2018 ASHS Annual Conference

Cercospora leaf spot of roses is a fungal disease caused by the pathogen Rosisphaerella rosicola (teleomorph: Mycosphaerella rosicola, syn: Passalora rosicola, Cercospora rosicola Pass.). It mainly causes lesions on the leaves, and in severe cases, chlorosis and defoliation. This disease seems to be especially prevalent in the southern region of the USA. Disease severities and landscape traits were evaluated monthly in replicated field trials in both College Station and Overton, Texas during 2016-2017. The rose trials were a mix of species, modern hybrid and modern shrubs. They were rated on a 0-9 scale which is percentage based with respect to the number of cercospora lesions on the rose canopy. Overton had higher cercospora ratings during both years (2.66 and 3.45) than College Station (0.44 and 1.28). This is due to the higher inoculum and precipitation in Overton. The temperature and humidity were similar in both locations. There was a wide range for cercospora incidence on roses for both years, in which ~20% of the accessions had less than a rating of 5 in Overton and in College Station, only one rose accession had a rating over 5. Only seventeen and seven rose accessions showed no cercospora lesion in College Station in 2016 and 2017 respectively, while in Overton, five rose accession had no cercospora lesions in 2016 and three showed no lesions in 2017. The incidence of cercospora leaf spot was negatively correlated with black spot and weakly negatively correlated with defoliation in both locations. This may be explained by the effects of black spot and other environmental factors on defoliation.