Evaluation of the build-up of Pythium populations highly virulent to corn and soybean in long-term rotation fields

Abstract

The goal of this research was to determine whether Pythium isolates highly virulent to both corn and soybean occur in a higher frequency in corn/soybean rotation fields than in the monocultural fields of the two crops. The first objective in this study was to determine whether individual Pythium isolates from long-term corn/soybean rotation can attack both corn and soybean. A total of 163 Pythium hyphal cultures were collected from 73 commercial corn/soybean rotation fields and five experiments were conducted to test the pathogenicity of the Pythium isolates on the two crops. The results showed that, regardless of their sources of Pythium isolates: soil, diseased soybean seedlings, or diseased corn seedlings, many of them were pathogenic, even highly virulent to both crops. The second objective was to determine whether there is a greater frequency of Pythium isolates highly virulent to both corn and soybean in corn/soybean rotation fields than in moncultural fields of corn or soybean. The difference in number of highly virulent isolates to both crops was found between CS and CC, but not between CS and SS, with more highly virulent isolates in SS and CS than in CC. Finally, the occurrence and relative abundance of Pythium species was quantified and compared among the three cropping systems by using a cucumber baiting assay and P10VP agar medium. The soil samples were collected from the same fields as above. Four Pythium species were isolated: P. ultimum, P. torulosum and P. paroecandrum occurred in soils of all three cropping systems, and P. spinosum occurred in one SS and one CS field. More Pythium isolates were recovered from CS and SS composite soil samples than from CC samples (P=0.001). P. torulosum was isolated most frequently.