Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/1998
Publication Date: N/A
Interpretive Summary: The bacterial pathogen causes two diseases of wheat, bacterial leaf streak and black chaff. These diseases occur sporadically, but they can cause severe damage to yield and quality of wheat when weather conditions favor epidemics and the bacteria are present in wheat fields. Yield losses up to 40% have been reported. Research on these diseases has been hampered by the lack of reliable techniques to induce the disease consistently in controlled experiments. For example, it has been difficult to breed for resistance to bacterial leaf streak, because natural epidemics are sporadic in occurrence and artificial inoculations with the pathogen generally produced inconsistent results. We developed an inoculation procedure that produces consistent levels of bacterial leaf streak on wheat leaves. The procedure is quick and simple. It involves spraying suspensions of bacterial cells onto the leaves of wheat plants and then keeping the plants sin an environment of high relative humidity. Most importantly, the procedure allows us to mimic the natural conditions in which bacteria multiply on the leaf surface for several days before infecting the leaves. If the number of bacteria on the leaf surface does not exceed a threshold amount, little or no infection will occur. Beyond that threshold number, increasing numbers of bacteria on the leaf lead to increasing severity of disease. This information is a key to developing disease control methods, such as biological control, that can keep the pathogenic bacteria from exceeding their disease threshold on the leaves. Our inoculation procedure is now being used in such studies.
Technical Abstract: The relationship between leaf-associated population sizes of Xanthomonas translucens pv. translucens (Xtt) prior to disease development and subsequent bacterial leaf streak (BLS) severity was investigated. Evaluation of assay conditions indicated that both pre- and post-inoculation environmental conditions influenced pathogenesis. In three experiments, Xtt was spray inoculated onto 10-day-old wheat seedling over a range of inoculum densities (108, 107, 106, 105, 104 cfu/ml). Lesions developed most rapidly on plants inoculated with higher inoculum densities of Xtt. The relationship between pathogen population size on leaves and subsequent BLS severity was best described by the logistic model. Leaf-associated pathogen population sizes recovered 48 h after inoculation were highly predictive of BLS severity 7 days after inoculation (R2=0.970,P<0.0001). Leaf-associated Xtt population size and BLS severity from a particular pathogen inoculum density often varied among experiments; however, the disease severity level caused by a particular leaf-associated Xtt population size was not significantly different among experiments. Biological and disease control implications of the Xtt population size-BLS severity relationship are discussed.