Influence of Ozone on the Severity of Phytophthora Root Rot of Azalea and Rhododendron Cultivars

The susceptibility of 21 azalea and rhododendron cultivars to colonization by Phytophthora cinnamomi and to injury caused by ozone was determined. Resistance to P. cinnamomi was lacking in most cultivars with only 3 rhododendron cultivars ‘Caroline,’ ‘Chionoides’ and ‘English Roseum’ and 3 azalea cultivars ‘Hinodegiri,’ ‘Sweetheard Supreme’ and ‘Tradition’ having resistance. All of the rhododendron cultivars, except ‘Nova Zembla,’ were highly resistant to ozone injury. Of the azalea cultivars, only ‘Delaware Valley White,’ ‘Roadrunner’ and ‘White Water’ exhibited injury following fumigation with 0.20 ppm ozone for 6 hours on 3 consecutive days. The fumigation of P. cinnamomi-inoculated plants with ozone significantly increased the severity of Phytophthora root rot only in ‘Hinodegiri’ plants, but a trend towards greater disease severity was evident in many cultivars.