Transport Mechanisms of Motile and Nonmotile Phytophthora cactorum Zoospores in Unsaturated Porous Media

Phytophthora is a genus of plant pathogens that infects vegetables, crops, ornamentals, and trees. Phytophthora diseases in soil are facilitated by the rapid dispersal of zoospores, which come in two different forms, that is, motile biflagellate zoospores and nonmotile encysted zoospores. However, little is known about the mechanisms controlling zoospore dispersal, especially under unsaturated flow and in natural soil. Hence, we investigated the transport and retention of motile and nonmotile P. cactorum zoospores under unsaturated conditions in inert sand, disturbed soil, and undisturbed soil. Transport of carboxylate-modified polystyrene microspheres in sand was studied to compare the zoospores to abiotic particles. Similar to microspheres, motile and nonmotile zoospores were affected by pore straining, wedging, settling, and fluid drag during transport due to their large size (6-10 mu m). Decreased water saturation of porous media reduced accessible pore space, enhancing the retention of zoospores. Nonuniform flow in the undisturbed soil facilitated the transport of zoospores. Derjaguin-Landau-Verwey-Overbeek calculations indicated unfavorable attachment of both motile and nonmotile zoospores to the solid-water interface, and repulsion from the air-water interface. Microscopic visualizations showed that motile zoospores bounced off the air-water interface, and swam randomly around root and soil surfaces without being attached; however, they actively moved toward baits. Motile zoospores showed enhanced transport in porous media relative to nonmotile zoospores, suggesting that the flagella helped zoospores to actively avoid retention sites. The transport of Phytophthora zoospores in natural soil was limited, and natural soil acted as an effective filter confining Phytophthora dispersal in the top soil. However, macropores can be a conduit for the transport of zoospores, especially motile zoospores, to deeper soil layers.