Isolation of a natural herbicidal constituent from culture filtrates of Drechslera hawaiiensis for management of Rumex dentatus

Rumex dentatus is a problematic broad-leaf weed of wheat in Pakistan. The present study was carried out to identify natural herbicidal constituents from culture filtrates of a phytopathogenic fungus Drechslera hawaiiensis for the management of R. dentatus. Culture filtrates of the fungus prepared in M-1-D broth, significantly reduced germination and seedling growth of R. dentatus in a laboratory bioassay conducted in 9-cm diameter Petri plates. Fungal culture filtrates were extracted successively with n-hexane, chloroform, ethyl acetate and n-butanol. Bioassays were carried out on punctured leaf discs of the weed using different concentrations (2, 1, ..., 0.0625 mg mL^-1) of the crude organic solvent fractions. Chloroform fraction exhibited the best herbicidal activity in terms of necrotic spot formation and leaf discolouration. Five compounds from chloroform fraction were separated and purified by preparative thin layer chromatography followed by reversed phase high performance liquid chromatography (HPLC). Efficacy of different concentrations (2, 1, ..., 0.03125 µg µL^-1) of the isolated compounds was assessed by leaf disc bioassays using a well known synthetic herbicide 2 – 4, D as reference. Structure of compound with the highest herbicidal activity againstR. dentatus was determined by various spectroscopic techniques viz. NMR and MS. The compound was identified as (Z)- docos-5-en-1- acid. This is the first report of this herbicidal compound from genus Drechslera.