Effects of ambient pH and nutritional factors on antifungal activity of the mycoparasite Coniothyrium minitans

Antifungal activity (AA) of the mycoparasite Coniothyrium minitans (CM) against Sclerotinia sclerotiorum (SS), designated as AACM–SS, was detected by measuring the effect of cell-free cultural filtrates of CM on inhibition of growth (IG) of SS on potato dextrose agar. Using this bioassay method, effects of ambient pH and nutritional factors on AACM–SS in CM cultures grown in various liquid media were investigated. Results showed that production of AACM–SS by CM was closely related to ambient pH. After incubation for 9–15 days, AACM–SS was high in modified Czapek–Dox broth (MCD) with IG values being 78–90%, but low in potato dextrose broth (PDB) with IG values being 9–18%. In contrast, pH value was as low as 3.3–3.6 in the MCD cultures, whereas as high as 6.0–7.2 in the PDB cultures. In the pH-buffered experiment, AACM–SS was detected in MCD cultures of CM buffered at pH 2–8 with the optimum pH being 3–5, under which AACM–SS in PDB cultures of CM was greatly enhanced with the IG values reaching up to 46–67%. Results also indicated that AACM–SS in CM cultures can be affected by nutritional factors and their effects were also related to ambient pH. In the basal medium Czapek mineral salts, most carbon and nitrogen sources investigated were favourable to CM to produce AACM–SS with the IG values being 73–91% and the final pH values being 2.7–4.1. In the other basal medium potato extracts, a few carbon and nitrogen sources were favourable for CM to produce AACM–SS with the IG values being 40–60% and the final pH values being 2.7–5.5. In the pH-buffered experiments with selected carbon and nitrogen sources, or with mycelial or sclerotial extracts of SS, AACM–SS was much higher in CM cultures buffered at pH 3 than that buffered at pH 6. Additional experiments showed that AACM–SS could be induced by the phytotoxin oxalic acid (OA), but not by sodium oxalate, further suggesting the importance of acidic pH in production of AACM–SS by CM. Meanwhile, leaf blight incidence on Chinese cabbage caused by SS was greatly reduced in the treatment of CM conidia pre-germinated in PDB plus OA (16 mM) than that in the treatment of CM conidia pre-germinated in PDB alone. This study suggests that ambient pH plays an important role in regulation of the antifungal activity of CM against SS.