The Q&A Archives: Rose disease

Question: I have a few roses that are getting whitish yellow spots on the leaves and then they shrivle up. they dont really dry up they just go limp and shrivle. any idea what It could be and what I could do for it??

Answer: It sound like the fungal disease called powdery mildew. Powdery mildew is named for the grayish-white, powdery mat consisting of fungus mycelium and masses of spores present on the surface of plant tissues. The mildew mat may vary from white to light gray to light tan, but is distinct and not easily confused with other rose diseases.

Reaction of roses to powdery mildew varies considerably. Age of host tissue at the time of infection, susceptibility of the variety, rate of plant growth, race of the mildew fungus, and environmental and cultural conditions determine the extent of symptoms and injury.

Newly unfolded leaves are the most susceptible to infection. Mature leaves are resistant to mildew and usually show no symptom development or, at most, only small local lesions.

Leaves of garden roses often are attacked first on the lower surface and then later on the upper surface. First symptoms are small, raised, blister-like distortions on the leaf that may or may not be accompanied by a slight purpling and curling.

As symptoms continue to develop, much of the leaf surface becomes covered by the grayish-white mildew (Figure 1), and the leaves are twisted or distorted. The coating of the leaf by the mildew reduces the leaf surface area available for photosynthesis.

When young canes become infected, they are dwarfed and distorted. Severe infections even may kill the tips of tender young canes.

Unopened flower buds sometimes become partially covered with mildew before the leaves show extensive symptoms. The petals are usually not affected, but the sepals can be covered with mildew. Infection of flower buds causes poor quality flower formation.

Environment plays a major role in powdery mildew development. The disease occurs during cloudy, humid conditions when days are warm and nights are cool. Powdery mildew is common in Alaska greenhouse grown roses, however, on outdoor grown roses, it often is not seen until late summer.

Day temperatures in the 80s and high night humidity induce mildew formation. Unlike most foliar blights or leaf spot diseases, powdery mildew does not require free moisture on the foliage to infect the plant; however, high humidity is important for infection.

Powdery mildew is common in crowded plantings, in damp areas, or in shaded sites where air movement is restricted.

The powdery mildew fungus survives the winter months as mycelia in rudimentary leaves of buds or in the inner bud scales. The initial infection of roses comes from spores, called conidia, produced on this mycelium.

Secondary or repeating inoculum are conidia produced on mildewed plant tissues during the growing season. Conidia are formed in chains on infected host tissues and are wind-blown or rain-splashed from leaf to leaf and plant to plant.

Mildew can spread rapidly since the disease cycle can be completed in as little as 72 hours. It commonly takes seven to 10 days from the time of infection to the development of symptoms and secondary spore production.

IPM for Powdery Mildew: Rose growers should use an integrated pest management (IPM) approach to control powdery mildew. Diligent monitoring for powdery mildew development is required because the pathogen has developed new races with the ability to attack certain mildew-resistant rose varieties.

Although resistance should continue to be the primary method of controlling powdery mildew, growers should be aware that a variety may be resistant to powdery mildew in one geographical area but not in another. Fortunately, powdery mildew is relatively easy to control by following a few simple practices:

Select powdery mildew resistant roses.
Plant roses in full sunlight.
Do not crowd plants.
Adequately fertilize roses but avoid stimulating succulent growth.
Apply a fungicide at first evidence of mildew and repeat applications as necessary.
Prune infected canes and rake and discard mildewed leaves and flowers during and after the growing season.
Avoid getting the leaf surface wet during irrigation.

As an alternative to fungicides, you can use a Baking Soda Spray. Sodium bicarbonate commonly known as baking soda has been found to posses fungicidal properties. It is recommended for plants that already have powdery mildew to hose down all the infected leaves prior to treatment. This helps to dislodge as many of the spores as possibly to help you get better results. Use as a prevention or as treatment at first signs of any of the diseases.

To make: Mix 1 tablespoon baking soda, 2 1/2 tablespoons vegetable oil (canola oil, soybean oil, olive oil, or neem seed oil) with one gallon of water. Shake this up very thoroughly. To this mix add 1/2 teaspoon of pure castille soap and spray. Be sure to agitate your sprayer while you work to keep the ingredients from separating. Cover upper and lower leaf surfaces and spray some on the soil. Repeat every 5-7 days as needed. (see: Homemade Fungicides for more information)

Hope this information helps!

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