The biology and control of Phytophthora root and stem rot of apple rootstocks from stool beds Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/f1881q530

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  • The present studies were conducted to better understand the etiology, epidemiology and potential control of the Phytophthora stem root rot as it occurs in apple stool beds. Symptoms of Phytophthora root and stem rot occurred as early as May and continued to develop throughout the growing season. Sunken brown-black lesions were visible on shoots, some of which wilted, or exhibited rust-brown or purplish bronzed leaves. Infected roots were red-brown. Some stem lesions were yellow-brown with narrow dark brown margins forming concentric, wavy-margined rings up the stem. Phytophthora cactorum and P. cambivora were both recovered from the stool beds and identified as the probable causal agents of the disease. P. cactorum was more frequently isolated with peak recovery in early September. P. cambivora recovery peaked in mid summer. Both species were pathogenic at either 15 or 24°C, but P. cactorum was more virulent than P. cambivora at both temperatures and is probably the more active of the two species in the field as well. Fosetyl-Al and metalaxyl applied to apple rootstocks in the field reduced the severity of disease subsequently induced by P. cactorum in an excised stem assay. These field treatments also reduced the development of disease in trees graded as healthy and held in cold storage for 3 to 9 months. Root dipping pre-, post-, or pre- and post-cold storage of trees graded either as healthy or mildly infected, with the chemicals benalaxyl, etridiazol, fosetyl-Al or metalaxyl gave variable results. Any chemical dip treatment reduced disease development compared to the untreated controls, but fosetyl-Al and metalaxyl at 12000 or 4000 ppm, respectively, applied pre-cold storage gave the best disease control over all. The development in apple stool beds of stem and root rot caused by P. cactorum and P. cambivora was reduced in the field by applications of fosetyl-Al and/or metalaxyl applied to trees during the growing season, and reduced further by a root dip before cold storage.
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