Received for publication 21 Sept. 1988. This paper is part of a thesis submitted by I.A.M. in partial fulfillment of the requirements for the MS degree. Thanks to Elizabeth Loder for technical assistance, to W.F. Mai for guidance and provision of growth chamber and laboratory space in the Dept. of Plant Pathology, and to Harris Seeds, Rochester, N.Y., and International Seeds, Halsey, Ore., for providing ‘Sparky’ marigold and ‘Companion Mix’ grass seed, respectively. USDA Hatch Project funds (NYC J55414) helped support this research. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.