Plectosphaerella cucumerina as a bioherbicide for Cirsium arvense: proof of concept

Plectosphaerella cucumerina is a fungal pathogen of several plant species with a global distribution. The recent discovery in New Zealand of Cirsium arvense as a natural host of the fungus led to the studies reported here where its potential as a bio-herbicide was evaluated under controlled environment and field conditions in New Zealand and Canada. Isolates obtained from C. arvense plants in New Zealand and Canada caused similar levels of disease under the same environmental conditions indicating global opportunities for its use as a bio-herbicide. P. cucumerina defoliated and killed C. arvense under greenhouse and field conditions when applied at 1013 conidia ha-1 with water volumes ranging from 250-1500 L ha-1. Lower rates of application produced insufficient disease to harm the weed. However, other factors enhanced the disease and may allow for reduction in the application rate. Multiple applications within a season and the adjuvant Pulse® penetrant both increased disease severity. The conditions for optimal disease expression occurred at 20°C with a 48 h leaf wetness period. This requirement for 48 h of continuous leaf wetness may limit the use of P. cucumerina in environments with little natural dew. However, temperature and leaf wetness interactions may compensate since a 16 h leaf wetness period was sufficient at 25°C. Conversely, results indicate that cooler temperatures during application necessitate longer periods of leaf wetness. This study shows that P. cucumerina can defoliate C. arvense when applied under a range of environmental conditions implying that the fungus has potential as a bio-herbicide.