Degradation and transformation of a potential natural herbicide in three soils.

The methyl ester of fusaric acid (ME) is one of four toxins produced by the fungus Fusarium nygamai, which could be used as a natural herbicide against Striga hermonthica, a parasitic weed of sorghum and corn in a vast zone of West and Central Africa. A laboratory study was performed to measure the degradation of ME in three soil types and under different temperature and soil moisture conditions, so as to ascertain whether a single ME treatment would protect the crops against this weed during the critical phases of growth. The results show that the persistence in all soils and under all incubation conditions is long enough to protect the crops for the first week of growth, excluding the trial at 30 degrees C in the humic soil, where the half-life of 6 days would require more than one treatment. A degradation product of ME (butylpyridine, BP) was identified by gas chromatography/mass spectrometry and its degradation measured. The sum of ME and BP residues for the first 7 days was almost 100% of the applied compound in all soils and incubation conditions, thus indicating that BP may be the only transformation product of ME at this stage.