Infection of Brassica napus (canola) by the fungus Sclerotinia sclerotiorum is usually mediated by airborne ascospores, which are produced when soil-borne sclerotia germinate carpogenically. However, basal infection of B. napus stems by mycelium derived from sclerotia that have undergone myceliogenic germination has also been observed (albeit less frequently) and can be very damaging. In this study the effects of artificial injury, hydration, incubation temperature, exogenous nutrients and conditioning temperature on sclerotia grown in vitro were assessed.
Four base treatments: scarred/imbibed, scarred/unimbibed, imbibed/unscarred and unimbibed/unscarred were applied to sclerotia to study their effects on myceliogenic germination. Imbibed sclerotia incubated at 15 °C on potato dextrose agar (PDA) produced more vigorous radial growth compared to unimbibed sclerotia. However, no difference in phenotype between the treatments was observed at 25 °C. Incubation of sclerotia at 25 °C increased pigmentation of colonies compared to incubation at 15 °C.
Subsequently, the four treatments were used in combination with three different conditioning temperatures: 17 °C, 4 °C and -20 °C for two weeks. After the conditioning treatments, sclerotia were placed on either 12.5% PDA or sterile moist sand, at 15 °C. When conditioned at -20 °C, imbibed/scarred and imbibed/unscarred sclerotia germinated myceliogenically after two and four days respectively, on moist sand. Myceliogenic germination on moist sand was characterised by dense white outgrowth from the scarred ends of the imbibed/scarred sclerotia and the formation of small dense white clumps on the upper surface of imbibed/unscarred sclerotia. Scarred/imbibed sclerotia conditioned at 17 °C and 4 °C germinated on moist sand after 11 days, producing radial hyphae that extended across the sterile sand. Together these results indicate that imbibing sclerotia promotes myceliogenic germination at 15 °C with or without exogenous nutrients. Myceliogenic germination on moist sand is enhanced when imbibed sclerotia are scarred and cold conditioned at -20 °C.