Characterization of sporulation of Alternaria alternata f. sp sphenocleae

Studies were conducted on agar media to characterize the factors for the op timization of sporulation of Alternaria alternata f. sp. sphenocleae, a fun gal pathogen being evaluated as a biological control agent for Sphenoclea z eylanica (gooseweed). A. alternata f. sp. sphenocleae conidiation was affec ted by nutrition, temperature, light conditions, and moisture. On all agar media tested, except for half-strength potato dextrose agar (1/2 PDA) and V -8 juice agar (VJA), exposure to different light conditions did not have an y significant effect on conidia production. However, when comparing 1/2 PDA and VJA, sporulation under constant near-ultraviolet (NUV) light at 28 deg rees C increased markedly on VJA, but decreased substantially on 1/2 PDA. T his trend, however, was opposite under dark conditions since 1/2 PDA produc ed the greatest number of conidia whereas a 75% reduction in conidia produc tion occurred on VJA in the dark. On all the standard agar media evaluated, the most virulent conidia were obtained on 1/2 PDA at 28 degrees C under c onstant NUV incubated for 4 weeks. Sporulation of A. alternata f. sp. sphen ocleae using the sporulation medium (S-medium) technique was rapid. Conidia were produced within 24 h and continuous sporulation was still observed un til 120 h. The best primary agar media for conidia production were PDA, 1/2 PDA and VJA, while water agar was the poorest. Conidia production was opti mized with the addition of 20 g l(-1) of calcium carbonate (CaCO3) and the addition of 2 ml of sterile distilled water on the medium. The most virulen t conidia were produced when the primary agar was 1/2 PDA, the CaCO3 concen tration was 20 g l(-1), and the cultures were incubated at 18 degrees C in the dark. Conidiophore induction occurred on nutrient rich media and was st imulated by NUV, while formation of conidia proceeded in darkness after nut rients were depleted under warm dry or cool moist conditions. Culture media , growth conditions, and CaCO3 affected the inoculum potential of A. altern ata f. sp. sphenocleae conidia.