Host Range, Temperature Response, Survival, and Overwintering of Alternaria cirsinoxia

Abstract

Alternaria cirsinoxia was evaluated for its host range, the influence of temperature on mycelial growth, and survival and overwintering on Canada thistle (Cirsium arvense) in Saskatchewan. With the exception of leafy spurge, the host range of A. cirsinoxia was limited to species within the Asteraceae. Canada thistle, safflower, and sunflower were most susceptible to A. cirsinoxia, the latter two being crop species of lesser importance in Saskatchewan. Mycelium of A. cirsinoxia grew best at a constant temperature of 25°C and in temperature cycles which alternated around a mean of 20–25°C. Mycelium did not grow when exposed to constant temperatures of 0, 40, or 45°C for 7 days. However, at 0°C, mycelium survived and was able to resume growth, whereas at 40 or 45°C, mycelium was killed. In the field, A. cirsinoxia produced viable conidia on senescent, basal Canada thistle leaves for at least 3–4 months after inoculation in 1998 and 1999. Sporulation tended to be higher in 1998 than in 1999, possibly favored by the warmer, drier, and sunnier conditions prevailing during July to mid-September in 1998. A. cirsinoxia also overwintered and produced viable conidia on infected Canada thistle leaves in the field, and at constant 4°C, when sampled from November 1998 until April 1999. Sporulation of leaves overwintering in the field was lowest in April 1999, probably due to inoculum degradation as a result of surface flooding in the plots. Clusters and chains of chlamydospores were abundant on overwintering leaf and stem debris of Canada thistle in field plots inoculated 10 months previously. A. cirsinoxia subsequently sporulated on this infected debris. Based on these host-range tests, the risks to major nontarget crop species in Saskatchewan should be minimal after the inundative application of A. cirsinoxia as a bioherbicide for Canada thistle. However, this pathogen appears able to persist and remain potentially infectious in the field for a prolonged period of time after inoculation. Hence, longevity and spread of A. cirsinoxia should be evaluated further to minimize the potential risks to susceptible minor crop species.