Effect of water potential on mycelial growth and perithecial production of Monosporascus cannonballus in vitro

Abstract

The effects of osmotic water potential (Ψs) on mycelial growth and perithecial production of Monosporascus cannonballus, the cause of root rot and vine decline of melons, were examined at 25°C on potato dextrose agar (PDA) amended with KCl, NaCl or sucrose. Patterns of the growth responses of four isolates to decreasing Ψs were similar for each of the osmotica. Compared with growth on nonamended PDA (−0·3 MPa), growth of all isolates increased as Ψs was reduced to −0·8 MPa. Maximum growth occurred at Ψs values of −0·6 to −0·8 MPa. Growth was not reduced below that on nonamended PDA until Ψs was reduced to −1·8 MPa, and a 50% reduction in growth did not occur until Ψs was reduced to < −2·5 MPa. Reproduction was much more sensitive to reduced Ψs than was mycelial growth, and perithecia were produced only at Ψs ≥ −0·7 or −0·8 MPa on PDA amended with KCl or NaCl, respectively. Three isolates produced perithecia on PDA amended with sucrose only at Ψs ≥ −0·6 MPa, but the fourth isolate produced perithecia at ≥ −1·9 MPa. Colonization of the xylem early in disease development may provide an essential source of water for subsequent reproduction in the root cortex during plant senescence. Postharvest cultivation to expose and desiccate roots may prevent reproduction even when temperatures lethal to hyphae are not attained.