EFFECTS OF WHEAT VOLUNTEERS AND BLACKGRASS IN SET-ASIDE FOLLOWING A WINTER-WHEAT CROP ON SOIL INFECTIVITY AND SOIL CONDUCIVENESS TO TAKE-ALL

Two experiments were carried out in France in which disease indices we re used to evaluate the effects of wheat volunteers and blackgrass (Al opecurus myosuroides) on soil infectivity and soil conduciveness to ta ke-all caused by Gaeumannomyces graminis var. tritici. Soil infectivit y was evaluated by measuring the disease index on susceptible wheat pl ants grown on soil samples collected from the field. Soil conducivenes s to the disease was obtained by measuring disease indices on plants g rown on soil samples to which different amounts of take-all fungus ino culum were added. One experiment(Expt. 1) was carried out using soils from farmers' fields (two fields in 1994 and two in 1995); soil infect ivity and soil conduciveness were evaluated for three experimental sit uations: bare soil, soil with wheat volunteers and soil with blackgras s plants. In 1994 the soil infectivity was zero in bare soil, high wit h the wheat cover, and intermediate with the blackgrass cover. In 1995 the soil infectivity was uniformly low for all three conditions. Soil s bearing wheat were less conducive than bare soil, soils bearing blac kgrass and bare soils were similarly conducive. A second experiment (E xpt. 2) carried out in 1995 compared the soil infectivity and soil con duciveness to take-all of soils planted with wheat or blackgrass in se t-aside land after periods of wheat monoculture of 0-6 yr. The soil in fectivity was low for all treatments. The soil was more conducive afte r blackgrass than after wheat. In both cases, the soil conduciveness w as less when the monoculture had continued for more than 4 yr. The dec line was less after blackgrass than after wheat. Thus, whenever set-as ide is set up during the increase phase of the disease in fields with cereal successions, abundant wheat volunteers might hinder the expecte d positive effect of a break in cereal successions on take-all develop ment. The presence of blackgrass in a set-aside field, with significan t soil infectivity and high soil conduciveness, might increase the ris ks of take-all development in a wheat crop following set-aside.