A foliar pathogen identified as a member of the genus Alternaria was isolated from blighted Sphenoclea zeylanica (gooseweed) collected in 1991 from a rice field near Los Banos, Laguna, Philippines. Inoculum density, dew period, and plant height are factors influencing biocontrol of S. zeylanica with this indigenous pathogen. Significantly higher percent reductions in plant height and dry weight were obtained and all plants were killed at higher inoculum concentrations with 8 h of dew. The number, germination, and virulence of conidia were significantly affected by production techniques, temperature, light condition, and incubation period. Exposure to continuous near-ultraviolet (NUV) light at 28$ sp circ$C stimulated sporulation on agar media and on solid substrates. Overall, the best production technique was the use of sorghum seeds using an equal quantity of sorghum seeds and water (w/v) incubated for four weeks. Another conidia production method using the sporulation medium (S-medium) technique was evaluated with the addition of 20 g L$ sp{-1}$ of calcium carbonate (CaCO$ sb3)$ and 2 ml of sterile distilled water. Primary 1/2 PDA at 18$ sp circ$C in the dark produced the most virulent conidia. This technique produced conidia relatively rapid, but was labour intensive. Host range studies using 49 plant species in 40 genera representing 20 families, selected by using a modified centrifugal phylogenetic and variety strategy indicated that only S. zeylanica was susceptible in the absence and presence of supplemental dew. On the basis of morphological and cultural characteristics, pathogenicity on the host, host specificity, and the absence of a previous record of this fungal pathogen on S. zeylanica, the binomial A. alternata f. sp. sphenocleae is proposed.

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