Spatio-temporal analysis of soybean root colonization by Fusarium solani f. sp. glycines in fields

Soybean roots were sequentially collected from two no-till fields from June 1997 through December 1998. Roots were ground to isolate and enumerate the fungus Fusarium solani f. sp. glycines, the causal agent of soybean sudden death syndrome (SDS), to obtain CFU per gram of root. The log CFU [log10(CFU + 1)] versus sampling time was used to produce the pathogen population curve, and the area under the pathogen population curve (AUPC) was calculated for each plot. The average log CFU from all plots for each sampling date was used to fit the logistic equation. Plot data of log CFU at each sampling time of the growing season, AUPC, and foliar disease index (FDX) were correlated with each other. Correlations among the log CFU in root residue in the winter of 1997, the log CFU and FDX in the 1998 growing season were also conducted. Geostatistics was applied to determine the spatial dependence in root colonization for different lag distance in the fields using semiviograms. Spatio-temporal autocorrelations of root colonization were studied using a computer model STAUTO. During the growing season, pathogen population in roots followed logistic growth in both fields. Pathogen populations in root residue decreased during the winter of 1997 and increased slightly in the spring of 1998 prior to planting. AUPC significantly correlated with FDX in both fields in 1997. Pathogen populations in root residue at three sampling dates in the winter of 1997 significantly correlated (r = 0.47-0.53) with FDX of the 1998 growing season in one field. No spatial dependence in root colonization was detected early in the growing season. However, some spatial dependence in certain directions of the fields was detected later in the growing seasons. Spatial dependence in AUPC in the across-rows direction was detected in both fields in 2 years. Spatial lag orders 0 and 1 were significantly correlated with temporal lag order 1 in both within-row and across-row directions in field 1 in 1997.