Vertical and temporal distribution of Fusarium solani and Heterodera glycines in fields with sudden death syndrome of soybean

Sudden death syndrome (SDS) is an important soilborne disease of soybean ca used by the fungus Fusarium solani. This disease is also associated with th e soybean cyst nematode (SCN), Heterodera glycines. To quantify the vertica l and temporal distribution of these pathogens in the soil profile, a study was conducted at the University of Arkansas Pine Tree (PTS) and Cotton Bra nch Stations (CBS) in 1991. Soil samples were taken at six depths (0 to 7, 7 to 15, 15 to 30, 30 to 50, 50 to 75 and 75 to 100 cm) at four growth stag es (planting, R2, R6 and harvest) and numbers of F. solani, SCN and total f ungi were determined. Soil physical and chemical characteristics were measu red at planting and soybean root densities were measured at R2 and R6. Dise ase development was followed weekly in August and September. Soil numbers o f F. solani were greatest in the top 15 cm of soil throughout the season. S CN egg numbers also were greatest in this zone until harvest. At harvest, n umbers significantly increased at depths of at least 50 cm. The greatest ro ot densities were in the 0- to Ij-cm zone. At both locations, the 0- to 15- cm zone was over a plow pan under which the pH decreased from 6.4 to 7.0 ab ove the pan to 3.8 to 4.5 below the pan. Total fungal numbers were greatest in the upper soil layers at planting but declined until there were no sign ificant differences in numbers of total fungi with depth from R2 to harvest . SDS developed sooner and was more severe at PTS than at CBS. PTS had grea ter numbers of F. solani but supported lower numbers of SCN at the end of t he season than CBS. (C) 1998 Elsevier Science Ltd. All rights reserved.