Phytotoxicity of Fosetyl Al and Phosphonic Acid to Maize During Production of Vesicular-Arbuscular Mycorrhizal Inoculum. N. P. SEYMOUR, Soil Microbiologist, Queensland Wheat Research Institute, P.O. Box 2282, Toowoomba, Q. 4350. J. P. THOMPSON, Senior Principal Soil Microbiologist, and M. L. FISKE, Field Assistant, Queensland Wheat Research Institute, P.O. Box 2282, Toowoomba, Q. 4350. Plant Dis. 78:441-446. Accpted for publication 14 Decembr 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0441.

Research into the function of vesicular arbuscular mycorrhizae (VAM) requires production of sufficient quantities of high-quality inoculum. Contamination of one of our Glomus cultures on maize with Pylhhim myrioiylum prompted us to assess the effects of anti-oomycete fungicides on colonization of maize by strains of Glomus mosseae, G. macrocarpum, G. elunicalum, and G. microcarpum. Following mycorrhizal inoculation, partially sterilized soil in pots was treated with fungicides at rates recommended for Pythium control: I) metalaxyl (0.05% Ridomil 250WP, 100 ml per pot as a soil drench, or 2.5 g of Ridomil 50G per 10 L of soil), 2) fosetyl Al (0.1% Aliette 740WP, 200 ml per pot as a soil drench), and 3) phosphonic acid (0.25% Foli-R-Fos 400, 100 ml per pot as a soil drench), all applied before planting the maize. Plants treated with fosetyl Al or phosphonic acid had stunted roots and tops that were rosetted and spindly with white variegated streaking of the leaves, symptomatic of induced zinc deficiency. Percent colonization of the roots was slightly decreased by fosetyl Al and phosphonic acid, but the total length of mycorrhizal root was markedly decreased. Metalaxyl had no phytotoxic effects on maize. Excellent mycorrhizal colonization of the Glomus-inoculated pots was obtained with the metalaxyl treatments, indicating that this fungicide did not adversely affect VAM cultures. Melalaxyl (granular form) is recommended for routine addition to open-pot cultures of VAM fungi on maize.