Toxin production by melon root rot fungus, Monosporascus cannonballus

A study was conducted on the possible in vitro production of phytotoxins by the melon root rot pathogen, Monosporascus cannonballus. The culture filtrate of a highly virulent isolate of the fungus was tested for its phytotoxic activity on cucumber by detached leaf assay, electrolyte leakage assay and seedling vigour test. In detached leaf bioassay, necrotic symptoms developed on cucumber leaves 5–7 days after application of the culture filtrate. The culture filtrate of M. cannonballus caused substantial reduction in seed germination, vigor of cucumber seedlings and an increase in electrolyte leakage from leaf tissues compared to the control. Studies on the effect of heat and dialysis on the phytotoxic activity of the culture filtrate revealed that the toxic metabolites are thermostable and have low molecular weight (<14 kDa). To purify the toxin, the culture filtrate was extracted with different solvents which were then tested separately for their phytotoxicity. The results revealed that the water fraction after extraction with chloroform retained the highest phytotoxic activity. The toxin was partially purified by thin layer chromatography (TLC) and the fluorescent compound eluted with an R_f value of 0.6 was analyzed by gas chromatography-mass spectrometry (GC-MS) technique. Mass spectral analysis revealed the presence of two metabolites viz., octocrylene and squalene in the toxin fraction.