UC Davis

Bacterial pathogens in the Ralstonia solanacearum species complex cause widespread bacterial wilt diseases throughout the humid tropics and subtropics, causing significant damage to numerous susceptible crop species. Ralstonia are diverse plant pathogenic bacteria with thousands of distinct lineages. DNA sequence analysis reveals the species complex is comprised of four primary branches (phylotypes) of continental or archipelagic origins in Africa, Asia, the Americas, and the tropical islands of the South Pacific. The Ralstonia pathogen family has a remarkably broad host range, infecting hundreds of host species in dozens of taxonomic families (Álvarez 2010). This breadth, however, is not uniformly distributed across strains. The IIB-4 clade is known to exhibit a particularly broad host range. When introduced to new geographic locations, IIB-4 strains can proliferate rapidly and infect crop species not susceptible to previously endemic strains (Wicker 2007).

In this investigation, we quantified the virulence and host range of 19 strains in the IIB-4 clade through stem and soil inoculations on four diverse host species: tomato (Solanum lycopersicum cv Moneymaker), melon (Cucumis melo cv. Sweet Granite), Impatiens (Impatiens walleriana cv. Beacon Orange), and banana (Musa acuminata cv. Dwarf Cavendish). The strains we analyzed originate from a broad geographic swath of the ancestral range of phylotype II, including Florida, the Caribbean, Central America, and South America. 18 strains were isolated from monocotyledonous plantain (Musa spp.), Heliconia (Heliconia spp.), pothos (Epipremnum aureum), and Anthurium (Anthurium spp.) plants. One strain (IBSBF1503) is a eudicot isolate from cucumber (Cucumis sativus). To differentiate patterns of host specificity and virulence amongst the closely related strains, we subjected each host-strain combination to disease progress assays in a controlled growth chamber environment. We calculated area under disease progress curve (AUDPC) values individually for each of the 3,005 plants in the study and aggregately for each host-strain combination. Highly virulent strains caused disease progressions with mean AUDPC values of 8  MAUDPC. Moderately virulent strains generated mean AUDPC values of 4  MAUDPC < 8. Strains with low virulence generated mean AUDPC values of 0  MAUDPC < 4. Avirulent strains generated mean AUDPC values of MAUDPC = 0.

Tomato plants were the most broadly susceptible. 17 strains were highly virulent on tomato plants, and two strains were moderately virulent. Virulence on melon plants was more variable. Eleven strains were highly virulent on melon plants, four strains were moderately virulent, and four strains exhibited low virulence. Virulence on Impatiens plants was also variable. Twelve strains were highly virulent on Impatiens plants, four strains were moderately virulent, and three strains exhibited low virulence. Banana plants were the most broadly resistant. One strain was highly virulent on banana plants, two strains were moderately virulent, five strains exhibited low virulence, and eleven strains were avirulent. Despite a high degree of genetic similarity, R. solanacearum strains in the IIB/4 clade exhibit considerable variation in virulence and pathogenicity on diverse hosts.