Transport of Phytophthora cactorum Zoospores in Unsaturated Sand Columns

Phytophthora cactorum, is an economically important plant pathogen causing sever damage on a wide range of crops, fruits, and ornamental plants. P. cactorum's infective potential and its spatial dissemination relies on rapid and copious production of asexual zoospores. However, little is known about P. cactorum zoospore transport behavior in porous media, impeding the development of effective technology in controlling  infection and disease spreading. In this study, we investigated the transport and retention of P. cactorum zoospores in sand columns at three levels of water saturation (100%, 65%, 48%). Both motile and encysted zoospores were studied as well as carboxylate-modified polystyrene microspheres with the same size as the zoospores (10 um). We hypothesized that (1) motile zoospores are more readily transported than polystyrene microspheres under the same conditions, implying the minor role of size (straining) and the critical role of zoospore motility in controlling zoospore transport; (2) the higher mobility of motile zoospores compared to that of encysted zoospores is caused by the lack of sticky chemicals and the existence of flagella on the surface of motile zoospores; and (3) transport of motile and encysted zoospores, as well as of polystyrene microspheres is enhanced as column saturation increases.