Preventing Black-Spot Fungus

By Jeff Rugg

June 15, 2016 5 min read

Q: I've had many rose bushes for years. They look great during the year, but every summer I begin to see black spots developing on all of them. By mid- to late summer most of the leaves have fallen off, and the plants really look sad. Do you know what's happening?

A: Your bushes have black-spot fungus, a fungus that causes black spots to grow as big as 3/4 inch in diameter on rose bush leaves, and sometimes the stems. The infected leaves then turn yellow around the spots and fall off. The environmental conditions that promote black-spot fungus are moisture and warm temperatures. Plants that lose their leaves produce less food and have a harder time surviving winter.

There are certain preventative measures you can take to prevent black-spot fungus and other funguses common to roses like powdery mildew.

First, pick a variety of roses that's resistant to black-spot. If you are interested in a particular variety, research it in books or ask the florist if it is resistant before you buy it. Every rose is pretty, but who wants a naked rose bush? Rugosa roses, shrub roses and many newer hybrid roses are often quite resistant to black-spot fungus and other fungal diseases.

In addition, you must plant roses in the proper microenvironment — an area with well-drained soil and lots of organic matter. Good air circulation within and around the bushes is important, too. You may need to move or remove fences or other shrubs, or even remove some of the interior branches from each bush. Roses need at least six hours of direct sunlight per day, preferably in the morning or first half of the day so the morning dew can dry.

Roses can withstand a lot of heat if they are properly watered. Their roots grow deep if the soil is significantly wet more than a foot deep. Avoid getting the leaves wet when watering the plants, especially late in the day because the leaves could stay wet overnight. Use a soaker hose, or just soak the ground with a light stream from a garden hose.

Infected leaves do not heal; they just spread the disease. So, remove every diseased leaf from infected plants or the ground immediately to prevent further spreading.

Since black-spot fungus is transmitted by water, remove all leaves from the plant close to the ground, since they are more susceptible to being splashed with water. Mulch around the bushes to minimize splashes. Remove all leaves in the fall, and replace the old mulch with new mulch in early spring.

Any plant that needs sufficient growing conditions also needs good culturing, so the next step is to feed the roses properly. If fed a high-nitrogen fertilizer too early in the spring, rose bushes will send out lots of new growth that is susceptible to black-spot fungus. Instead, use 5-10-10 fertilizer a couple of weeks after new growth begins in the spring. Fertilize the plants again when the roses are in full bloom, and fertilize a third time no later than two months before the first fall frost. Slow-release and organic fertilizers tend to work best.

Since your roses regularly get this disease, you should try a preventative fungicide spray. There are two types of fungicides — preventative fungicides and fungicide treatments. Use a preventative fungicide on plants that aren't infected, and use a fungicide treatment if the roses become infected. You can spray the plants more than once at one time, but test a double dosage on a few plants first to see if it causes too many leaves to fall off. Alternating sprays also seems to be more effective than repeating the same one. Additionally, using fungicide sprays together with baking soda and horticultural oil seems to work well.

Remember, any growth that comes out after a spray is not protected, so treatments must be repeated weekly. Many chemical sprays are designed to break down rapidly in sunlight or with water so they don't contaminate the soil. They do not stay at full strength if rained or watered on. Spraying too much at once is wasteful, ineffective and can harm the leaves even more.

Email questions to Jeff Rugg at [email protected] To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

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