Examination of sclerotial germination in Sclerotinia minor with an in vitro model

Abstract

An in vitro model was developed to examine the effect of surface microflora and exogenous nutrient on sclerotial germination of Sclerotinia minor. Germinated sclerotia were identified by the presence of appressoria on the plastic Petri dish. Surface sterilised field sclerotia germinated at maximum rates on water agar, but unsterilised sclerotia failed to germinate, even after drying and rewetting, indicating that sclerotia of S. minor are unlikely to germinate spontaneously in the presence of soil microflora. Partial surface sterilisation gave intermediate germination rates, which were enhanced two- to four-fold by treatment with sunflower root sap. There was no significant difference in the germination response to root sap from susceptible and resistant sunflower lines, but germination rates were highest with root nutrient obtained from plants during vegetative growth. A positive germination response to peptone and an amino acid mixture implicated amino acids as germination stimuli. Germination of field sclerotia required incubation with root sap for more than 3 days, suggesting that prolonged exposure of sclerotia to root exudates may be required for the onset of stem rot in sunflowers. This is discussed in relation to the dynamics of root growth. Keywords: exogenous nutrient, field sclerotia, germination stimulus.