Extension News

Beat Black Spot with Resistant Roses

Note to media editors: This is the Garden Column for the week of May 18, 2007.

5/11/2007

By Mark Gleason
Plant Pathologist
Iowa State University Extension

Roses are the supermodels of many Iowa gardens.  Hostas have a certain something, daylilies are nice in their own way, but nothing outshines roses when they are in bloom. Their shape, color and perfume put everything else in the shade. It’s no surprise that Iowa State University’s Reiman Gardens, the largest landscape garden in Iowa, relies heavily on its rose collection to entice visitors.

Like other supermodels, roses can be temperamental and high maintenance. Their thorny ways intimidate gardeners, many of whom refuse to plant them. Hostas and daylilies may lack the pizzazz of roses, but are much better behaved.

How did roses get this blotchy reputation? A disease called black spot deserves much of the blame. Caused by the fungus Diplocarpon rosae, black spot occurs almost everywhere in the world where summertime weather is moist and humid. 

Infected leaves develop coal-black spots up to half an inch across, turn yellow and drop prematurely from the plant. Besides their dark color, the spots are distinctive because they have “fringed margins.” In other words, the edges of the spots seem to be bleeding into the rest of the leaf like an ink blot.

In severe cases of black spot, nearly all the leaves fall off.  Premature defoliation depletes the plant’s energy reserves, resulting in fewer flowers. The plant doesn’t die, but you may fantasize about killing it out of pure frustration. In fact, many gardeners end their rose-induced torment with a shovel.

It doesn’t have to be this way. Happier experiences can begin with something as simple as choosing the right rose cultivar. Luckily, many rose cultivars now have a high level of resistance to black spot. Growing resistant or tolerant roses means that you will get better disease-control results with less frustration and less fungicide spraying.

Ironically, most roses offered for sale in garden centers and big-box stores are susceptible to black spot. Is this evidence of a dark conspiracy by retailers? Not really.  Susceptible cultivars are sold because the public continues to buy and request them.  Most gardeners, and even some retailers, are unaware that there are better alternatives.

Where can you find lists of black-spot-resistant cultivars? A good place to start is the Web. For starters, an article from a Purdue University Extension newsletter offers an extensive list (http://www.ppdl.purdue.edu/ppdl/weeklypics/3-22-04.html).

 Dozens of field trials have been conducted in recent decades to find the most resistant cultivars, and rose breeders release new ones continually. There are now many choices of resistant cultivars in every rose category, including hybrid teas, floribundas and grandifloras, shrub roses, climbing roses, miniature roses and Rugosa hybrids. 

The next question is where to buy them.  Local garden centers often stock several of the more popular resistant cultivars, but you may have to go to the Web to find particular favorites. Keep in mind that garden centers will respond to rising customer demand for resistant cultivars by stocking more of them.

Some of the most popular shrub roses with black spot resistance were developed right here at Iowa State University. The late Dr. Griffith Buck, a long-time faculty member in the ISU Department of Horticulture, released dozens of cultivars during his career. Among the best-loved Buck roses is Carefree Beauty.

At Reiman Gardens, Daren Mueller and I compared 25 Buck rose cultivars for black spot resistance during the 2005 and 2006 growing seasons. Carefree Beauty came out on top, but lesser known Buck cultivars such as Aunt Honey, Honeysweet, Earthsong, Pearlie Mae, Les Sjulin, Piccolo Pete, Prairie Squire, Applejack, Prairie Breeze and Pipedreams also showed good levels of resistance. 

Carefree Beauty is widely sold in garden centers, but many of the Buck roses can be obtained from Sam Kedem roses (http://www.kedemroses.com/avail%202005.htm) among other Web sources.

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Contacts :

Mark Gleason, Plant Pathology, (515) 294-0579, mgleason@iastate.edu

Jean McGuire, Extension Communications and Marketing, (515) 294-7033, jmcguire@iastate.edu