<i>Ustilaginoidea virens</i> Infection of Rice in Arkansas: Toxicity of False Smut Galls, Their Extracts and the Ustiloxin Fraction

Abstract

Cool, wet conditions in the southern US during the maturing stages of rice in 1998 contributed to outbreaks of false smut caused by Ustilaginoidea virens . Water extracts of false smut galls in Asia have been reported to contain ustiloxin toxins, cyclic peptide antibiotics that interfered with microtubule function and caused “lupinosis”-like lesions in mice. Cell-free extracts from false smut galls on rice grown in Arkansas were fractionated by a published procedure for the purification of ustiloxins. The ustiloxin fraction was phytotoxic to Lemnapausicostata (duckweed) at ≥19 μg/ml, but the host plant, rice, was much less susceptible, exhibiting phytotoxic effects in germinating seeds at ≥1000 μg/ml. The aqueous extract of rice false smut galls showed no cytotoxicity to mammalian cell cultures at 200 μg/ml, but the ustiloxin fractionwas cytotoxic at 10 - 100 μg/ml. However, rice false smut galls were not toxic when fed to mice at 10% of chow, but caused feed refusal at higher concentrations. We conclude that for 1) the U. virens which causes false smut in southern USA differs from Asian isolates in that does not produce detectable ustiloxins; and 2) false smut affects the appearance, but not the food safety of rice in the United States.