Evaluation of Endemic Foliar Fungi for Potential Biological Control of Johnsongrass (Sorghum halepense): Screening and Host Range Tests

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Abstract
Diseased johnsongrass leaves were collected during 1985-1986 in North Carolina in a search for leaf-infecting fungi as potential biocontrol agents of johnsongrass. Seven pathogenic fungi were isolated and tested. After preliminary evaluation of virulence, dew requirments, and sporulation capacity. Exserohilum turcicum, Colletotrichum graminicola, and Gloeocerospora sorghi were selected for second-stage tests. Multiple isolates of these three fungi were compared in separate tests; isolates of each fungus differed in their virulence and host specificity. Data on E. turcicum isolates suggests physiologic specialization with regard to virulence to sorghum hybrids and johnsongrass from different seed collections. The most virulent isolate, with the best or average specificity to johnsongrass, was selected for each fungus for subsequent broad-spectrum host range tests. Data on leaf sporulation from inoculated plants indicate that G. sorghi was more host-specific than the other two fungi; it was compatible only with Sorghum spp., whereas E. turcicum and C. graminicola were compatible with reactions. Dicotyledonous species were immune to all three fungi. Among the graminaceous plants, oats, wheat, and barley were undamaged or the least damaged, whereas all Sorghum spp. (sorghum, johnsongrass, sudangrass, and shattercane) were severely injured by inoculation with high spore concentrations suitable for mycoherbicide application. Overall results suggest that the three tested fungi are too virulent to be used as mycoherbicides in corn and sorghum but may be safe in dicotyledonous crops.