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Physiologic Specialization in Trichometasphaeria turcica f. sp. zeae and T. turcica f. sp. sorghi in Hawaii. R. R. Bergquist, Assistant Professor, Department of Plant Pathology, College of Tropical Agriculture, University of Hawaii, Kapaa 96746; O. R. Masias, Graduate Research Assistant, Department of Plant Pathology, College of Tropical Agriculture, University of Hawaii, Kapaa 96746. Phytopathology 64:645-649. Accepted for publication 19 November 1973. DOI: 10.1094/Phyto-64-645.

A new race of Trichometasphaeria turcica f. sp. zeae, virulent to corn lines carrying Ht resistance genes (Ht₁^A, Ht₁^B, Ht^Bw, and Ht^Mol) is currently reported from Hawaii. This race is designated race 2 to distinguish it from race 1, already known in Hawaii, that is avirulent on corn lines with these monogenic sources of resistance. Resistance gene Ht₂ conditioned a chlorotic-lesion reaction in seedlings and mature plants to both races. Non-host-specific and host-specific isolates of race 2 were identified from nature; i.e., pathogenic to both corn and sorghum or to corn alone, respectively. Races 1 and 2 of T. turcica f. sp. sorghi were identified on corn differentials. Corn isolates of T. turcica were generally characterized as having gray to green-white, profuse aerial mycelium whereas colonies of sorghum isolates were dark olivaceous, with scant, appressed aerial hyphae. T. turcica isolates from corn or sorghum could not be distinguished on the basis of conidial morphology or position of germ-tube protrusion. A phytoalexin-like substance obtained from corn and sorghum leaves of noncompatible, host-pathogen interactions was inhibitory to spore germination.