Austin Hagan, J. Randy Akridge, and Ken Tilt

Few flowering shrubs can match the beauty, versatility, and popularity of roses. In recent years, interest of rose fanciers and landscape designers has expanded to include hybrid tea, Florabunda, and Grandaflora roses but also newly released cultivars of shrub and ground cover roses. Typically, diseases, particularly black spot, have a detrimental impact on the aesthetics and health of the vast majority of rose cultivars. Alabama’s often wet and warm climate along with a long growing season, which favors the rapid development and spread of disease, mandates the adoption of an intensive, season-long fungicide spray program to maintain the health and beauty of most cultivated roses. Although the susceptibility of most hybrid tea, Florabunda, and Grandiflora roses is well known, the reaction of newly released shrub and ground cover roses to diseases is largely unknown. Also, little or no information is available concerning their adaptability to Alabama’s hot summers.

Results of an AAES study started in the spring of 1998 indicate that selected cultivars of ground cover and shrub roses in a simulated landscape planting significantly differ in their susceptibility to black spot and Cercospora leaf spot. Although many of the cultivars tested proved highly susceptible to black spot, a few suffered only light to moderate damage and may be good choices in low maintenance landscapes.

METHODS
Bare-root roses were potted in a pine bark/peat moss (3:1 by volume) medium amended with 14 pounds of 17-7-12 Osmocote, 6 pounds of dolomitic limestone, 2 pounds of gypsum, and 1.5 pounds of Micromax per cubic yard. One month later, the potted roses were transplanted into raised beds at the Brewton Experiment Field where the fertility and pH had been adjusted according to the results of a soil assay. The beds were mulched with aged pine bark and a drip irrigation system was installed. During the growing season, the roses were watered as needed.

A tank mixture of 1 pound of Gallery 75DF and 2 quarts of Surflan T/O per acre was broadcast over the beds for preemergence weed control. Hand weeding and directed applications of recommended rates of Roundup or MSMA were used to control escaped weeds. Ammonium nitrate at the rate of 40 pounds per treated acre was broadcast monthly during the growing season over the beds. The severity of black spot and Cercospora leaf spot was rated on a scale of 1 to 10 on April 25, June 3, August 5, October 16, and December 3, 1998. The rating scale is described in the footnote to the table.

RESULTS
In 1998, weather patterns from late April through much of August were unusually hot and dry. As a result, the development and spread of black spot and other leaf diseases during that time was suppressed. In September, an excess of 30 inches of rain was recorded from two tropical storms. The weather, which remained relatively wet and unseasonably mild from October into early December, favored the onset and rapid spread of disease.

Although none of the 25 cultivars screened were immune to both diseases, considerable differences in the severity of black spot and Cercospora leaf spot were seen (see table). By far, black spot was the predominate disease observed. Black spot-related spotting of the foliage and early leaf shed was recorded on all but one cultivar ‘Petite Pink Scotch’, which remained free of this disease all season. With disease ratings of 2.6 to 3.8 at seasons-end, ‘Flower Carpet’, ‘Ralph’s Creeper’, ‘Magic Carpet’, ‘Happy Trails’, Rosa. wichuraiana, R. mutabulis, and ‘Red Cascade’ generally suffered light to moderate spotting of the leaves in the lower to mid-canopy and possibly some light defoliation around the base of the plant. Moderate spotting of the leaves with increasingly heavier leaf shed was recorded for ‘White Flower Carpet’, ‘Nozomi’, and ‘Fushia Meidiland’. The heaviest spotting and yellowing of the leaves and defoliation levels exceeding 50% (as indicated by disease ratings of 6.0 to 6.6) were noted for ‘Cherry Meidiland’, ‘Bonica’, ‘Carefree Delight’, ‘First Light’, ‘Sevillana’, ‘Betty Prior’, and ‘Royal Bonica’. On the remaining cultivars with disease ratings of 5.0 to 5.8, black spot-related leaf spot and premature leaf shed was extensive.

Development of Cercospora leaf spot was also noted on two cultivars. On ‘Petite Pink Scotch’ (disease rating of 5.6), symptoms of only Cercospora leaf spot were observed while the ‘The Fairy’ (disease rating of 5.0) may have been damaged by both black spot and Cercospora leaf spot. On both cultivars, heavy spotting of the leaves in the lower and mid-canopy along with noticeable defoliation was observed.

Observations concerning the appearance of the foliage and plant vigor were also recorded during the growing season. Some cultivars tolerated the hot summer of 1998 while others did not. ‘Magic Carpet’, which demonstrated resistance to black spot in the December ratings, suffered greatly from the high summer temperatures. By August, this cultivar had shed nearly all of its leaves and shoot growth had ceased. Extensive bronzing or yellowing of the leaves along with some premature defoliation was noted that same month on ‘White Flower Carpet’ and ‘Flower Carpet’. Like ‘Magic Carpet’, both of these cultivars had partial to full recovery by early December from summer heat stress. Some light bronzing or discoloration of the older leaves, particularly those around the base of the plant, was recorded on the majority of the remaining cultivars. The foliage of ‘Fushia Meidiland’, which appeared to have suffered from some mineral deficiency, remained yellow or chlorotic throughout much of the growing season. Among the cultivars tested, R. mutabulis, R. wichuraiana, ‘Bonica’, ‘Jeeper’s Creeper’, ‘The Fairy’, ‘Ralph’s Creeper’, ‘Nozomi’, ‘Petite Pink Scotch’, ‘Livin’ Easy’, ‘Nearly Wild’, and ‘Mystic Meidiland’ appeared to have the best heat tolerance.

In summary, significant differences in the disease severity and general adaptability were noted among the 25 cultivars of ground cover and shrub roses. Those cultivars with the best resistance to disease and heat stress were ‘Ralph’s Creeper’, ‘Happy Trails’, R. mutabulis, and R. wichuraina; all would be excellent choices in a low maintenance landscape. Although some light bronzing of the inner canopy was seen on ‘Red Cascade’, this cultivar also demonstrated disease resistance. The heat sensitive cultivars ‘Flower Carpet’, ‘White Flower Carpet’, and ‘Magic Carpet’ which appeared unsightly and unthrifty throughout much of the growing season, would be poor choices for landscape plantings across much of Alabama. To maintain plant health and beauty of the remaining roses, an intensive, season-long fungicide spray program would be required to keep black spot or Cercospora leaf spot in check.