Tillage Modifications of Dry Edible Bean Root Colonization by VAM Fungi1

Soils compacted by excessive secondary tillage and traffic restrict the growth and function of plant root systems. The influence of soil compaction on root and the symbiotic vesicular‐arbuscular mycorrhizal (VAM) fungal associations is essentially unknown. The VAM fungal colonization of dry edible bean (Phaseolus vulgaris L.) roots was investigated on Aeric Haplaquept clay, Typic Hapludalf loam, and Typic Haplaquoll sandy loam soils. Three cultivars were subjected to secondary tillage and traffic treatments on several field experiments. The influences of soil porosity, bulk density, depth and type on the colonization of dry edible bean roots by VAM fungi were characterized 45 days after planting. Excessive secondary tillage and traffic increased the bulk densities of all soil types, decreased plant growth and root development. Colonization of dry edible bean root systems was a function of soil type and decreased with increasing secondary tillage and depth within the soil profile. Dry edible bean cultivars having vigorous root systems which penetrated below the Ap horizon of compacted soils maintained high levels of colonization by VAM fungi. Roots of the cv., Seafarer, which are susceptible to both soil compaction and root rot diseases, had the lowest amount of colonization when growing below the Ap horizon.