Screening of Tomato (Solanum lycopersicum L.) Lines for Bacterial Spot (Xanthomonas spp.) Resistance

Bacterial spot (BS), caused by four species of Xanthomonas in tomato (Solanum lycopersicum L.), is challenging to manage by chemical methods under favorable environmental conditions. Development of resistant genotypes is a desirable solution to limit losses. A total of 64 genotypes were planted in the field at the Mountain Horticultural Crops Research and Extension Center (MHCREC), NC in a randomized complete block design with two replications consisting of six plants per plot. The genotypes were inoculated with a local and prevalent strain of Xanthomonas and phenotypic data on foliar disease severity was scored using the Horsfall-Barratt (HB) scale and on fruit incidence. Foliar severity ranged from 0 to over 75% defoliation and fruit incidence ranged from 0% to 100% and these traits were not correlated. The response of tomato genotypes for Xanthomonas-specific pathogen associated molecular pattern (PAMP) molecules in production of reactive oxygen species (ROS) was also scored for these lines.  A luminol-based assay was conducted to measure the ROS by using four bacterial flagellin and one cold shock protein (CSP) peptides. In addition, observations were taken for seven morphological characters including vegetative and reproductive growth stages. Based on the principal component analysis (PCA), six principal components explained 83 percent of total morphological variability. Cluster analysis resulted into three major clusters of which one consists of genotypes NC946, NC50-7, IRAT-L3 and Hawaii 7997 which produced high ROS when Xanthomonas-specific flagellin was used in luminol-based assay. These genotypes could be useful in developing BS resistance in breeding programs.