APHANOMYCES-EUTEICHES INOCULUM POTENTIAL - A ROLLED-TOWEL BIOASSAY SUITABLE FOR FINE-TEXTURED SOILS

A sensitive measure of soil inoculum potential is needed res evaluate field management of common root rot (aphanomyces euteiches) in peas (P isum sativum). A modified rolled-towel (RT) bioassay had been proposed to measure soil inoculum potential in fine-textured soils used for pe a production in Minnesota, Homogenized soil was used because organic d ebris containing the inoculum could not be separated by wet sieving. T he poor precision prompted an evaluation of procedures to improve this modified RT bioassay, Seed treatment with a 5% solution of sodium hyp ochlorite before pea seed germination and plant isolation procedures d uring the RT bioassay preparation/incubation reduced seedborne contami nation and seedling loss to less than 5%. Tests conducted with pasteur ized soil that was artificially infested with oospores showed the regi on of the pea taproot 1 to 2 cm below the seed to be more susceptible to infection (33% compared with 15% infected seedlings) than the regio n 1 to 2 cm above the root tip. A soil volume of 1.0 cm(3) increased i noculum potential compared with 0.5 cm(3) applied to each seedling but did not influence the random error; the 40-seedling compared with the 20-seedling RT bioassay reduced random error from 18 to 12%. The modi fied RT bioassay conducted on soil that was artificially infested afte r steam treatment or without steam treatment showed superior performan ce when using 40 seedlings compared with 20 seedlings when evaluated f or accuracy and precision. Multiple infection theory demonstrated more multiple infections in the RT bioassay with a 0.5 cm(3) soil volume a pplied to each seedling, which shows that soil mass is a factor preven ting a higher percentage of infected seedlings, These modifications to the RT bioassay improved the method enough to reduce the random error by one-half compared with using homogenized soil without the proposed modifications.