Biological treatments affect Ascochyta rabiei growth and apparent solanapyrone A levels in chickpea anthracnose

Background Chickpea anthracnose, caused by Ascochyta rabiei , remains a major constraint to chickpea production. In addition to fungal growth and tissue colonization, A. rabiei produces solanapyrone A, a phytotoxic secondary metabolite associated with toxin-mediated plant damage. Biological control candidates are commonly evaluated on the basis of visible fungal growth inhibition; however, this criterion alone may be insufficient if treatments also modify toxin-associated responses. This study evaluated the effects of Ruta montana and Carthamus caeruleus extracts, together with Bacillus megaterium and Pseudomonas fluorescens cell-free filtrates, on A. rabiei growth and spectrophotometrically estimated solanapyrone A-associated levels. Results The solanapyrone A-associated signal was strongly influenced by culture medium and reached its highest estimated level in Czapek Dox medium supplemented with metal cations. Solanapyrone A inhibited chickpea hypocotyl elongation in a genotype-dependent manner, with FLIP 97–220 C showing the lowest sensitivity. Both plant extracts inhibited (A) rabiei growth, with R. montana showing stronger activity than C. caeruleus . Live bacterial antagonism reduced fungal growth by 43.17% for (B) megaterium and 50.31% for P. fluorescens . However, fungal growth inhibition and the apparent solanapyrone A-associated signal did not follow parallel profiles. High treatment concentrations often produced visible antifungal activity but did not necessarily reduce the 327 nm signal. In contrast, selected bacterial filtrate dilutions, particularly B. megaterium at 1/4 and P. fluorescens at 1/8 of the crude filtrate, combined residual visible inhibition with lower apparent solanapyrone A-associated levels than the untreated control. Conclusions These findings support a dual-response screening approach for biological control candidates against chickpea anthracnose, combining fungal growth inhibition with the evaluation of toxin-associated responses. Under the tested conditions, selected bacterial filtrate dilutions combined visible antifungal activity with lower apparent solanapyrone A levels. However, because solanapyrone A was estimated spectrophotometrically at 327 nm, the observed decrease should be interpreted cautiously as an apparent reduction in the solanapyrone A-associated signal. Chromatographic confirmation by HPLC-DAD, HPLC-MS or LC-MS/MS is required to determine whether this decrease reflects reduced biosynthesis, conversion, degradation, binding or metabolization of solanapyrone A.