Factors influencing biocontrol of jimsonweed (Datura stramonium L.) with the leaf-spotting fungus Alternaria crassa

In greenhouse tests, Alternaria crassa (Sacc.) Rands killed > 80% of inoculated jimsonweed (Datura stramonium L.) seedlings within 14 days following a 9-h dew period at 25°C with 1 × 10⁵ spores/ml, and after 8 h of dew at 1 × 10⁶ spores/ml. At least 10 h of dew with 1 × 10⁵ spores/ml and 9 h of dew with 1 × 10⁶ spores/ml were required to obtain 100% mortality of fungus-inoculated plants. Growth stage and inoculum concentration studies revealed that higher concentrations of inoculum were required to obtain 100% mortality of larger plants. Weed control was significantly reduced by day/night air temperatures of 35°C and 24°C, respectively, at all inoculum concentrations as compared to the controls at lower air and dew temperature regimes. The results of these studies indicate that A. crassa has potential as a biological herbicide for the control of jimsonweed.