Limitations to using benomyl in evaluating mycorrhizal functioning

Arbuscular mycorrhizal (AM) grasses compete for nutrients with ectomycorrhizal (EM) pine in the southeastern United States. Our objective was to determine if benomyl could be used to selectively inhibit the function of AM and thereby reduce grass competition in the filed. The effects of Benlate (active ingredient: benomyl) in the greenhouse and field were evaluated. No effect was observed on pine inoculated with Pisolithus tinctirius in the greenhouse. Colonized root length of benomyl-treated Zea mays L. plants inoculated with Glomus sp. in the greenhouse remained static over time and the response was not dose dependent at concentrations of 0, 20, 60 and 150 kg benomyl ha^-1 equivalent. In contrast, colonization of non-treated plants increased over time. In the field, a minimal reduction of grass colonization was observed following four applications of benomyl ranging from 5 to 20 kg ha^-1. We conclude that benomyl can successfully inhibit development of AM fungi under controlled conditions in the greenhouse with no inhibitory effects on the EM fungus P. tinctorius; however, in the field several factors may interfere with the effect of benomyl on AM fungi. These factors include: (a) the presence of ground cover which obstructs penetration of the fungicide to the soil, (b) timing of application in relation to mycorrhizal development, and (c) the application method of benomyl, a soil drench being preferable to a foliar spray.