Influence of Moisture and Temperature on Infection of Canada Thistle by Alternaria cirsinoxia

Relative humidity (RH), temperature, continuous leaf wetness, and intermittent leaf wetness were evaluated for their influence on conidial germination, appressoria formation, and infection of Canada thistle by Alternaria cirsinoxia. Conidia germinated and formed appressoria at 98% RH, but required at least 100% RH, and preferably free water, to penetrate leaves. In free water, conidia germinated equally well at 10, 15, 20, 25, or 30°C. Appressoria formation and leaf penetration also occurred at all temperatures evaluated from 10 to 30°C, with the highest values at 20 to 25°C and 20°C, respectively. Conidia required 8 h of continuous leaf wetness to establish visible symptoms of infection on Canada thistle. Exposure of conidia of A. cirsinoxia to up to five intermittent leaf wetness cycles (30 min wet/30 min dry) reduced germination, appressoria formation, and leaf penetration, but conidia remained infective after all cycles. Exposure to cycles of 4 h dew/20 h dry was most detrimental to infection, compared with a 72 h dry period or cycles of 1 h dew/23 h wet or 2 h dew/22 h wet, indicating greater sensitivity of more fully germinated conidia to drying. Such nonspecific temperature requirements and survival during repeated intermittent leaf wetness periods are beneficial characteristics for a bioherbicide. However, high moisture requirements for infection may limit the potential of using A. cirsinoxia to control Canada thistle in the semi-arid prairie provinces of Canada.