The effects of Fusarium oxysporum f.sp. lycopersici (Sacc.) Snyder & Hansen on tomato in diphenamid-treated soil

Pre-emergence soil application of the herbicide diphenamid in concentrations exceeding the normal field rate increased the resistance of tomato plants towards infection by the wilt fungus Fusarium oxysporum f.sp. lycopersici. This was detected as significant increases in the percentage emergence of seedlings although growth parameters of the raised seedlings were reduced. Treated plants exhibited no wilt symptoms, although the pathogen maintained its population at detectable levels in the rhizosphere of tomato plants. However, the growth inhibition caused by diphenamid alone was much less than that reported for the combined application of pathogen and herbicide. Growth activities of F. oxysporum f.sp. lycopersici were inhibited by high concentrations of diphenamid in vitro. It is possible that the biodegradation of this herbicide by species such as Aspergillus candidus (present in substantial counts in treated rhizospheres) was one of the causes of increased tolerence of the pathogen to the herbicide in situ.