Abstract
During the summer of 1986, experiments were conducted under semiarid conditions in Southwestern Puerto Rico to evaluate yield losses in grain sorghum inbred lines (TAM42B and SC307, moderately resistant and susceptible to rust, respectively) and forage sorghum hybrids (Haygrazer and FS25A, moderately and very susceptible to rust, respectively). A split plot design was used in which half of each plot was sprayed with triadimefon (0.5 kg/ha) at boot stage followed by an application of oxycarboxin (1.0 kg/ha) 2 weeks thereafter, and the other half was nontreated (control). In all sorghum entries, except TAM 428, yields increased significantly (P = 0.05) with fungicidal rust control over those of nontreated plots. When treated with triadimefon and oxycarboxin, FS25A, Haygrazer, and SC307 showed 23, 16, and 39% greater yield than respective nontreated plots. Rust control increased 100-seed weights over the nontreated controls by 19, 25, and 45% for the respective varieties. Nonsignificant increases in seed density (approximately 5%) were found in the fungicide treatment in ail lines. TAM 428, which did not respond to rust control, had over 75% functioning leaf area at physiological maturity (PM). In FS25A, Haygrazer, and SC307, all of which had less than 70% functioning leaf area remaining at PM, yield increased with increasing levels of functioning leaf area after fungicide treatment (r = 0.62*). Use of an eradicant nonpersistent rust fungicide (triadimefon) followed by a persistant systemic rust fungicide (oxycarboxin) gave excellent sorghum rust control during the critical stages for sorghum seed development. One-time use of different classes of fungicides should reduce development of fungicide resistant rust populations.