Production of phytotoxins by Phoma exigua var. exigua, a potential mycoherbicide against perennial thistles

The potential of the different Phoma exigua var. exigua strains for the biocontrol of the perennial weeds Sonchus arvensis and Cirsium arvense, occurring throughout temperate regions of the world, has been evaluated in previous studies. The majority of the above strains produced ascosonchine, a newly discovered enol tautomer of 4- pyridylpyruvic acid, whereas strains C-177 and S-9, though virulent to weeds, did not produce the above metabolite. In this study, it was demonstrated that the above two strains, grown in liquid and solid cultures, produced p-hydroxybenzaldehyde, cytochalasins B, F, Z2 and Z3, and deoxaphomin. When assayed on the leaves of both C. arvense and S. arvensis, p-hydroxybenzaldehyde was inactive, whereas deoxaphomin demonstrated the highest level of toxicity on leaves of S. arvensis. Cytochalasin Z2 appeared to be the less toxic cytochalasan on both plants according to the lack of the secondary hydroxyl group on C-7. Production of cytochalasins by P. exigua var. exigua strains isolated from C. arvense and S. arvensis is discussed in relation to chemotaxonomy and the biocontrol potential of the fungus.