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Downy Mildew

Rx FOR HEALTHY ROSES - NASTY DISEASES
by Nanette Londeree, Master Rosarian

For most rose gardeners, the biggest challenge to growing healthy roses is keeping them free of disease, especially anthracnose, blackspot, downy mildew and rust. Caused by nasty fungi, this quartet of diseases is considered to be the pestilence of the wet spring and fall months, but can infect your garden during our dry summer months given the right conditions. Effectively managing these invaders employs planning, vigilance and patience!

Anthracnose, also known as purple spotting, spot anthracnose or shot-hole fungus is caused by Sphaceloma rosarum. Symptoms include red or sometimes brown to purple circular spots, 1/16” to 1/8” in diameter that are mostly on the leaves and occasionally on the stem. As the disease progresses, the spots may enlarge with the centers of the spots turning gray, tan or white with a dark red margin. The spot itself drops out of the leaf, leaving a circular hole, thus creating a “shot hole” effect, while most of the leaf remains green. In severe cases, the entire leaf will turn yellow and drop.

Diplocarpon rosae is the fungus that causes blackspot. It produces characteristic round black spots with fringed or feathery margins on the upper surface of leaves or stems. On some varieties of roses, there may be yellowing around the spots that extend to the entire leaf. The spots are generally seen first on leaves close to the ground. Infected leaves will drop off, and the plant may be almost completely defoliated. Such plants are badly weakened and may die over the winter.

One of the most insidious of the fungi that attack roses is downy mildew, caused by the fungus Peronospora sparsa. It can defoliate a plant in just a day or two. Contrary to its name, it really doesn’t look “downy” on roses. Symptoms on leaves vary from yellow, purple to brown irregularly shaped interveinal blotches surrounded by a yellowing of the leaves to a scorch-like burn. The spores are produced on the undersides of the plant’s leaves, but the effects are often first observed on the upper sides.

There are nine different species of the rust fungus Phragmidium found on roses. The orange, powdery pustules on the undersides of leaves and other plant parts make this disease fairly easy to identify. Early in the season the pustules may appear as yellow to light orange, with the color deepening to dark orange to red brown in the summer. As the disease progresses, the upper sides of the leaves may discolor and drop.

All these diseases need free water in order to infect your plants. Their spores are spread by air currents and splashing water, from rain, heavy dew or irrigation, to newly expanding leaves and stems. Blackspot spores need about seven hours in contact with water, and temperatures of 65 – 75°F to germinate, while rust prefers slightly cooler temperatures of 65–70°F, but only two - three hours in free water.

As with other diseases, employing preventive measures is the most effective means of managing these nasty diseases. Some options include:

  • Cultural methods – hybridizers continue to develop new roses that are resistant to disease; when buying new roses, select varieties that aren’t susceptible and individual plants that are disease-free. Plant in a location that has good drainage and gets plenty of sun. Avoid shady spots and dense plantings and ensure the plants get lots of air movement to dry any moisture off the leaves. Maintain a “clean” garden (pick up and dispose of any infected leaf material).

  • Mechanical and physical methods – Take care when doing any overhead watering; do it early in the day so plant surfaces have time to dry before temperatures cool in the evening. If possible, switch to alternate methods of irrigation that don’t wet leaf surfaces.

  • Chemical methods - there’s a plethora of chemical products to treat the fungal diseases. While one may take care of the problem in the short term diseases, along with insects, can develop resistance to a chemical if exposed repeatedly. Select at least two products, each from a different chemical class and that have different modes of action; rotate application of the products to reduce the potential for the pathogen to develop resistance.

  • Horticultural oils can be effective in managing mild cases of some fungal infections; EcoSmart Garden Fungicide, a contact fungicide, is a blend of rosemary, thyme and clove oils labeled for control of anthracnose, blackspot, botrytis and rust, while OMRI listed Organocide from Organic Labs, is a mixture of sesame oil, fish oil, lecithin and soy extract for treatment of blackspot. (Labeled Caution)

  • Natria from Bayer is labeled to treat anthracnose, blackspot, downy mildew and rust; the active ingredient in this biological based fungicide is the bacterium, Bacillus subtilis Q713. Natural Industries Actinovate is another biological fungicide for control of botrytis and downy mildew. (Labeled Caution)

  • Potassium bicarbonate is the active ingredient in Greencure; it’s registered for treatment of anthracnose, blackspot, botrytis, leaf spots, downy mildew, and rust. Bonide’s Copper Fungicide is also listed for managing those diseases; it’s a formulation of copper salts and fatty acids. (Labeled Caution)

  • Both Subdue MAXX, and Aliette are systemic fungicides labeled for control of Phytophthora and downy mildew. They belong to different classes of chemicals with different active ingredients and modes of action. (Labeled Caution)

  • Bayer Advanced Disease Control (triazole class) is one of many chemical fungicides available for treatment of blackspot and rust, along with a range of other fungal diseases. Eagle 20EW from Dow AgroSciences, is a protectant and curative formulation with the active ingredient myclobutanil. Pine Cone Fungus (Labeled Caution)

  • A relatively new class of chemicals, the strobilurins, is based on a natural, anti-fungal chemical produced by a Basidiomycete fungus. BASF’s Pageant™ , with the active ingredient pyraclostrobin, claims to be “a broad-spectrum fungicide combining two fast acting active ingredients, boscalid and pyraclostrobin, into one product that offers more control on more diseases than any other currently registered fungicide.” (Labeled Caution)
  • Managing the quartet of nasty fungal diseases can be a challenging task, especially when Mother Nature decides to keeps that liquid sunshine coming. The good news is that hybridizers are introducing more disease resistant varieties of roses into the market in a wide range of colors and forms, surely ones to catch your fancy. Additionally, chemical companies continue to develop new, environmentally safer materials for the rose gardener. Starting with the least toxic measure, you’re likely to find something that will work for you and your garden while being kind to the environment.

    Photo of downy mildew courtesy of Baldo Villegas’ webpage at: http://buggyrose.tripod.com/irosepests.htm and photo of pine cone cone fungus courtesy of Pageant website at: http://betterplants.basf.us/products/pageant-fungicide.html.

    Author’s disclaimer: Inclusion of a product in this series of articles does not constitute endorsement either by the author or the Marin Rose Society. Products are included for informational and descriptive purposes only.


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