Terpenoid aldehydes and naphthofuran precursors are synthesized in diseased tissues as part of an active defense mechanism. The terpenoid aldehyde hemigossypol (HG) is derived by oxidation of the terpenoid naphthofuran desoxyhemigossypol (dHG). The enzyme dHG O-methytransferase (dHG OMT) catalyses the transfer of a methyl group from S-adenosyl-L-methionine to dHG to form desoxyhemigossypol-6-methyl ether (dMHG). dMHG can undergo the same oxidation as dHG to yield the methylated derivative of HG (i.e., hemigossypol-6-methyl ether). The methylated terpenoids are only about one-half as toxic as the unmethylated parents to microbial pathogens. Antisense constructs of dHG OMT gene may prevent the conversion of dHG to dMHG by blocking the synthesis of the enzyme, and thereby increase resistance of the cotton plant to pathogens. To facilitate cloning of the dHG OMT gene, the dHG OMT was purified for sequence information.