Factors Influencing Biological Control of Sphenoclea zeylanica with Alternaria alternata f. sp. sphenocleae

Alternaria alternata f. sp. sphenocleae is an endemic disease causing a few small necrotic foliar lesions and occasionally a foliar blight on Sphenoclea zeylanica, a problem weed in paddy rice. The effects of inoculum concentration, dew period, and plant age on the biocontrol of S. zeylanica with A. alternata f. sp. sphenocleae were studied under controlled environmental conditions. S. zeylanica plants are susceptible from seedling to maturity. When higher inoculum concentrations (10⁶ conidia ml^-1, 10⁸ conidia m^-2) were applied to plants of different ages with a long dew period, 100% leaf area damage occurred earlier than when lower inoculum concentrations (10⁴ conidia ml^-1, 10⁶ conidia m^-2) were used. At the higher inoculum concentrations, a significantly (α < 0.01) higher percent reduction in plant height was obtained, and all plants were killed. A dew period of 8 h was sufficient to cause 100% mortality when plants were inoculated with 10⁶ conidia ml^-1, whereas a dew period of 16 h was required with 10⁴ conidia ml^-1 to achieve the same effect. At all dew periods, a significantly (α < 0.01) higher percent dry weight reduction was obtained with higher inoculum concentrations. Field performance of this bioherbicide candidate relies on the primary infection established by inoculum application, as secondary infections have not been observed. Effective use rates can be reduced during the rainy season, while higher rates are required during the dry season.