EFFECT OF THE STEM BLIGHT PATHOGEN, PHOMOPSIS-EMICIS, AND THE WEEVIL,PERAPION-ANTIQUUM, ON THE WEED EMEX-AUSTRALIS

The stem blight pathogen, Phomopsis emicis, and the weevil, Perapion a ntiquum, are two potential biological control agents for the annual we ed, Emex australis. Neither pathogen nor weevil affected the developme nt of rosette (5-wk-old) E. australis, but both significantly reduced stem length and number of new fruits in mature plants (10-wk-old) in g rowth room experiments. Stems grew two-thirds less than controls in pl ants attacked by weevils, had no net increase when both weevils and fu ngus were present, and collapsed in the presence of the fungus alone. Attack by weevils elicited a host response that slowed the development of the disease. Phomopsis emicis and Perapion antiquum reduced the ab ility of E. australis to produce new fruits by 77% and 68%, respective ly. Pathogen and weevil together reduced the number of new fruits by 8 3%. Perapion antiquum did not carry or provide infection sites for P. emicis. The lack of damage by pathogen and weevil to rosettes could co mpromise their effectiveness as biological control agents.