In the Garden:
Northern & Central Midwest
June, 2003
Regional Report

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1069

Champlain, one of the Canadian Explorer roses, is draped in crimson glory all summer.

Keeping Beautiful Roses Disease-Free

Roses are a favorite in so many gardens. But with those grand hybrid teas and glorious grandifloras come the dreaded rose diseases and pests. Is there an alternative to pouring tons of toxic pesticides to keep these beauties producing the blossoms we so covet? It is possible, but it will take a little more attention to monitoring and prevention than to just fixing a problem once it has engulfed the plant.

Resistant Varieties
The first practice of a good rose grower should be to grow resistant roses. Visit public rose gardens, botanic gardens, or home gardeners who grow roses. If you see a rose that looks particularly clean, find out if it has been sprayed. This will start you on your way to selecting resistant roses for your own garden.

Read the most recent literature and catalogs. Breeders are constantly working on developing resistant roses, and the varieties available are expanded every year. One tip: a rose with a particularly leathery, puckered leaf is usually more resistant to leaf problems than one with smooth leaves.

The most common rose diseases we face in the Midwest are black spot and powdery mildew. Both are fungal diseases, spread by spores being washed or blown onto leaves and stems.

Learn to Recognize Diseases
Black spot starts as tiny black flecks that turn into larger circular black spots. Eventually the spots coalesce into large lesions and cause the leaf to yellow and fall off. The spots appear on leaves and stems, and spend the winter on fallen leaves and infected canes.

Powdery mildew appears first as very small blister-like areas on young leaves, causing them to pucker. As spores reproduce, the leaf and flower buds become covered with a gray-white powder.

Preventive Strategies
Begin monitoring roses for these two diseases as soon as the leaves begin to emerge. Research has shown that black spot spreads most readily when foliage is wet. Therefore, planting in full sun, pruning to keep the shrubs open to circulation, and watering only at ground level will help prevent infection.

Powdery mildew, however, is best prevented by wetting the leaves since it needs dry conditions to thrive. A strong water spray several times a week will help prevent infection. As a bonus, this also helps control pests such as aphids and spider mites.

Another preventive method for both diseases is to remove all foliage on the plant and on the ground in late fall, and to prune the tips of the canes. Both pathogens overwinter in fallen tissue and in the leaf buds just below the flowers. Pruning and clean-up get rid of many overwintering spores. If the canes are badly infected, a rose can be pruned to 4 to 6 inches from the graft with no harm.

Rose growers also frequently have good luck using a baking soda/detergent spray for disease prevention. Research done at Cornell University showed that roses sprayed with the Cornell Fungicide Formula every three or four days had little to no black spot or powdery mildew.


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