June 18, 2016
Gorgeous roses are a wonderful addition to any garden. But throw in a few days of spring rain and voila, you might end up with nearly naked plants. This dramatic effect is likely a result of infection by either blackspot or rust, the gardener’s pestilence of wet spring or fall months. Effectively managing these invaders employs planning and vigilance.
Blackspot is aptly named; 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. Caused by the fungus Diplocarpon rosae, infected leaves drop off, potentially defoliating the whole plant. Miniature roses seem to be more susceptible than their larger relatives.
The orange, powdery pustules on the undersides of leaves make rust fairly easy to identify. Early in the season they’re yellow to light orange, with the color deepening to reddish-brown in summer. Over time, the upper sides of the leaves may discolor and drop. There are numerous species that can cause this disease; Phragmidium mucronatum is the most common. If leaf drop is severe, it may weaken the plant over time, reducing flower production and overall vigor.
Both of these diseases need free water to infect your plants. Their spores are spread by air currents and splashing water from rain, heavy dew, fog or irrigation, to newly expanding leaves and stems. For spores to germinate, blackspot needs about seven hours in contact with water and temperatures of 65 to 75 degrees, while rust prefers slightly cooler temperatures of 65 to 70 degrees, but only two to three hours in free water.
Managing these diseases can be challenging, especially during rainy periods with mild temperatures. Preventing infection is your best bet. Start with disease-resistant varieties; there are many new cultivars of roses that are less susceptible to blackspot and rust, with more being introduced into the market each year. If you have a rose that seems to be a magnet for either of these maladies, dig it up and toss it. Replace it with a variety that is known for overall good health and resistance to this dastardly infectious duo. Ten varieties that generally do well in Marin include Ballerina, Bonica, Gourmet Popcorn, Graham Thomas, Hannah Gordon, Iceberg, Julia Child, Penelope, Sally Holmes and Westerland.
Healthy plants can generally withstand a bout of either rust or blackspot, so keep your roses vigorous. Plant them in a location with good drainage that gets at least six hours of sun each day, and has adequate air movement through the bush. As water is the main conduit for spreading the disease, 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.
Good garden sanitation is a vital part of disease management. What does that mean? Remove any infected leaves, stems and canes from plants and the leaf litter on the soil around them; prune out infected, unhealthy or dying stems and canes whenever you see them. These infectious agents can survive through winter on dead leaves and debris and attack the new leaves as they emerge in spring. And don’t add the infested material to your compost pile unless you know it gets hot enough to kill these nasty pathogens.
If cultural methods aren’t sufficient to control the diseases, you may want to try chemical treatments. Be warned; effective control requires a preventative spray program, usually every seven to 14 days to protect new growth. A home remedy for blackspot (it also works for powdery mildew) is 4 teaspoons of baking soda per gallon of water mixed with a 1 percent solution of narrow-range horticultural oil. Neem oil can also be effective for preventing both diseases.