Rose Disease and Fungi

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Fungi cause most rose disease. The good news is that no matter which fungus is damaging your roses, the treatment options are similar.

yellow rose

pink rose

orange rose

Types of Fungal Diseases 
The fungus diplocarpon rosae causes blackspot. This rose fungus starts with small black spots surrounded by a yellow halo on the leaves and can eventually cause a complete defoliation if you do not treat it. This fungus spreads through splashing water, and infection can occur if the leaves are wet for a few hours.

Powdery mildew is caused by the sphaerotheca pannosa fungus. Powdery mildew is characterized by white or grayish powdery growth on the leaves, shoots, buds, and sometimes the petals of rose bushes. These small patches of mildew spread in white strands across the plant’s foliage. As the fungus draws moisture and nutrients from the leaves, the leaves begin to crinkle and fall. Powdery mildew thrives in cloudy, humid conditions when days are warm and nights are cool. However, this mildew occurs on DRY, not wet, plants. Direct overhead watering during the midday may actually help the plant by breaking up spores that are released during the day.

The other common type of mildew is downy mildew, caused by the peronospora sparsa fungus. Downy mildew thrives in cool, humid conditions and most commonly occurs under the leaves of the plant. Other characteristics of this downy mildrew include purple, red, or brown spots on leaves, usually followed by yellowing of the leaves, then loss of leaves.

The fungus Botrytis Blight affects the flower buds, often causing them to decay and die. This grayish-black fungus develops below the bloom’s head. You need to cut off and remove any decaying or dead blossoms to prevent the spread of Botrytis Blight.

Rust, which looks just like the rust that develops on your car, can first appear on the underside of leaves. If this fungal rose disease is not treated, orange and brown spots can also develop on the top surface of leaves.

The brown canker fungus can attack any part of the plant that is above the ground, sometimes killing an entire stem. If you see red or purple spots on the new canes or gray-white lesions on more mature stems, you might be dealing with brown canker.

Preventing Rose Diseases

Prevention is always the best treatment. Take steps to prevent rose disease caused by fungi:

    • Plant your roses in an area with good drainage and air circulation. Shady spots encourage moisture and fungus growth.
    • Water the soil, not the actual plant. Most fungi are attracted to moisture on leaves, blooms, and stems.
    • Check regularly for exposed canes because they let fungi find their way into the plants.
  • Keep the plant clean and remove plant litter, particularly fallen leaves and petals that gather at the base. Also, remove canes, leaves, or blooms that you suspect have fungus. Throw them away or burn them; putting diseased plant litter in your compost pile will only add to your fungus problems.

Learn more about preventing rose disease.

Getting Rid of Fungus

If you discover fungal rose disease on your plants, remove infected leaves, blooms, and canes. Not removing infected foliage makes it easy for any fungus to spread from part of the plant to another or even from one plant to another one.

For a homemade treatment and prevention option for fungus problems, a mixture of water, baking soda, and dishwashing liquid is effective. Combine one tablespoon baking soda and either two tablespoons horticultural oil or a few drops of dishwashing liquid detergent with 1 gallon of water. Mix thoroughly, and spray both sides of the leaves once a week.

You could spray with a fungicide, but choose carefully. Always look out forenvironmentally friendly fungicide options.

Damp, humid climates are more prone to fungal rose disease than other locations, so if you live in a humid area, you need to be alert to fungus. You may prefer to plant disease-resistant species of roses. And remember that dealing with fungus problems as soon as you spot them makes all the difference.

Rose photos courtesy of www.cepolina.com/freephoto/.


Updated: June 14, 2013 — 9:34 am

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