Carbon loss by sclerotia of Sclerotium rolfsii under the influence of soil pH, temperature and matric potential and its effect on sclerotial germination and virulence

Abstract

Germinability and virulence of sclerotia of Sclerotium rolfsii were assessed after 50 days of exposure of 14C-labeled sclerotia to soil at 0, −5 and −15kPa and pH 6.9, or to soil at 15, 25 or 30°C, pH 5 or 8 and −1kPa. Evolution of 14CO2 accounted for the greatest share of endogenous carbon loss from sclerotia under all soil conditions, except in water-saturated soil (0kPa), in which sclerotial exudates contributed the major share of carbon loss. Total evolution of 14CO2 from sclerotia in soil at −15kPa (42.4% of total 14C) and at −5kPa (38%) was significantly higher than at 0kPa (23.8%). Evolution of 14CO2 in soil at 25 or 30°C was more rapid than at 15°C with regardless of pH. Loss of endogenous carbon by sclerotia was the greater after 50 days of exposure to soil at 0kPa, or at 25 or 30°C and pH 8, than at other soil conditions. Sclerotia exposed to water-saturated soil (0kPa) showed a more rapid decline in nutrient independent germinability, viability and virulence, than to those exposed to −5 or −15kPa. Sclerotia became dependent on nutrient for germination and lost viability and virulence within 30–40 days in soil at 25 or 30°C, pH 8. However, more than 60% of sclerotia retained viability in soil at 15°C regardless of pH, even after 50 days. Radish shoot growth was increased significantly by the sclerotia that had been exposed to soil at 0kPa, or to soil at 25 or 30°C and pH 8 for 50 days. In conclusion, carbon loss by sclerotia during incubation on soil at different pH levels, temperatures and water potentials was inversely correlated with sclerotial ability to infect radish seedlings. The relationship between carbon loss by sclerotia and radish shoot length was positive.