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Isolation of Phytoalexins from Corn with Monogenic Resistance to Helminthosporium turcicum. S. M. Lim, Research Associate, Department of Plant Pathology, University of Illinois, Urbana 61801; A. L. Hooker(2), and J. D. Paxton(3). (2)(3)Processor, and Assistant Processor, respectively, Department of Plant Pathology, University of Illinois, Urbana 61801. Phytopathology 60:1071-1075. Accepted for publication 17 February 1970. DOI: 10.1094/Phyto-60-1071.

Two phytoalexins, A1 and A2, were found in monogenic resistant corn. These phytoalexins were produced when spore suspensions of Helminthosporium turcicum from corn were incubated on detached corn leaves. An isolate of H. turcicum from Johnson grass did not stimulate production of these phytoalexins. Similar phytoalexins were also isolated from extracts from infected, intact leaves. No phytoalexin was found in diffusates or extracts from infected, susceptible corn or noninfected corn leaves of either genotype. Phytoalexin A1 has a paper chromatography R>f value of 0.87 in upper phase of BAW, and an ultraviolet absorption spectrum with a maximum at 280 nm. Phytoalexin A2 has an Rf value of 0.97 and an ultraviolet absorption spectrum with a maximum at 270 nm. Both phytoalexins are blue fluorescent compounds. Reaction with diazotized sulfanilic acid reagent indicates that they are probably phenolic. Fungal isolates varied in their response to phytoalexin. At the same phytoalexin concn, spores of a highly pathogenic isolate germinated better than those of a weakly pathogenic one. Also, highly pathogenic isolates induced the formation of a higher concn of phytoalexins than did weakly pathogenic isolates. Results indicate that monogenic resistance of corn to pathogenic isolates of H. turcicum probably is chemical in nature and is due to phytoalexins produced in response to infection by this fungus.

Additional keywords: Zea mays, leaf dropdiffusate technique, leaf extracts, bioassay of phytoalexin, dosage response effects.