The Q&A Archives: Help needed

Question: I have an indoor/outdoor Parade mini Rose bush (which I have indoors) that has some sort of dust/powder on some of the leaves. I also found a white/gray spider on it yesterday (which is no longer living on the plant ~ sigh) and today found the web. Also, new growth is wilting. What's wrong?

Answer: It sounds like you may have powdery mildew on your mini-rose.

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 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 of the leaf.

As symptoms continue to develop, much of the leaf surface becomes covered by the grayish-white mildew, 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 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. (As temperature goes down at night, relative humidity increases.) 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.

There are fungicides registered for use against powdery mildew on roses. There are also alternatives to fungicides: Baking Soda Spray, Oil Sprays, Soap Sprays, and Garlic Oil Sprays for powdery mildew has been used successfully as a general fungicide.

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 possible 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.

Although I've never tried it, many gardeners swear by a milk spray to control powdery mildew. The milk recipe is simple. You mix one part whole milk and two parts water. ( Try 2 oz. of milk to 6 oz. of tap water and put it in a small spray bottle.) Spray the milk mixture on the upper and lower leaf surfaces and all the stems of your mini-rose. I'd set the plant in the kitchen sink or bathtub so you can spray throughly without worry of having to mop everything up afterwards!

Best wishes with your mini-rose!

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