When black spots mar the 'Queen of Flowers'

The rose is the "Queen of Flowers." Few would dispute this. Then why isn't the rose the first choice of plants for the landscape? Seldom is it the center around which people plan their landscape.

As revered as the rose flower may be, there is no claim that the rose is the "Queen of Shrubs." It has a reputation as a fussy, temperamental grower that is subject to various and sundry life-threatening problems. Chief among those is the propensity of the rose bush to be stricken with black spot, a fungus disease that can and often is fatal to the plant if left unchecked. Must we be forever consigned to carry a sprayer full of harsh chemicals with us every time we visit with our rose shrubs?

Diplocarpon rosae, or black spot, is a most serious fungal disease that causes defoliation of the plant. Small, round spots, ranging in size from 1/16- to 1/2;-inch in diameter, appear on the upper sides of leaves. Leaf tissue adjacent to the spot turns yellow. Whole leaves eventually turn yellow and fall prematurely. Black spot can be distinguished from other leaf spot diseases by the generally fringed margins and the darker and consistently black color of the leaf spots. Similar spots may appear on petioles and fruit. Raised, reddish-purple spots may also appear on canes. If black spot is left uncontrolled and early defoliation occurs, bushes are weakened and cane dieback the following spring may be severe. Weakened plants may continue to die even after the plants leaf out.

There are two key elements to controlling black spot on roses. First, prevention is easier than cure. Second, find roses that have shown resistance to the fungus. The rosarians of the Medford Rose Society are always happy to share their experiences and offer helpful advice to people who want to grow roses. You can contact them at P.O. Box 132, Medford, OR 97501. Better yet, attend one of their meetings at Anna Maria Creekside, 822 Golf View Drive in Medford. The next meeting is May 19.

To prevent the fungus from growing, we must have at least a rudimentary understanding of the conditions that foster its growth.

During dormancy the fungus survives in infected canes and fallen leaves. Spores are spread to the highly susceptible, young, unfolding leaves in spring by splashing water. Infection takes place only when water remains on the leaves for seven or more hours. Because the fungus tolerates a wide range of temperatures, symptoms can continue to develop all season long if moisture is adequate.

A preventative program for black spot should begin in the fall with a thorough sanitation program. Diseased leaves on the ground should be raked and removed from the garden. All diseased canes should be pruned back to healthy wood. These practices will reduce the amount of overwintering fungus. During the growing season, overhead irrigation, which prolongs leaf wetness, should be avoided. If plants are overhead irrigated, watering should be done in the morning rather than the afternoon so leaves dry quickly. Do not plant roses where there is poor air circulation or in shaded locations. Don't allow the center of the plants to become overcrowded with canes and leaves. Thin at any time.

Fungicides registered for black spot control should be applied preventatively to susceptible roses starting in spring before the new leaves become spotted. Watch for rainy periods when the sprays will have to be re-applied. Fungicides registered for black spot control include Daconil 2787, Fore, Zyban, and Compass. These fungicides can be sprayed at 7- to 10-day intervals when rains are infrequent.

Doug Green, an organic specialist, suggests alternating sprays of baking soda — two tablespoons of baking soda to a gallon of water — with soap as a sticker, with liquid lime sulfur. This will create a confusion of pH levels on the leaves that will prevent the fungus from establishing.

Or, you can be like me and plant a manageable number of resistant varieties and let the rest of the world wield the sprayers. Works for me!

Stan Mapolski, aka The Rogue Gardener, can be heard from 9-10 a.m. Saturday mornings on KMED 1440 AM and seen in periodic gardening segments for KTVL Channel 10 News. Reach him at stanpolski@gmail.com.

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