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Growth of Magnaporthe poae and Gaeumannomyces incrustans as Affected by Temperature-Osmotic Potential Interactions. K. E. Kackley, Departments of Botany and Agronomy, University of Maryland, College Park 20742-5815, Present address: Monsanto Agricultural Company, 3015 Blueford Road, Kensington, MD 20895-2724; A. P. Grybauskas, and P. H. Dernoeden. Departments of Botany and Agronomy, University of Maryland, College Park 20742-5815. Phytopathology 80:646-650. Accepted for publication 2 January 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-646.

Two isolates of Magnaporthe poae and one isolate of Gaeumannomyces incrustans were grown at 20, 25, 30, and 35 C on a basal salts medium (?0.12 MPa) adjusted with KCl to obtain osmotic potentials from ?0.12 to ?2.35 MPa. Isolates of M. poae produced maximal growth at the highest osmotic potentials (?0.12 to ?0.35 MPa) at 25 and 30 C. At 20 C, no significant (P = 0.05) shift in optimal growth was observed between ?0.35 and ?1.03 MPa. Significantly reduced growth of M. poae occurred at 35 C with an optimum between ?1.03 and ?1.47 MPa. Optimal growth of G. incrustans generally occurred at osmotic potentials between 0.23 and 0.68 MPa lower than that of the basal medium. Maximal growth at 20 C occurred between ?0.80 and ?1.3 MPa, whereas at 25 and 30 C maximal growth occurred between ?0.35 and ?0.80 MPa. There was no difference in growth of G. incrustans among any of the osmotic potentials tested at 35 C. G. incrustans produced more growth than the isolates of M. poae at all temperatures except 35 C. Based on these results, growth of M. poae, causal agent of summer patch of Kentucky bluegrass (Poa pratensis), would be restricted by drought at temperatures supraoptimal for the growth of Kentucky bluegrass, and summer patch is most likely to be severe at temperatures >25 C when moisture is not limiting.