by Nanette Londeree, Master Rosarian
What happens when you combine our lovely spring weather – cool nights and warm days, and a rain that lasts for a day or two? Well, besides the proverbial May flowers, it is the appearance of the BIG THREE fungal diseases of roses – powdery mildew, blackspot and rust. And just in time for the big show! The bad news is that as long as we have rain, the opportunity for blackspot and rust exist. The good news is that as soon as our rainy season ends, they generally disappear, or at least can be controlled.
Blackspot and rust both require free water to reproduce – thus rainy weather, or overhead watering that wets leaves and doesn’t have enough time to dry produce optimum conditions for these diseases. Blackspot needs about 7 hours with temperatures of 65 – 75 degrees; rust prefers temperatures of 65–70 degrees with 2 – 3 hours of free water. Powdery mildew does not need free water to reproduce – in fact, free water can actually inhibit its growth. Its favored growing conditions are daytime temperatures in the low 70s with a relative humidity of 40-70 %, and nighttime temperatures near 60 degrees.
In Marin’s climate, powdery mildew is the most common of the three diseases. It is caused by the fungus Sphaerotheca pannos and produces a white, talcum-powder-like growth on the top and bottom of the leaves and stems, also on buds or flowers. New leaves may become curled or twisted and the shoots may look badly deformed. The fungus may also infect older leaves. The upper surface of the leaves often appears normal despite extensive fungus growth on the underside of the leaf. When the disease is severe, plants become stunted and leaves curl 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 may 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.
Rust is fairly easy to identify with its orange, powdery pustules on the undersides of leaves and other plant parts. Early in the season these may appear as yellow to light orange, with the color deepening to dark orange to red brown in the summer. There are nine different species of the rust fungus Phragmidium found on roses.
Here are some general controls for all three of these diseases:
- Buy and plant disease-free plants
- Choose resistant varieties
- Avoid wounding plants during transplanting
- Plant roses in areas with good soil drainage and ventilation
- Avoid shady spots and dense plantings
- Remove and destroy infected leaves and canes during the season, rake up and discard infected, fallen, dead leaves
- Avoid overhead watering – water on the leave surface increases the chances for rust and blackspot
If you choose to use sprays to prevent or eradicate these diseases, you have a wide variety to choose from – beginning with baking soda and horticultural oil, antitranspirants, botanicals such as Neem oil, systemics such as Funginex, or contact killers like Fungi-gard. See the article “The Fundamentals of Fungicides” for more detailed information on fungicides.