Variability and geographical distribution of Fusarium oxysporum f. sp. lycopersici physiological races and field performance of resistant sources in Brazil.

Fusarium wilt, caused by three physiological races of Fusarium oxysporum f. sp. lycopersici (FOL), is a major tomato disease in tropical and subtropical regions of Brazil. A collection of FOL isolates from Brazil was characterized via inoculation assays using a set of Solanum (sect. Lycopersicon) accessions employed for race discrimination: 'Ponderosa' (susceptible to all races), LSQUOIPA-5RSQUO (race 1 resistant due to the I locus); 'Floradade' (races 1 and 2 resistant; I-2 gene), Solanum pennellii 'LA 716' (resistant to race 3; I-3 gene) and 'BHRS 2-3' (race 3 resistant; I-7 gene introgressed from S. pennellii 'PI 414773'). Race-specific molecular marker systems were also employed in these studies. Surveys indicated that races 1 and 2 are widespread in Brazil. However, because of the massive use of hybrids with both I and I-2 genes, no new isolates of these two races have been incorporated into the Brazilian FOL collection since 2002. Race 3 isolates were initially restricted to commercial fields located in mild climate areas encompassing the Atlantic Forest biome in Espírito Santo and Rio de Janeiro states (southeast region). However, recent surveys indicated that FOL race 3 isolates were also present in the states of Minas Gerais (in the continental southeastern region) and Bahia (northeastern region), thus expanding the geographical distribution of this pathogen. The fast and widespread dissemination of race 3 in Brazil suggests its introduction into new producing areas via contaminated propagative material. The severe outbreaks of race 3 were the basis for the substantial replacement of susceptible hybrids by resistant ones (mainly with the I-3 gene). 'BRS Imigrante' (released by Embrapa) was the first commercial hybrid with the I-7 gene. 'BRS Imigrante' and the I-3 gene-carrying hybrids show high levels of resistance in distinct production regions even when the tomato crop is established in heavily infested soils. Multi-race-resistant hybrids are now prevalent in all FOL-infested areas, increasing the selection pressure in favor of new FOL variants.