LIMITATIONS TO USING BENOMYL IN EVALUATING MYCORRHIZAL FUNCTIONING

Arbuscular mycorrhizal (AM) grasses compete for nutrients with ectomyc orrhizal (EM) pine in the southeastern United States. Our objective wa s to determine if benomyl could be used to selectively inhibit the fun ction of AM and thereby reduce grass competition in the field, The eff ects of Benlate (active ingredient: benomyl) in the greenhouse and fie ld were evaluated. No effect was observed on pine inoculated with Piso lithus tinctorius in the greenhouse. Colonized root length of benomyl- treated Zen mays L. plants inoculated with Glomus sp. in the greenhous e 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 nontreated plants increased over time. In th e field, a minimal reduction of grass colonization was observed follow ing four applications of benomyl ranging from 5 to 20 kg hd(-1) We con clude that benomyl can successfully inhibit development of AM fungi un der 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 i nclude: (a) the presence of ground cover which obstructs penetration o f 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.