Phosphate and the fungicide, benomyl, were applied to two alpine grasslands in the Kananaskis Valley, Alberta, Canada, to test the hypothesis that a reduction in leaf phosphorus concentration should result from reduction in vesicular-arbuscular mycorrhizal (VAM) infection if mycorrhizal hyphae are the main route of P uptake by roots.
Benomyl successfully reduced infection in the roots of several species. At one site the result was an increase and not a decrease in leaf N and P concentrations, which was directly proportional to the original infection level, implying a benefit to the plant from the reduction in infection. At the other site, opposite trends were observed before and after a severe drought episode, implying that infection was initially deleterious and later beneficial. These results are adduced as evidence to support a hypothesis that VAM infection is only beneficial to plants under particular environmental conditions or at specific times in the year or life-cycle.