first 2 2days after inoculation. The best disease control was by non-pathogenic bacteria that can live on many of the nutrients, particularly sucrose and inositol, that occur naturally on surfaces of wheat leaves. The bacterial leaf streak pathogen uses sucrose for growth but cannot utilize inositol. We believe that the successful non-pathogenic bacteria competed with the pathogen for available sucrose and that their ability to use inositol, as well, gave them a competitive advantage over the pathogen. This information will be used to continue the search for even more effective biological control agents that may be applied to wheat crops in the field to ward off attacks of bacterial leaf streak.
Technical Abstract: The abilities of 44 bacterial antagonists to reduce leaf-associated population sizes of Xanthomonas translucens pv. translucens strain XttRif-2 and bacterial leaf streak were quantified. Bacterial antagonists were inoculated individually onto 10-day-old wheat seedlings at a density of 108 cfu. Pathogen strain XttRif-2 was inoculated onto wheat seedlings 2 days later at a density of 107 cfu/ml. Antagonist and pathogen population size were quantified 2 days after pathogen inoculation and disease severity was assessed 7-8 days after pathogen inoculation. Thirteen successful antagonists were identified by their ability to significantly reduce XttRif-2 populations, disease severity, or both compared to a pathogen-only control. The potential roles of antibiosis and competition for nutrient resources in mediating the observed antagonist-pathogen interactions were also investigated. Only one antagonist inhibited Xtt in vitro; thus, it was concluded that antibiosis did not play a large role in reducing pathogen populations or disease severity. Similarity in nutrient utilization between antagonists and pathogen strain XttRif-2 was estimated using nutrient overlap indices. Nutrient overlap indices were not predictive of the ability of antagonists to reduce pathogen populations or disease severity. Ability to utilize both sucrose and inositol was significantly more frequent among successful antagonists than poor antagonists. The patterns of resource utilization among antagonists and interactions between antagonists and the pathogen suggest mechanisms mediating the ability of bacterial antagonists to reduce pathogen population size and disease severity.