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Michael C. Long*, Stephen L. Krebs, and Stan C. Hokanson

Forty deciduous azalea (Rhododendron sp.) cultivars from commercial sources were evaluated for powdery mildew (Microsphaera sp.) resistance. Plants were established in two duplicate field plantings in Ohio and Minnesota and evaluated in 2002 and 2003. Plants were scored using a disease symptom rating based on the percent of leaf area infected, evaluating both ab- and adaxial leaf surfaces. Highly significant differences were found for cultivar, location, year, cultivar × location and cultivar × year for disease severity. Calendulaceum × speciosum, `Fragrant Star', `Garden Party', `Late Lady', `Millennium', `Parade', and `Popsicle' showed no powdery mildew symptoms in both locations. Another group of plants with only minimal symptoms (<25% leaf area affected) included `Jane Abbott', `Magic', `Northern Hi-Lights' and `Snowbird'. The symptom-free cultivars exhibited glaucous foliage, suggesting a potential, common resistance mechanism. The mean scores for the abaxial and adaxial leaf surfaces were 2.34 and 1.64, respectively, although four cultivars had more disease symptoms on the adaxial surface. `Arneson Gem' showed nearly a two-point difference between abaxial and adaxial scores. Evaluations of azalea powdery mildew susceptibility should consider both leaf surfaces and use the highest score as the best estimate of host resistance.

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Michael C. Long, Stephen L. Krebs, and Stan C. Hokanson

Forty-one deciduous azalea (Rhododendron subgen. Pentanthera G. Don) cultivars were assessed for powdery mildew (PM) resistance in a two-location, 3-year field trial. Disease severity (percent leaf area affected) on abaxial leaf surfaces was used to rate the level of field resistance. This measure was proportional to (r = 0.83) but higher than estimates from corresponding adaxial surfaces. Eleven of these cultivars (27%) appeared to be highly resistant under field conditions, i.e., evidence of PM on the leaves was zero or near zero. Twenty-three of the cultivars evaluated in the field experiment were also evaluated in a growth chamber experiment. In contrast to the field study, PM was more severe on the adaxial leaf surface in the growth chamber but still highly correlated with the abaxial response (r = 0.93). Based on adaxial disease scores, no cultivars in the growth chamber experiments were completely resistant. Growth chamber disease ratings based on either leaf surface were predictive of field performance (r 2 = 0.62), suggesting use of the chambers could serve as a low-cost, off-season, early selection component of a deciduous azalea PM resistance breeding program.