Mycelial growth and production of oxalic acid by virulent and hypovirulent isolates of Sclerotinia sclerotiorum

Abstract

Hypovirulent dsRNA-containing, and virulent dsRNA-free, isolates of Sclerotinia sclerotiorum were compared for mycelial growth and oxalic acid production in liquid culture. When harvested at 14 days post-inoculation, hypovirulenr isolates 91 and 215 produced less dry mycelium than virulent isolates 191 and 275C in potato-dextrose broth (PDB) and PDB amended with sodium succinate (PDBSS). All isolates produced more oxalic acid per gram dry mycelium When grown in PDBSS than in PDB. In a time course study, hypovirulent isolates 91 and 275 produced less mycelium (0.23 and 0.03 g dry wt.) and oxalic acid (0.22 and 0.01 mg/mL) than virulent isolates 191 and 275C (0.81 and 0.91 g dry wt., and 0.50 and 0.63 mg oxalic acid per millilitre, respectively) at 15 days post-inoculation. The amount of oxalic acid produced per gram dry mycelium reached a maximum for virulent isolates 191 and 275C at 3 and 5 days post-inoculation. Hypovirulent isolate 91 reached a maximum at 9 days post-inoculation, and hypovirulent isolate 275 was still increasing at 15 days post-inoculation. The mycelial dry weight of individual isolates was positively correlated with production of oxalic acid and inversely correlated with pH of culture filtrates at harvest. Three near-isogenic pairs of hypovirulent and virulent isolates were compared for mycelial growth and oxalic acid production in PDB and PDBSS. All hypovirulent isolates produced considerably less dry mycelium and oxalic acid than virulent isolates at 2 weeks post-inoculation. The mycelial growth of hypovirulent isolates was still less than the virulent isolates at 4 weeks post-inoculation, but the production of oxalic acid by two hypovirulent isolates increased to levels comparable to that of the virulent isolates at 2 weeks post-inoculation. The results are consistent with the hypothesis that dsRNA-associated hypovirulence in isolates of S. sclerotiorum was associated with reduced or delayed production of oxalic acid, or both, in comparison to virulent isolates, particularly during the first 3 to 7 days of growth.