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Rita L. Hummel, Marianne Elliott, Gary Chastagner, Robert E. Riley, Kathy Riley and Annie DeBauw

Growth and susceptibility of evergreen Rhododendron ‘English Roseum’, ‘Cunningham’s White’, and ‘Compact P.J.M.’ to Phytophthora ramorum in response to biweekly nitrogen (N) fertilizer application at rates of 25, 75, and 150 mg N per 11.4-L container was evaluated during two growing seasons. At the end of both growing seasons, horticultural evaluation of the different plants showed that 150 mg N-fertilized cultivars had superior shoot growth, visual quality, leaf color, and the highest leaf N concentration, whereas the 25-mg N cultivars were inferior for these characteristics. Plants fertilized with the 75-mg N rate were typically intermediate to the 150- and 25-mg N plants for the measured characteristics. During the first growing season, the number of flower buds on ‘Cunningham’s White’ and ‘English Roseum’ was not influenced by N rate but the second season bud numbers increased with increasing N fertilizer. Foliar susceptibility to P. ramorum was influenced by N fertilizer application rates in the most susceptible cultivars, ‘English Roseum’ and ‘Cunningham’s White’, in which lesion size and infection frequency both increased at higher N rates. The results were variable in ‘Compact P.J.M.’, the most resistant cultivar.