Fungicide resistance in Pythium and Phytophthora from ornamentals in Georgia
2014
William-Woodward, Jean L. | DeMott, Max E.
The majority of root and crown diseases on ornamental crops are caused by oomycete pathogens, including species of Pythium and Phytophthora. Both Pythium and Phytophthora cause root, crown, stem, and foliage blights. Symptoms often include root softening, sloughing, darkening of roots, crowns and stems, wilting, foliage chlorosis, leaf drop, stem dieback, and leaf and petiole blighting. Oomycete pathogens or “water molds” as they are commonly called, which also includes downy mildew causing pathogens, are unique and are not true fungi. They are more closely related to brown algae than fungi. One of the major differences between oomycetes and true fungi is in their cell wall components. Oomycete cell walls are composed of β-1,3 and β-1,6 glucans, whereas true fungi cell walls are composed of chitin. This is an important distinction because the mode of action of many fungicides is to act on and inhibit chitin cell wall biosynthesis. Since Oomycete cell walls do not contain chitin, these products have no activity on these pathogens. This has resulted in a limited number of commercially available fungicides with activity against Pythium, Phytophthora, and downy mildew diseases.
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