Resistance of Shrub and Groundcover Roses to Black Spot and Cercospora Leaf Spot, and Impact of Fungicide Inputs on the Severity of both Diseases
Authors: Hagan A.K. , Rivas-Davila M.E. , Akridge J.R. , Olive J.W. 
Published: August 2020

Reaction of selected shrub and ground cover roses to black spot, Cercospora leaf spot, and powdery mildew, as well as the impact of fungicide inputs on the control of the above diseases, was assessed from 1999 through 2003 in a simulated landscape planting in Brewton, AL. Chlorothalonil at 1.25 g ai/liter was applied at 2- and 4-week intervals from mid-March until October to randomly selected plants in each replicate. An unsprayed control was also included in each replicate. Although black spot was the predominate disease observed, a number of rose selections suffered from objectionable Cercospora leaf spot-induced leaf spotting and premature defoliation. Few mixed outbreaks of black spot and Cercospora leaf spot on a single selection were seen. In all years, significant differences in the reaction of rose selections to black spot and Cercospora leaf spot were noted. Of the roses damaged by black spot, the least leaf spot and defoliation were noted on the unsprayed Ice Meidiland®, Mystic Meidiland®, Red Cascade™, ‘Hansa’, ‘Pink Grootendorst’, ‘Pink Pet’, and to a lesser extent Carefree Wonder™ and Pearl Sevillana™. In a residential planting, monthly applications of chlorothalonil or other recommended fungicide would be needed to protect the above rose selections from a destructive black spot outbreak. ‘Betty Prior’, Bonica®, Cherry Meidiland®, First Light™, Kent™, Jeepers Creeper™, ‘Livin’ Easy™, Lilian Austin™, ‘Nearly Wild’, ‘Nozomi’, Butterfly rose, Ralph's Creeper™, Raven™, Royal Bonica™, ‘Sea Foam’, Sevillana™, and Sweet Chariot™ were susceptible to black spot. While black spot did not appreciably damage Carefree Delight™, Flower Carpet®, White Flower Carpet®, Fire Meidiland®, Fuchsia Meidiland®, Happy Trails™, ‘Petite Pink Scotch’, Polar Ice™, R. wichurana, The Fairy™, and ‘Therese Bugnet’, considerable Cercospora leaf spot development occurred on all of the above rose selections. Of these roses, Polar Ice™, Fuchsia Meidiland®, and Fire Meidiland® exhibited the highest resistance to Cercospora leaf spot and may not require any fungicide inputs to maintain plant health and vigor. Magic Carpet™ and Knock Out™ roses, which were susceptible and resistant to black spot, respectively, as well as Flower Carpet®, and White Flower Carpet® appeared to be poorly adapted to the hot and sometimes dry summer weather patterns of South Alabama. In nearly all years, chlorothalonil gave better control of both diseases when applied on a 2-week than on a 4-week schedule. Significant chlorothalonil-induced leaf burn was seen on First Light™, Flower Carpet®, ‘Hansa’, Happy Trails™, Magic Carpet™, Mystic Meidiland™, ‘Nozomi’, and Raven™. Consistent powdery mildew development was found only on ‘Therese Bugnet’ and to a lesser extent on Red Cascade™ and ‘Petite Pink Scotch’. Canopy spread of the roses that were heavily damaged by black spot and Cercospora leaf spot often was often reduced in size when compared with that of adjacent chlorothalonil-treated plants of the same selection. In contrast, little if any increase in growth was obtained with fungicide inputs for the more disease resistant rose selections.