By C. Rae Hozer
Selecting a rose to add to your landscape this fall? In addition to good looks, consider disease resistance. Using a rose that doesn’t require a lot of pesticides and/or fungicide lowers your maintenance time, saves money and is good for the environment.
Many now available in the marketplace are said to be disease resistant. University of Tennessee researchers have tested a number of those roses to see whether they perform as advertised. Key "soilandpest.utk.edu/pdffiles/gardensolutions/nosprayroses.pdf" into the address line of your web browser to get a copy of a two-page brochure with an overview of the trial results and color photos of 24 roses in the test.
The trials started in 2006 at the West Tennessee Research and Education Center (WTREC) in Jackson. Test plots at the Plateau Research and Education Center (PREC) in Crossville and the Thad Cochran Horticultural Research Laboratory (TCHRL) in Poplarville, MS, were added in 2007. In these tests, roses claiming to be resistant to the fungal diseases black spot and cercospora leaf spot were first exposed to infective agents then set out for field trials. The treated roses which showed less than two percent foliage infection demonstrated superior resistance and were considered "no-spray" roses. They should not require fungicides to maintain clean leaves. These cultivars were Blushing Knockout, Golden Eye, Hansa, Knockout Rose, My Girl, Pink Knockout, White Dawn and Wildberry Breeze.
Roses designated as moderately resistant demonstrated less than 10 percent foliage infection. Only limited fungicide spraying would be needed to keep the leaves clean on moderately resistant roses. Moderately disease resistant roses were identified as being Carefree Sunshine, Como Park, Fiesta, Homerun, Kashmir, Moje Hammarberg, My Hero, Palmengarten Frankfurt, Pink Double Knockout, Super Hero, Wild Spice and Wild Thing.
Tolerant roses might show signs of black spot or cercospora leaf spot and may drop their leaves but should bloom well all summer and any infection should not be fatal. They will come back the next year. Roses considered to be tolerant reported in 2009 were Double Knockout, Forty Heroes, About Face, Bonica, Carefree Delight, Crystal Fairy, Lovely Fairy, Fairy Queen, Snowcone, Carefree Celebration, High Voltage, Red Drift and Crimson Meidiland.
As someone who rarely uses commercial sprays in her gardens, I didn’t try growing roses until the introduction of knockout roses changed my thinking. A rose that was extremely resistant to pests and diseases was something I just had to try. I bought two knockout rose bushes. Knockout roses were said to need less sunlight than most other roses. Good news, I thought. The beds at the front of our house got about five hours of sun each day — just over the four hours of sunlight minimum knockout roses require. They were properly planted and looked good the first few years.
In 2013 the weather changed things. It was a case of buying the right plants but not putting them in the right place. Clouds rolled in. Day after day there was too little sun. Showers and warm temperatures encouraged plants to grow like crazy, elbowing each other for space. Overcrowding led to poor air circulation. Dampness encouraged mildew and other fungus problems. Not a good environment for roses.
The rose bushes weakened and became infested by insects. The foliage was riddled with holes. The leaves eventually grew back. There are some blooms, but the bushes seem to have lost their vigor. I tell this tale on myself to caution other gardeners not to mistake a plant said to be "disease resistant" with one "immune from disease." There are no bullet-proof plants.
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Plateau Gardening is written by Master Gardeners for gardeners in Tennessee’s Upper Cumberland Region. Contact UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (484-6743) for answers to horticulture questions, free publications and how to become a Master Gardener. Send email comments or yard and garden inquiries to Master Gardener Rae, MGardenerRae@frontiernet.net.