Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/7/2000
Publication Date: N/A
Technical Abstract: Certain strains of Trichoderma (Gliocladium) virens are effective agents for controlling cotton seedling diseases caused by Rhizoctonia solani. To gain an understanding of the biocontrol mechanisms of T. virens, interactions of R. solani and T. virens with cotton are being investigated. Recently, it has been reported that R. solani and T. virens cause increased dphytoalexin (PA) levels in cotton seedling roots. The PA's that were affected include hemigossypol (HG), desoxyhemigossypol (dHG), hemigossylic acid lactone (HGAL), hemigossypol-6-methyl ether (MHG), desoxyhemigossypol- 6-methyl ether (dMHG), gossypol (G), gossypol-6-methyl ether (MG) and gossypol-6,6'-dimethyl ether (DMG). To determine the possible importance of PA stimulation to the biocontrol mechanism, the toxicities of the cotton PA's towards R. solani were evaluated. In lethal dose bioassays, the LD100's were found to be 7-10ug/mL for dMHG, 7-10ug/mL for dHG, 10-15ug/mL for MHG and 20-25ug/mL for HG. Due to solubility problems, the LD100's fo G, MG and DMG could not be determined exactly but the results do indicate that the values are greater than 60ug/mL. Results from bioassays determining the effective dose required to reduce growth by 50percent (i.e., the ED50) correlate with the LD100 test findings. dMHG and dHG have very similar toxicities towards R. solani with ED50s of 0.65 +/- 0.11ug/mL and 0.87 +/- 0.10ug/mL, respectively, while MHG with an ED50 of 3.60 +/- 0.55ug/mL is more potent than HG with an ED50 of 5.79 +/- 0.72ug/mL. G with an ED50 of 15.6 +/- 2.3ug/mL is much less toxic to R. solani than are dMHG, dHG, MHG or HG. As in the LD100 work, PA solubility limitations prevented the determination of ED50 values for MG and DMG; however, the results did show that the ED50's for MG and DMG are greater than 23ug/mL.