What happens when you combine 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, black spot, and rust. The bad news is that as long as we have rain, the opportunity for black spot and rust exist. The good news is that as soon as our rainy season ends, they generally disappear, or at least can be controlled.
Black spot 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. Black spot 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.
Diplocarpon rosae is the fungus that causes black spot. 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.
Here are some general controls for all three of these diseases:
- Buy and plant disease-free plants
- Choose resistant varieties
- 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 black spot
By Nanette Londeree, Master Rosarian