Effect of flooding on root and foliar disease severity on Rhododendron caused by Phytophthora ramorum

It is generally thought that extensive periods of flooding can predispose plants to infection by pathogens. We evaluated the effect of 0, 1, 3, and 7 days of flooding before infection of Rhododendron plants through either wound inoculation of leaves or infestation of the potting mix using two hybrid cultivars 'Catawbiense Boursault' and 'Minnetonka'. Foliage was inoculated with sporangia and potting mix was infested with both sporangia and mycelial agar plugs. Lesion area was quantified using digital imaging. Flooding had no effect on lesion area of foliar infections. Sporangia were retrieved from infected leaves after 10 days of incubation under 20C ambient containment growth chamber conditions. Root rot developed after about 4 weeks on most plants where the potting mix was infested. Above ground symptoms included wilting, yellow or red discoloration, and at times development of lesions similar to those observed in foliar inoculations.