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Two genetically diverse groups of strains were identified among cultures of Xunthomonas campestris pv. vesicatoria isolated from plants with bacterial spot of pepper and tomato. Group A strains do not pit pectate gels or hydrolyze starch, whereas group B strains are strongly positive for these reactions. Group A strains cause a hypersensitive reaction in plants of tomato breeding line Hawaii 7998, but group B strains do not. Other differences between the two groups of strains were discovered in tests for utilization of carbon compounds, serology, fatty acid profiles, silver-stained protein bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels, and DNA restriction enzyme digestion profiles. The levels of DNA homology between strains belonging to the same group were more than 74%, but the levels of DNA homology between strains belonging to different groups were less than 46%. The two groups of strains have different genetic backgrounds, but cause essentially the same disease of tomato and pepper.The bacterial spot disease of tomato and pepper is caused by the bacterium Xanthomonas campestris pv. vesicatoria. Necrotic spots on leaves, stems, peduncles, and fruits are distinctive symptoms of the disease (5, 17). Lesions may enlarge or coalesce and cause leaves to become chlorotic and eventually die. When prevalent, lesions on fruits result in loss of marketability. Bacterial spot occurs in all regions of the world where tomatoes and peppers are grown (17).A large collection of strains of the bacterial spot pathogen of tomato and pepper was accumulated for study (4), and two characteristics, pitting of pectate gels and hydrolysis of starch, were determined for each strain as aids for identifying possible opportunistic xanthomonads in the collection (14). Strains that were pectolytic and amylolytic, characteristics expected for opportunistic xanthomonads, were discovered, but these strains were pathogenic for tomato and/or pepper. One of the pectolytic and amylolytic strains was from Brazil and was designated a new race of X. campestris pv. vesicatoria on the basis of its ability to cause disease in the resistant tomato breeding line Hawaii 7998 (29). Of 522 strains tested (4), 104 were pectolytic and amylolytic. Most of these 104 strains were from Argentina, but similar strains were received from Australia, Brazil, New Zealand, and Spain. Only 1 of 331 strains from the United States was pectolytic and amylolytic, and no such strain was among the 28 strains from Taiwan tested. The pectolytic and amylolytic strains were not evenly distributed in all regions of the world where bacterial spot of tomato and pepper occurs, but seemed to be most prevalent in countries in the southern hemisphere.The two groups of strains, which are placed in X. campestris pv. vesicatoria because of their host specificity, were further characterized to determine the extent of genotypic

Xanthomonas campestris strains that cause disease in citrus were compared by restriction endonuclease analysis of DNA fragments separated by pulsed-field gel electrophoresis and by DNA reassociation. Strains of X. campestris pv. citrumelo, which cause citrus bacterial spot, were, on average, 88% related to each other by DNA reassociation, although these strains exhibited diverse restriction digest patterns. In contrast, strains of X. campestris pv. citri groups A and B, which cause canker A and canker B, respectively, had relatively homogeneous restriction digest patterns. The groups of strains causing these three different citrus diseases were examined by DNA reassociation and were found to be from 55 to 63% related to one another. Several pathovars ofX. campestris, previously shown to cause weakly aggressive symptoms on citrus, ranged from 83 to 90% similar to X. campestris pv. citrumelo by DNA reassociation. The type strain ofX. campestris pv. campestris ranged from 30 to 40% similar in DNA reassociation experiments to strains of X. campestis pv. citrumelo and X. campestris pv. citri groups A and B. Whereas DNA reassociation quantified the difference between relatively unrelated groups of bacterial strains, restriction endonuclease analysis distinguished between closely related strains.

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Crop domestication events followed by targeted breeding practices have been pivotal for improvement of desirable traits and to adapt cultivars to local environments. Domestication also resulted in a strong reduction in genetic diversity among modern cultivars compared to their wild relatives, though the effect this could have on tripartite relationships between plants, belowground beneficial microbes and aboveground pathogens remains undetermined. We quantified plant growth performance, basal resistance and induced systemic resistance (ISR) by Trichoderma harzianum, a beneficial soil microbe against Botrytis cinerea, a necrotrophic fungus and Phytophthora infestans, a hemi-biotrophic oomycete, in 25 diverse tomato genotypes. Wild tomato related species, tomato landraces and modern commercial cultivars that were conventionally or organically bred, together, representing a domestication gradient were evaluated. Relationships between basal and ISR, plant physiological status and phenolic compounds were quantified to identify potential mechanisms. Trichoderma enhanced shoot and root biomass and ISR to both pathogens in a genotype specific manner. Moreover, improvements in plant performance in response to Trichoderma gradually decreased along the domestication gradient. Wild relatives and landraces were more responsive to Trichoderma, resulting in greater suppression of foliar pathogens than modern cultivars. Photosynthetic rate and stomatal conductance of some tomato genotypes were improved by Trichoderma treatment whereas leaf nitrogen status of the majority of tomato genotypes were not altered. There was a negative relationship between basal resistance and induced resistance for both diseases, and a positive correlation between Trichoderma-ISR to B. cinerea and enhanced total flavonoid contents. These findings suggest that domestication and breeding practices have altered plant responsiveness to beneficial soil microbes. Further studies are needed to decipher the molecular mechanisms underlying the differential promotion of plant growth and resistance among genotypes, and identify molecular markers to integrate selection for responsiveness into future breeding programs.

Cucurbit downy mildew (CDM), caused by Pseudoperonospora cubensis, is one of the most important diseases affecting cucurbits worldwide. In the USA, host resistance in cucumber had adequately controlled the disease with very minimal application of fungicides from the late 1960s to 2004. In 2004, there was a resurgence of the disease that devastated the cucumber crop in several states in the eastern USA. Since then, host plant resistance alone has not been sufficient to adequately control the disease and now control relies heavily on application of fungicides. To effectively apply fungicides in a timely manner, cucurbit growers, extension personnel, and crop consultants and advisors can now utilize information on disease occurrence and predicted spread disseminated through the United States Department of Agriculture's CDM ipmPIPE decision support system developed by scientists at North Carolina State University. Based on a survey conducted in Georgia, North Carolina, and Michigan, the CDM ipmPIPE resulted in an average reduction of 2 to 3 fungicide applications in 2009 compared to calendar-based fungicide sprays. With approximately 122,000 acres of cucurbits in these three states, this translates to more than $6 million in savings to the producers in these three states. Economic savings and positive environmental implications of reduced fungicide applications demonstrate the value of a coordinated national monitoring network for management of a plant disease that is disseminated aerially over long distances. Accepted for publication 31 January 2011. Published 11 April 2011.