During 1996, soil sampling was performed three times (February, April, and June) in a crop rotation experiment sown in 1963 at INIA La Estanzuela. The objectives of this study were: 1. to adjust methodologies for studying Fusarium population in the soil; 2. to measure the soil population of Fusarium species under four different cropping systems; and 3) to study the seasonal fluctuation in Fusarium spp. population. Four systems differing in the pasture type and duration were selected: System 2, crop rotation without pasture; System 3, crop rotation including birdsfoot trefoil pastures (50% of the time); System 4, crop rotation including legume pastures (66% of the time); and System 5, crop rotation including legume-grass pastures (50% of the time). Twenty soil cores were randomly taken with a soil tube and pooled to form a composite sample from each plot. Soil samples were processed in the laboratory with the soil-dilution plate technique, using two selective media for Fusarium, Nash-Snyder and Komada, and testing two moments of plating. Fusarium colonies were identified at the species level, and fungal density was expressed as colony forming units per gram of soil (CFU/g). Recovery of Fusarium spp. was significantly greater on Komada than on Nash-Snyder. Eight Fusarium taxa were recovered on both selective media (F. oxysporum, F. merismoides, F. equiseti, F. solani, F. tricinctum, F. chlamidosporium, F. graminearum and F. nivale), while one species (F. moniliforme) was isolated only on Komada, and two species (F. poae and F. avenaceum) only on Nash-Snyder. F. oxysporum was the most frequently isolated species and its relative density accounted for 64% and 45% of the total on Komada and Nash-Snyder, respectively. Recovery of Fusarium spp. and other soil fungi was significantly affected by the plating moment, the larger the time between sampling and plating date, the less the recovery of Fusarium spp. and the greater the recovery of other genera. Fusarium spp. were recovered from all cropping systems and their relative density ranged from 1518 to 3784 CFU/g of soil. Regardless of the cropping system, F. oxysporum was the most frequently isolated species (54%). Recovery of Fusarium spp. and F. oxysporum was significantly greater at System 4 (66% legume pastures) and significantly less at System 2 (continuous crop without legume pastures), for the three sampling dates. The statistically significant association of these fungi with the crop rotation systems demonstrated that the crop sequence that characterizes each system has a long-term effect on the soil fungal populations. Thus, crop rotation may contribute to reduce or maintain lower populations of Fusarium spp. and F. oxysporum in the soil. Seasonal fluctuation in recovery of Fusarium spp. occurred over the three sampling dates (2876, 1730, and 2803 CFU/g, for February, April, and June, respectively).

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