Detection of Fusarium oxysporum f. sp. melonis Race 1 in Soil in Colima State, Mexico.

Abstract

During the winters of 2002 and 2003, a wilt occurred in melons cultivated on 1,500 ha in Colima State, Mexico. Yield losses reached 25% of final production, despite soil disinfestation with 60% methyl bromide and 40% chloropicrin. On the basis of the observation of plants with necrotic xylem, yellowing, and wilting of leaves, this disease was identified provisionally as Fusarium wilt. During February 2003, four soil samples from affected fields were plated onto a Fusarium-selective medium (1), which resulted in the detection of 2,260 ± 357, 179 ± 76, 668 ± 357, and 1,391 ± 256 CFU/g of F. oxysporum (3). Thirty-one randomly chosen isolates were used to inoculate differential cultivars of melon as described by Risser et al. (4). The cultivars were Amarillo Canario (susceptible to all races), Diana (resistant to races 0 and 2), Tango (resistant to races 0 and 1), and Vulcano (resistant to races 0, 1, and 2) (2). Ten plants of each cultivar, grown on sterilized vermiculite, were inoculated at the first true-leaf stage by drenching with 200 ml of a conidial suspension (1 × 105 CFU/ml) of each isolate. Noninoculated plants of each cultivar served as controls. Plants were maintained in a growth chamber with a 16-h photoperiod (18 × 103 lux) and temperatures at 23 to 25°C. Yellowing, wilt, and vascular discoloration symptoms developed on cvs. Amarillo Canario and Diana following inoculation with each of the 31 isolates, while noninoculated plants remained symptomless. F. oxysporum was consistently reisolated on potato dextrose agar from the affected plants. On the basis of the combination of affected cultivars, all isolates were identified as F. oxysporum f. sp. melonis race 1. To our knowledge, this is the first report of F. oxysporum f. sp. melonis race 1 in Colima State, Mexico.