Vascular wilting diseases have become one of the most serious diseases of tomato (Lycopersicon esculentum) throughout the Baja California Peninsula. Since the winter of 2004, a disease with symptoms characteristic of those caused by a Fusarium species has been observed in commercial fields near La Paz and Todos Santos in the state of Baja California Sur (BCS). Symptoms include typical one-sided wilting and dark brown vascular discoloration. Upper stem tissues and wilted seedlings were disinfested by immersion in a 1.0% aqueous solution of sodium hypochlorite for 2 min, rinsed in sterile water, and placed on Komada's medium (pH 6.8) at 22 ± 3°C. After 72 h, hyphal growth was recovered and subcultured on carnation leaf agar and potato dextrose agar and incubated at 25°C in 12-h light/dark cycles. Identification was based on colony morphology, conidial characteristics, and molecular techniques. White cottony mycelium, reddish coloration of the medium, ovoid two-celled macroconidia, and large macroconidia, all characteristic of F. oxysporum, were observed (2). Polymerase chain reaction and restriction fragment length polymorphisms with restriction enzymes EcoRI, RsaI, and HaeIII were used to characterize 24 isolates (sampled during January 2005) from La Paz (Fol-LaP) as formae speciales lycopersici and assigned to vegetative compatibility group 0030 (1). Confirmation of pathogenicity and race determination for the Fol-LaP isolates were as described previously (3). Mexican isolates of races 2 and 3 (one each) were included as positive controls. Conidial suspensions of 7 × 105 CFU/ml were used to inoculate differential tomato cvs. Bonny Best (Millington Co., universally susceptible), Tequila F1 (Vilmorin, race 1 resistant), Rio Grande (Harris Moran, race 1 and 2 resistant), and Sebring (Rogers, race 1, 2, and 3 resistant). Plants at the first true-leaf stage were inoculated by dipping their roots in the conidial suspension. Inoculated seedlings were transplanted into pots containing a sterile 5:1:1 mixture of sand/vermiculite/soil (v/v/v) and maintained in the greenhouse at 25 to 28°C under natural daylight. An equal number of plants of each cultivar dipped in water were used as controls. The experimental design was a completely randomized type with six replications (pots) containing four seedlings per pot. The test was done twice. The most susceptible plants inoculated by root-dipping developed typical symptoms of wilt, slight vein clearing on outer leaflets, stunting, dark brown vascular discoloration, and death. F. oxysporum was recovered from all symptomatic plants, whereas noninoculated tomato seedlings showed no symptoms. According to differential infection and symptomatology observed on infected cultivars, 62.5% of the isolated strains were identified as race 2, 25% as race 3, and 12.5% as an undetermined race isolated from Sebring. The presence of race 3 in BCS has important epidemiological implications since it has been reported on tomato in Sinaloa (4). The potential spread of the pathogen on introduced transplants represents a risk to tomato crops on the peninsula. To our knowledge, this is the first report of F. oxysporum f. sp. lycopersici race 3 in the state of BCS, Mexico.
References: (1) G. Cai et al. Phytopathology 93:1014, 2003. (2) P. E. Nelson et al. Fusarium species. Pennsylvania State University Press, University Park, 1983. (3) B. A. Summerell et al. Plant Dis. 87:117, 2003. (4) J. G. Valenzuela-Ureta et al. Plant Dis. 80:105, 1996.