In the Garden:
Western Mountains and High Plains
June, 2014
Regional Report

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Growing healthy roses like this Harrison's yellow rose is easy with the right cultural conditions.

Keep Your Roses Healthy

Growing roses in our region is often a challenge, but with the proper soil preparation, site placement, and common-sense care techniques, roses will thrive with vigorous green foliage and an abundance of blooms.

Roses can be temperamental about their growing conditions, and if you're like many gardeners, you don't have time to baby the bushes along throughout the season. To protect your rose garden from a severe outbreak of some of the most common diseases, follow some sensible growing practices.

Powdery Mildew
One of the most common rose diseases in our region shows up as a white, powdery substance on the surface of the leaves. Infected leaves turn purplish and curl upwards, and flower buds die without opening. Powdery mildew is perhaps the most frequently occurring disease on roses -- both shrub and tea roses -- in our climate because it doesn't need water on the leaves for an infection to start, although high humidity promotes the disease. It is especially common when the day temperatures are hot and nights are cool. The disease spores are spread by wind.

To control powdery mildew, space rose bushes far enough apart to allow for good air circulation. Avoid drought stress by mulching with shredded cedar, and water at ground level in early morning so leaves are dry by nightfall, which will reduce the relative humidity around leaves. Clean up all infected plant debris well at the end of the season to reduce the amount of fungus that overwinters.

If powdery mildew is a big problem on your roses, you can spray with a fungicide containing neem oil or potassium bicarbonate at the first sign of the disease, repeating at the intervals as directed on the label. This will not cure existing infections, but will prevent new ones from occurring.

Black Leaf Spot
If tiny black spots appear on the leaves in early to late summer, followed by fringed margins on the leaves, you are seeing an infection of black leaf spot disease. Infected leaves eventually turn yellow and fall to the ground.

Choose resistant varieties of roses, if possible. In our region, blackspot is most often seen on some of the old-fashioned roses; hybrid teas as not usually troubled by this disease. Remove fallen leaves to reduce the spread of the disease to new leaf growth. Applying water at ground level so the foliage stays as dry as possible will help reduce the outbreak of blackspot.

Rust
A common disease that covers rose leaves with tiny, orange/black spores, rust is appropriately named. It causes the foliage to fall off and weakens the plants. As with blackspot, remove the leaves that show symptoms of infection, collect fallen leaves, and dispose of the debris. Do not compost infected leaves because most composting techniques will not heat up enough to kill the disease organisms. Look for rust-resistant varieties.

Rose Mosaic
One of the most frustrating diseases is rose mosaic. This problem is often identified by the characteristic bright lime green or yellow patterns on rose leaves. It's caused by a virus and most often attacks weakened or stressed plants. Infected plants are stunted and have reduced flowering. There are no cures for rose mosaic. Replace infected plants with virus-free rose bushes from a reputable grower.

Protect your rose garden from a severe disease outbreak this year by following good growing practices. Avoid overhead watering, especially at night, and use drip irrigation or a soaker hose that will soak the soil while avoiding excess moisture on the foliage. Prune your roses on a regular basis to allow for good air circulation, which reduces the incidence of disease contamination. The best preventative is to select disease-resistant varieties when planning your rose garden.


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