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 well 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 effectiveness assessed using benomyl were compared at seven sampling times between the thaw and the autumn. The effect of benomyl dose and application time on mycorrhizal suppression and phytotoxicity in irradiated soil was studied. 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 sampling time, barley (Hordeum vulgare) was also used and phytotoxicity was studied additionally on red clover (Trifolium pratense), barley and pea (Pisum sativum) mutants. Sampling in the spring after the thaw resulted in the highest infectivity and AM response and the dearest differences between soils with varying AM potential. No evidence of temporal variation in benomyl effectiveness on mycorrhiza was found. The dose of benomyl sufficient to create a control with suppressed mycorrhization was 20 mg per kg soil at target moisture incorporated in the soil. Plant growth reduction in irradiated soil was observed with benomyl application 1 week after sowing only with flax and red clover. The most effective application time for benomyl was immediately before sowing.