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Laboratory, Greenhouse, and Field Assessment of Fourteen Fungicides for Activity Against Claviceps africana, Causal Agent of Sorghum Ergot

Commercial formulations of 14 fungicides representing seven chemical classes were assessed in vitro and in vivo for activity against Claviceps africana, causal agent of sorghum ergot. All fungicides markedly reduced spore germination in vitro on water agar, with effective concentration (EC)₅₀ values (based on active ingredient) that ranged from 0.01 μg ml^-1 for thiophanatemethyl and trifloxystrobin to 1.18 μg ml^-1 for cyprodinil. In vivo assays included applications of fungicide on full bloom panicles of a male-sterile line, ATx623, and then inoculating panicles with a conidial suspension immediately after the fungicide had dried. In greenhouse trials and in the 2000 field evaluation, a single application of propiconazole, tebuconazole, triadimefon, myclobutanil, or azoxystrobin at a rate of 25 μg ml^-1 markedly suppressed ergot severity on ATx623. In the 2001 field evaluation, tebuconazole applied at 25 μg ml^-1 was most effective in reducing levels of ergot severity. Ergot control was greater when fungicides were applied at 25 μg ml^-1 than at 5 μg ml^-1. Generally, triazole and strobilurin classes of fungicide were more effective in controlling ergot than benzimidazole, anilinopyrimidine, ethylenebisdithiocar-bamate (EBDC), pyridinamine, and organometallic classes. At the two fungicide rates used in this study, ergot severity increased between 7 and 10 days after inoculation. Results showed that in vitro fungicide screening by measuring spore germination inhibition was not a good predictor of performance in the field. Results from the field trials suggest that control, particularly under ergot-favorable environments, may require higher rates than used in this study.