Resistance of garden roses to cercospora leaf spot

Cercospora leaf spot (Cercospora rosicola) is a fungal disease that is prevalent in the southeastern United States. It causes leaf spotting and plant weakening, which may lead to premature defoliation. The current cercospora management practice is repeated fungicide applications throughout the season. Replicated garden rose cultivar field trials in south central (College Station) and northeastern (Overton) Texas were rated monthly for the percent of leaves with cercospora lesions from April through November in 2016 to assess the relative resistance of rose cultivars to cercospora leaf spot. Although both sites had a wide range of cercospora lesion incidence on the roses, the Overton site had a greater average (2.66 vs. 0.44) cercospora leaf spot rating than College Station due to more favorable conditions. At the higher disease pressure location, of the 263 roses accessions evaluated, similar to 13% were highly susceptible with >50% of the leaves infected with cercospora and 6 cultivars showed no cercospora symptoms. A restricted estimated maximum likelihood (REML) genetic variance analysis of field ratings for cercospora incidence of 16 interrelated diploid estimated the narrow sense and broad sense heritabilities were 0.51 and 0.72, respectively. This indicates that the selection for cercospora resistant cultivars should be possible. Further work to determine markers associated with the resistance is ongoing.