Effect of polyethylene glycol and composition of basal medium on the mycelial growth of Phytophthora spp.

Growth of Phytophthora cambivora, P. cinnamomi, P. citricola and P. quercina was evaluated on basal media supplemented with polyethylene glycol 3350 and 6000. Optimum growth were at approx. - 0.35 MPa, and declined with decreasing osmotic potential between -1.18 and -1.75 MPa. Mycelium growth was severely reduced at -3.00 MPa, and it was completely inhibited at -3.98 MPa. P. cinnamomi and P. cambivora showed the best performance under osmotic stress. No differences in growth rate were evaluated between the two polyethylene glycol preparations differing in molecular weight. The composition of basal medium had an additive and positive effect on growth of Phytophthora between -1.75 and -3.98 MPa, although variable responses were reported among the four species. This study suggests that minimum amounts of water are essential for mycelial growth of Phytophthora spp. The ability of these oomycetes to overcome osmotic and water stress for a long time period is supposed.