"Phytophthora and Pythium are commonly known as water molds, and can cause enormous damages to many floriculture and vegetable crops worldwide, including seedling damping-off, stunting, and crown, stem and root rot. It is challenging to control these pathogens because plants can be infected and do not show symptoms until the disease is too advanced to respond to treatment. The pathogens can also easily develop resistance to effective fungicides. As the zoospore movement with water flow is a major transmission pathway of these pathogens, understanding the transport of zoospores in natural and engineered systems is critical to developing strategies to control the pathogens in both field and greenhouse crops. Thus, the first study investigated the transport and retention of Phytophthora. capsici zoospores in saturated columns packed with iron oxide coated sand (IOCS) or uncoated sand in Na+ or Ca2+ background solution at pH 7.2 or 4.4, in combination with XDLVO interaction energy calculations and microscopic visualizations. Significantly more encysted zoospores were retained in IOCS than in uncoated sand, and at pH 4.4 than at pH 7.2, which likely resulted from increased electrostatic attraction between zoospores and grain surface. At pH 7.2, up to 99% and 96% of the encysted zoospores were removed in IOCS and uncoated sand, respectively, due to a combination of strong surface attachment, pore straining, and adhesive interactions. Motile biflagellate zoospores were more readily transported than encysted zoospores, thus posing a greater dispersal and infection risk. The second and third studies were conducted in a greenhouse to demonstrate a proof-of-concept of using fast-flow filtration to control Phytophthora and Pythium diseases in greenhouse floriculture and vegetable crops. The second study showed that Pythium aphanidermatum could be effectively removed by the fast-flow sand and AC filters at low water pressure. The rapid sand filter had the best performance because no decrease in the poinsettia quality was observed when compared to the non-inoculated control plants. Because the AC filter could also remove the essential nutrients from the irrigation water, and cause the Fe deficiency in the poinsettias, it is less desirable to be used unless the nutrients can be supplied separately instead of through irrigation water. The third study found that the filter with iron oxide coated media [IOCM] could effectively protect the squash plants from Phytophthora capsici, but caused the nutrient deficiency in the squash. The sand filter could not prevent, but only slow the disease development in the squash. Again, it shows that the IOCM filter has the potential to be used in treating irrigation water in the greenhouse vegetable production, but sufficient nutrients also need to be provided. Overall, the results suggested that physical removal of pathogens using fast-flow filtration can overcome many limitations of fungicide application, and may be a promising alternative for disease management in greenhouses."--Pages ii-iii.
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