Fungitoxicity of plant growth regulators (PGRs) and PGR/fungicide mixtures in soak treatments to Septoria apiicola pycnidiospores

Mixtures of the plant growth regulators (PGRs) gibberellins A4 and A7 (GA 4/7) and ethephon (2-chloroethylphosphonic acid), applied as soak treatments at 20° C for 24 h, were toxic to pycnidiospores of Septoria apiicola on agar, apparently as a result of low pH of their solutions. Ultraviolet spectroscopy showed that the aqueous solubility of the fungicides benomyl and carbendazim increased significantly when buffered at low pH or mixed with aqueous solutions of GA 4/7 and ethephon. This resulted in greater quantities of ¹⁴C-labelled fungicides entering celery seeds imbibed in PGR/fungicide mixtures in comparison with fungicides alone.
A 24-h seed soak at 20° C in a mixture of aqueous benomyl, GA 4/7 and ethephon eliminated S. apiicola in infected celery seeds. A thiram soak was also effective but PGR mixtures alone did not completely eliminate the pathogen.