Effects of water potential on mycelial growth, sclerotial production, and germination of Rhizoctonia solani from potato

The effects of osmotic and matric potential on mycelial growth, sclerotial production and germination of isolates of Rhizoctonia solani [anastomosis groups (AGs) 2-1 and 3] from potato were studied on potato dextrose agar (PDA) adjusted osmotically with sodium chloride, potassium chloride, glycerol, and matrically with polyethylene glycol (PEG) 6000. All isolates from AGs 2-1 and AG-3 exhibited fastest mycelial growth on unamended PDA (−0.4 MPa), and growth generally declined with decreasing osmotic and matric potentials. Growth ceased between −3.5 and −4.0 MPa on osmotically adjusted media, and at −2.0 MPa on matrically adjusted media, with slight differences between isolates and osmotica. Sclerotium yield declined with decreasing osmotic potential, and formation by AG 2-1 and AG-3 isolates ceased between −1.5 and −3.0 MPa and −2.5 and −3.5 MPa, respectively. On matrically adjusted media, sclerotial formation by AG 2-1 isolates ceased at −0.8 MPa, whereas formation by AG-3 isolates ceased at the lower matric potential of −1.5 MPa. Sclerotial germination also declined with decreasing osmotic and matric potential, with total inhibition occurring over the range −3.0 to −4.0 MPa on osmotically adjusted media, and at −2.0 MPa on matrically adjusted media. In soil, mycelial growth and sclerotial germination of AG-3 isolates declined with decreasing total water potential, with a minimum potential of −6.3 MPa permitting both growth and germination. The relevance of these results to the behaviour of R. solani AGs in soil and their pathogenicity on potato is discussed.