Ralstonia solanacearum Race 3 Biovar 2 Causes Tropical Losses and Temperate Anxieties

Introduction Bacterial wilts caused by Ralstonia solanacearum (Smith 1986) Yabuuchi et al. 1996 (formerly called Pseudomonas solanacearum) have long been a scourge of tropical agriculture because the pathogen is very widely distributed and has an unusually broad host range (7,13). Moreover, because R. solanacearum is a soilborne pathogen and host resistance is limited, bacterial wilt is very difficult to control (13,26). Bacterial wilt disease generally occurs in lowlands in tropical or subtropical areas, but one subgroup of R. solanacearum called Race 3 biovar 2 (R3bv2) attacks plants at higher altitudes or in temperate zones. R3bv2 is an extremely destructive potato pathogen, causing brown rot of potato in the highland tropics of Africa, Asia, and Latin America. It has recently appeared in Europe where it disrupted seed potato production and caused serious quarantine-related losses (8,15). In the United States, R3bv2 was listed in 2002 as a Select Agent plant pathogen, and is now subject to the strictest biosecurity regulations (20). R. solanacearum is considered a "species complex," due to significant variation within the group (11). It was historically subdivided into five races, based loosely on host range, and five biovars, based on ability to acidify a panel of 5 to 8 carbohydrate substrates, but this old classification system has proved to be unsatisfactory. There are no laboratory tests to define the "race" of an isolate because host ranges of strains are broad and often overlap. The biovars do not correspond to phylogenetically coherent groups, with the exception of biovar 2A, which corresponds to R3bv2. Moreover, a subgroup of biovar 2, known as biovar 2T, is present in strains from Asia and Africa as well as strains in the original "biovar 2" group from South America, which means that the traditional biovar test is not completely reliable for diagnostic purposes. Recently, a phylogenetically meaningful classification scheme was developed based on DNA sequence analysis (11,24). This scheme divides the species complex into four phylotypes that broadly reflect the ancestral relationships and geographical origins of the strains. Phylotype I strains originated in Asia, phylotype II strains originated in the Americas, phylotype III strains in Africa, and phylotype IV strains in Indonesia. Phylotypes are further subdivided into sequevars based on the sequence of the endoglucanase (egl) gene (11,24). Multi-locus sequence typing and other analyses have confirmed that this system of classification reflects the phylogeny of the group. R. solanacearum R3bv2 strains belong to phylotype II and sequevars 1 and 2 (11). R3bv2 probably originated in the Andes, and sequevar 1 was apparently disseminated worldwide on potato tubers; this group now occurs in tropical highlands and in subtropical and warm-temperate areas throughout the world, except in North America. It is responsible for an estimated $1 billion US in losses each year (8).