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Identification of Fusarium oxysporum f. sp. lycopersici Race 3 Infecting Tomatoes in Northeast Brazil

The three races of Fusarium oxysporum f. sp. lycopersici (FOL) are important tomato pathogens throughout the world, causing severe economic losses (1). In Brazil, races 1 and 2 are widespread, but the current geographic distribution of race 3 is restricted to the mild climate areas of Espírito Santo and Rio de Janeiro States in the southeast region (2,3). Here we report the spread of FOL race 3 to the warm northeast region of Brazil. Plants in commercial fields of the hybrid ‘Alambra’ (resistant to FOL races 1 and 2) were found displaying chlorosis, vascular browning, and wilt symptoms in Jaguaquara County, Bahia State, Brazil. Disease incidence ranged from 10 to 50%. The virulence profile of six isolates obtained from three distinct tomato-producing fields was investigated by root-dipping inoculation (10⁶ conidia/ml) of 21-day-old seedlings from a set of FOL race differential accessions: ‘Ponderosa’ (susceptible to all races), ‘IPA-5’ (FOL race 1 resistance; I-1 locus); ‘Alambra’ and ‘Floradade’ (FOL races 1 and 2 resistance; I-2 gene), and Solanum pennellii ‘LA 716’ (resistant to all three races; I-3 locus). All six isolates were able to induce severe wilt symptoms in 100% of the plants from all lines but S. pennellii ‘LA 716’. FOL race 3 identity was confirmed via PCR assays employing a specific set of primers that are able to discriminate all the three FOL races as well as F. oxysporum f. sp. radicis-lycopersici isolates (1). Total DNA was extracted from pure fungal colonies growing in agar medium. The typical FOL race 3 amplicon profiles (i.e. positive for the primers uni, sp13, and sp23 and negative for the primer sprl) were observed only in the six FOL 3 isolates from Bahia as well as in five reference isolates of race 3 (previously obtained from tomato in Espírito Santo and Rio de Janeiro States), thus confirming their race identities. This recent, fast, and wide geographic expansion of the FOL race 3 in Brazil suggests that the pathogen has been introduced into new tomato producing areas via either contaminated seeds or seedlings. Because of the complexity of establishing effective chemical and cultural control strategies, these epidemics caused by FOL race 3 in distinct areas of Brazil might cause the replacement of the currently grown susceptible hybrids by resistant ones.

References: (1) Y. Hirano and T. Arie. J. Gen. Plant Pathol. 72:273, 2006; (2) A. Reis et al. Fitopatol. Bras. 30:426, 2005; (3) A. Reis and L. S. Boiteux. Hort. Bras. 25:451, 2007.