Creation of a non-mycorrhizal control for a bioassay of AM effectiveness 2. Benomyl application and soil sampling time

The study aimed to determine the most appropriate soil sampling time as wel l as dose and application time of benomyl for assessing AM effectiveness in field soils in a bioassay in a growth chamber. AMF infectivity and AM effe ctiveness assessed using benomyl were compared at seven sampling times betw een the thaw and the autumn. The effect of benomyl dose and application tim e on mycorrhizal suppression and phytotoxicity in irradiated soil was studi ed. Doses of 10-100 mg kg(-1) and application times 2 weeks before sowing, at sowing and 1 week after sowing were investigated. Various Finnish field soils with their indigenous AMF communities were used. The main test plant species was oil-seed flax (Linum usitatissimum). In a comparison of samplin g time, barley (Hordeum vulgare) was also used and phytotoxicity was studie d additionally on red clover (Trifolium pratense), barley and pea (Pisum sa tivum) mutants. Sampling in the spring after the thaw resulted in the highe st infectivity and AM response and the dearest differences between soils wi th varying AM potential. No evidence of temporal variation in benomyl effec tiveness on mycorrhiza was found. The dose of benomyl sufficient to create a control with suppressed mycorrhization was 20 mg per kg soil at target mo isture incorporated in the soil. Plant growth reduction in irradiated soil was observed with benomyl application 1 week after sowing only with flax an d red clover. The most effective application time for benomyl was immediate ly before sowing.