Biological control of common cocklebur by Alternaria helianthi.
Greenhouse studies were carried out on the use of Alternaria helianthi as a biological control agent against Xanthium strumarium. A. helianthi conidia grown at 18°C were the most virulent in producing diseases on hosts. The dew period requirements could be reduced to 9 hours with an unrefined maize oil emulsion and the dew period could also be delayed for 24 hours after application. A. helianthi was effective in the control of imazaquin-, MSMA- and metribuzin-resistant biotypes of X. strumarium. A. helianthi was most effective on young seedlings (2- to 6-leaf stage), but higher doses (>50 000 conidia/ml) could also control older seedlings.