P&PDL Picture of the Week for
March 22, 2004

Reduce Defoliation From Rose Black Spot; Plant Tolerant Cultivars

Gail Ruhl, Senior Plant Disease Diagnostician, Interim P&PDL Director

Roses are a popular landscape and garden plant in Indiana. Black spot, caused by the fungus Diplocarpon rosae, is the major foliar disease of roses in Indiana. It is a serious problem every year under our warm, humid, growing conditions. Infected leaves become spotted, turn yellow and drop prematurely from the plant. Premature defoliation decreases plant energy reserves and results in reduced flowering of roses. Rose fanciers wanting to maintain good health of their susceptible rose cultivars are usually obligated to use repeated applications of fungicides throughout the growing season. If rose growers could grow genetically resistant or disease tolerant roses, they would benefit from improved performance and reduced fungicide use.

John Hartman, Extension Plant Pathologist at the University of Kentucky in Lexington, provided the following insights on managing rose black spot with diseases tolerant cultivars in a recent Kentucky Pest News article:

“Rose cultivars are evaluated for disease reactions. Plant Pathologists from around the U.S. periodically evaluate rose cultivars for their reaction to black spot disease. The following black spot-resistant rose lists were assembled from reports of evaluations done in Virginia, Louisiana, Florida, Texas, and Washington. Some of these test results were printed in recent issues of Biological and Cultural Tests for Control of Plant Diseases, an annual journal published by the American Phytopathological Society. Many of these cultivars are also listed in the publication Pest Resistant Ornamental Plants by Deborah C. Smith-Fiola of the Rutgers Cooperative Extension Service.”

“Disease reactions can vary from one location to another depending on disease pressure. Cultivars with resistance may still develop symptoms under high disease pressure, but they survive black spot without much damage to the plant. It is possible for rose disease resistance to break down due to the black spot fungus adapting to the plant’s resistance, so resistance can be lost. Nevertheless, the roses listed here should perform well during most Kentucky disease outbreaks and with less fungicide than most roses.”

“Are Kentucky growers trying these roses? No doubt some growers have tried the disease-tolerant roses. County Agents advising rose growers will want to advise their clients to try these improved rose cultivars. Rose growers could increase their leisure time and decrease their fungicide expenses by using disease resistant and tolerant roses.”

Black spot resistant hybrid tea roses:
“Alec’s Red, Auguste Renoir, Bride’s Dream, Canadian White Star, Carla, Cary Grant, Cayenne, Charlotte Armstrong, Chrysler Imperial, Dainty Bess, Duet, Electron*, Elina, Elizabeth Taylor, First Prize, Forty Niner, Frederic Mistral, Granada, Great Scott, Helmut Schmidt*, Just Joey*, Keepsake*, Lady, Lady Rose, Lady X, Las Vegas*, Love and Peace, Marilyn Monroe, Marijke Koopman, Mikado, Mirandy, Miss All-American Beauty, Mister Lincoln, Nantucket, New Day*, Olympiad, Otto Miller, Pascale, Peggy Rockefeller, Peter Frankenfeld*, Peter Mayle, Pink Peace, Polarstern*, Portrait, Precious Platinum*, Princess of Monaco, Pristine, Proud Land, Sheer Bliss, Silver Jubilee*, Smooth Lady, Sunbright, Sutters Gold, Tansinnroh, The McCartney Rose, Tiffany, Uncle Joe, and Voodoo*.”

Black spot resistant floribunda and grandiflora roses:
“Angel Face, Betty Prior, Bill Warriner, Bonica*, Brass Band, Candelabra, Carousel, Cathedral, Charlotte Ann, City of London, Class Act, Escapade, Europeana*, Fashion, First Edition, First Kiss, French Lace, Gene Boerner, Goldilocks, Gruss an Aachen, Hot Cocoa, Iceberg*, Impatient*, Ivory Fashion, Koricole, Lavaglut, Livin’Easy, Love*, Matangi*, Montezuma, Nearly Wild, New Year*, Orangeade, Pink Parfait, Playboy*, Playgirl*, Pretty Lady, Prima Donna, Prominent, Queen Elizabeth, Razzle Dazzle, Red Gold*, Regensburg*, Rose, Parade, Sarabande*, Sexy Rexy*, Showbix*, Simplicity, Sonia, Sun Flare, Sunsprite, Tournament of Roses*, Trumpeter*, and Viva*.”

Black spot resistant shrub roses:
“Shrub roses are normally pretty resistant to black spot disease. There are many more black spot resistant cultivars than those listed here. Alba Meidiland, All That Jazz, Baby Love, Belinda’s Dream, Caldwell Pink, Carefree Beauty, Carefree Delight, Carefree Sunshine, Carefree Wonder, Distant Drums, Else Poulsen, George Vancouver, Harrison’s Yellow, Katy Road Pink, Knock Out, Mrs. R. M. Fincn, Prairie Harvest, Prairie Sunrise, Robusta, Sea Foam, Simon Fraser, Simplicity, Sir Thomas Lipton, The Fairy, Wanderin’ Wind, and Winter Sunset.”

Black spot resistant climbing roses:
“Dortmund*, Dublin Bay*, Eden, John Davis, New Dawn, Pinkie, Prosperity, Rambling Red, Red Climber, Royal Sunset*, and William Baffin.”

Black spot resistant miniature roses:
“Always a Lady, Angel Darling, Anytime, Apricot Twist, Baby Betsy McCall, Beauty Secret, Black Jade, Centerpiece, Cinderella, Cuddles, Deep Velvet, Green Ice, Gourmet Popcorn, Jennifer, Linville, Little Artist, Loving Touch, Magic Carrousel, Minnie Pearl, Old Glory, Pacesetter, Queen City, Rainbow’s End, Red Cascade, Red Flush, Rose Gilardi, Simplex, Singles Better, Sweet Pickins, Watercolor and Work of Art.”

Black spot resistant Rugosa hybrid roses:
“Rugosa roses are normally resistant to black spot disease. These and other cultivars should do well. Blanc double de Coubert, F. J. Grookendorst, Frau Dagmar Hartopp, Linda Campbell, Polyantha, Rugosa Alba, Rugosa Magnifica, Rugosa Rubra, Rosecraie de l’Hay, The Fairy, and Therese Bugnet.”

Dr. Hartman also noted that the cultivars 'Peace' and 'Tropicana,' although listed as resistant in older literature, may not show disease resistance today. He explained that there are several reasons why these cultivars now fail to resist black spot, including: low disease pressure in the initial tests, inaccurate reporting of results, changes in the black spot fungus allowing it to overcome resistance, or very high disease pressure in locations where these cultivars now fail.

*Also resistant to powdery mildew and rust diseases.

NOTE: Trade names are used to simplify the information presented in this newsletter. No endorsement by the Cooperative Extension Service is intended, nor is criticism implied of similar products that are not named.

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Photos courtesy of Dr. Paul Pecknold

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service