Abstract
Bacterial spot caused by several Xanthomonas spp. is an economically important disease of tomato (Solanum lycopersicum L.). Host resistance to the disease is partially dominant or incomplete, which requires accurate assessment of disease severity for genetic studies of resistance. In the present study, three independent experiments were conducted to investigate the feasibility of using image analysis to estimate foliar disease severity of bacterial spot in tomato. The resistant line PI 114490 and the susceptible line OH 88119 were used in the first experiment, five tomato lines (PI 114490, PI 128216, Hawaii 7981, Hawaii 7998, and Fla. 7600) with a range of resistance and OH 88119 were used in the second experiment, and 439 F2 individuals from a cross between OH 88119 and PI 114490 were used in the third experiment. Tomato plants were spray-inoculated with bacterial spot race T3. Five diseased leaves from each plant were randomly collected and scanned to obtain digital images 21 days after inoculation. The disease severity (% leaf area) was measured using image analysis. The susceptible line OH 88119 showed the most severe disease. The resistant line PI 114490 showed the least severe disease, and was not significantly different to PI 128216 or Hawaii 7981. These results indicated that image analysis could be used to distinguish tomato lines with different resistance to bacterial spot. Marker-trait association analysis identified four quantitative trait loci conferring resistance to race T3 in PI 114490 using data obtained from image analysis, the Horsfall-Barratt (HB) category scale data, and HB midpoint converted values. However, the disease severity was slightly underestimated using the HB category scale and the phenotypic variation explained by each marker was overestimated using the HB category data compared to using the image analysis-measured disease severity data. Therefore, image analysis could provide a consistent, accurate and reliable method compared to the HB scale to estimate disease severity for genetic studies of foliar bacterial spot in tomato.
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