Effect of the stem blight pathogen, Phomopsis emicis, and the weevil, Perapion antiquum, on the weed Emex australis

Abstract

SummaryThe stem blight pathogen, Phomopsis emicis, and the weevil, Perapion antiquum, are two potential biological control agents for the annual weed, Emex australis. Neither pathogen nor weevil affected the development 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 growth room experiments. Stems grew two‐thirds less than controls in plants attacked by weevils, had no net increase when both weevils and fungus 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 ability of E. australis to produce new fruits by 77% and 68%, respectively. Pathogen and weevil together reduced the number of new fruits by 83%. Perapion antiquum did not carry or provide infection sites for P. emicis. The lack of damage by pathogen and weevil to rosettes could compromise their effectiveness as biological control agents.