INVERT EMULSION DROPLET SIZE AND MYCOHERBICIDAL ACTIVITY OF COLLETOTRICHUM-TRUNCATUM

When formulated and sprayed in an invert emulsion (IE), conidia of the mycoherbicide, Colletotrichum truncatum, controlled hemp sesbania in the absence of dew. To optimize hemp sesbania control, formulation droplet size influence upon the pathogen's germination and pathogenicity was investigated. Conidia germination in manually produced IE droplets decreased from 46% to 5% as droplet diameter decreased from 2700 to 900 mum. Droplet size did not affect appressoria formation. On a per conidium basis, 900-mum droplets were more pathogenic to detached hemp sesbania leaves than were 2100-mum droplets. An air-assist spray system produced droplet spectra with volume median diameters of 421 and 104 mum, respectively. The spectrum of smaller droplets covered the target better than did that of larger droplets. When applied to whole hemp sesbania plants in greenhouse trials, conidia in the smaller and larger droplet spectra gave 90 and 94% control, respectively. Sufficient conidia germinated in IE droplets of a variety of sizes to control hemp sesbania excellently.