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Development of an Infection Assay for Sporisorium reilianum, the Head Smut Pathogen on Sorghum

The practical value of dry teliospores of Sporisorium reilianum as inoculum in head smut infection assays was determined from a series of studies using three different isolates of the pathogen. Overall germinability of teliospores ranged from 8.0 to 28.0% on culture media after 48 h of incubation. Teliospores germinated at slower rates in the vicinity of sorghum seedlings growing on germination paper, reaching maximum values of 11.5 to 13.0% after 120 h. These results suggest adaptation in the germination strategy of S. reilianum, probably modulated by the variable soil environment. Fifteen percent (15%) of the seedlings of a head smut susceptible maize hybrid inoculated with teliospores of S. reilianum at root protrusion openings of the mesocotyl tissues became infected, while seedlings similarly inoculated with sporidial suspensions remained uninfected. Three sorghum lines susceptible to head smut were inoculated in three separate experiments with dry teliospores of isolates from Corpus Christi and Taylor, Texas. Seeds were imbibed for 18 h, and the seedlings were planted in soil at 40% moisture content (wt/wt) and covered with a 1:50 (vol/vol) mixture of teliospores and autoclaved soil. Infection levels obtained in these experiments were 2.5 to 2.8 times higher than field infection levels on susceptible lines RT×7078 and B1. In this study, average infection levels of 65 to 79.5% and 84 to 87% for RT×7078 and B1, respectively, were obtained with Corpus Christi isolates of S. reilianum; whereas Taylor isolates infected 91.9 and 82.3% of the plants in these two lines. It is postulated that the increased infection efficiency observed with this inoculation technique results from uniform and higher levels of inoculum, timely delivered under stable soil conditions that provide the pathogen with an environment more conducive for infection.