Population Dynamics of Soilborne Fungi in a Field Multicropped to Rye and Soybeans Under Reduced Tillage in Florida

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Abstract
Soil sampled to a depth of 5 cm in a reduced-tillage, multicropped field in Florida was assayed for palnt pathogenic and nonpathogenic fungi. During a 4-yr period prior to the start of the study, plots in the field were not tilled and were either subsoiled or not subsoiled; a multicropping sequence of winter rye followed by soybeans in the summer was maintained. At the beginning of the study, subsoiled and nonsubsoiled plots were either tilled to a depth of 15 cm or not tilled before the rye crop was planted. Soil samples were collected on 22 days over a period of 27 mo. Of the 16 genera of fungi identified, species of Aspergillus, Fusarium, Penicillium, Rhizopus, and Trichoderma accounted for up to 75% of the total fungal population detected. Plant pathogens in the genera Rhizoctonia and Pythium accounted for a much lower proportion of the total fungal population detected in the soil. Anastomosis group four (R. solani AG 4) and a binucleate anastomosis group of Rhizoctonia (CAG 3) were the predominant members of Rhizoctonia, and Pythium irregulare and P. acanthicum were the most common species of Pythium isolated from soil. Propagule densities of most of the fungi that were monitored were significantly higher (P = 0.05) in no-till plots than in plots tilled to 15 cm, regardless of subsoiling treatment. Differences in detected populations in no-till plots and plots tilled to 15 cm were usually greatest during the rye crop. Although tillage had a significant effect on propagule densities of Rhizoctonia spp., propagule densities of R. solani AG 4 were not affected by tillage. Instead, propagule densities of this pathogen were influenced by the presence or absence of a susceptible host (rye or soybean seedlings).