Sulfur's Potential to Decrease the Severity of Root Rot Caused by Aphanomyces euteiches

Aphanomyces root rot caused by the soilborne oomycete, Aphanomyces euteiches, is a major cause of reduced crop yields in alfalfa (Medigaco sativa).  The pathogen has overcome host resistance and there are no chemical treatments that prevent or remediate the disease.  There is a correlation between increased sulfur and decreased disease in the field. This study’s three objectives were to investigate whether sulfur: (1) directly inhibits A. euteiches growth, (2) impacts A. euteiches ability to infect alfalfa, and (3) fertilization effects on the severity of disease in alfalfa. In the first experiment we measured the diameter of A. euteiches growth on corn meal agar (CMA) plates with various CaSO₄ concentrations. Results showed no significant difference between treatments, suggesting sulfur is not directly toxic to A. euteiches. In experiment two A. euteiches was grown CMA plates with varying CaSO₄ concentrations and alfalfa seedlings were placed onto the plates. The results measured the percent of seedlings infected with oospores, which indicate disease.  The trend observed suggested possible correlation between increased sulfur and decreased disease in alfalfa.  The final experiment, currently in progress, measures the severity of alfalfa root rot in greenhouse grown plants.  The alfalfa seedlings will be exposed to A. euteiches mycelia inoculants and treated with or without CaSO₄.  Based on field observations, this technique of disease control is effective; however the mechanisms remain in the investigative phase.  The observed effect of sulfur on alfalfa infected with A. euteiches may be the result of the production of sulfur containing secondary root compounds.