Submerged culture of a mycelial formulation of a bioherbicidal strain of Myrothecium verrucaria with mitigated mycotoxin production

Abstract

A mycelial formulation of the fungus Myro- thecium verrucaria (IMI 361690) containing 0.20% Silwet L-77 surfactant was found to be highly efficacious in controlling the exotic invasive weed kudzu. The mycelium can be rapidly (48-72 h) produced in several media, including an inexpensive soy flour-corn meal medium. Mycelial yields were 2, 10, and 25 g dry weight l -1 in Czapek-Dox, Richard's V-8, and soy flour-corn meal media, respectively. Scale-up production in soy flour-corn meal medium using laboratory fermenters (10-25 l), resulted in a mycelial formulation that caused 90% mor- tality of naturally-occurring mature (0.9-1.0 m in height) kudzu within 48 h after application in field experiments. HPLC analyses revealed that the mycelium produced in this liquid culture contained no detectable amounts of the trichothecene mycotoxins roridin A and verrucarin A (limit of detection 2 l gm l -1 ). This has resulted in a safer, yet effective bioherbicidal product. We anticipate that these findings should improve the probability of EPA registration and subsequent commercial development of this bioherbicide.