Factors Influencing Pathogenicity of Myrothecium verrucaria Isolated from Euphorbia esula on Species of Euphorbia . Shawming Yang, Research Plant Pathologist, U.S. Department of Agriculture, Foreign Disease-Weed Science Research Unit, Ft. Detrick, Bldg. 1301, Frederick, Md. 21702. S. C. Jong, Head, Department of Mycology, ATTC, Rockville, Md. 20852. Plant Dis. 79:998-1002. Accepted for publication 8 June 1995. This article is in the public domain and not copyrightable, ll may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1995. DOI: 10.1094/PD-79-0998.

Leafy spurge (Euphorbia esula) inoculated with Myrothecium verrucaria had dead tops or shoots with blackened stems and blackened, curled, or wilted leaves. On leaves, circular spots with light brown centers and dark brown margins were produced. Pathogenicity was greatly affected by the conidial concentration and dew temperature, but not by the age of cultures from which conidia were used for inoculation. Conidia stored in sucrose solution at 4°C were viable for more than 14 months. Myrothecium verrucaria did not spread from diseased plants to healthy plants, even when the healthy plants touched the diseased plants in the dew tent for 7 days. Four-week-old or younger seedlings of leafy spurge grown from seeds were killed with one inoculation, but repeated inoculations were required to kill older plants. Pathogenicity tests on 89 collections of nine species of Euphorbia and on reed canarygrass (Phalaris arundinacea) showed that the pathogen severely infected all collections tested. The susceptibility of E. corollata, E. cyathophora, E. cyparissias, E. helioscopia, E. heterophylla, E. lathyrus, E. marginata Pursh varigata, E. virgata, and reed canarygrass to M. verrucaria is reported for the first time.

Keyword(s): host, invert emulsion