Abstract The involvement of allelopathy in citrus orchards is indicated by the discovery of a phenolic phytotoxin in soils and partly decomposed citrus root residues. The phytotoxin, which was tentatively identified as homovanillic acid, is produced by the anaerobic decomposition of citrus roots in the deeper soil horizons. It caused an arrest in rough lemon (Citrus jambhiri) radicle elongation and a severe swelling of the root tip.