KENNEWICK -- Last week, we talked about when and how to prune roses.
Since spring is right around the corner, now is a good time to talk about the two rose problems that local gardeners often face: aphids and powdery mildew.
Aphids are small, green or pinkish soft-bodied insects found in clusters on succulent new bud and stem growth. The aphids suck sap from the plant. When they are present in high numbers, they damage growth. Rose aphids overwinter as eggs on buds and stems, emerging at the same time that new growth begins in the spring.
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Horticultural oils can be used to help minimize aphids problems by smothering aphids eggs before the young aphids emerge. The oils are applied at the delayed dormant stage, when the buds start to emerge.
There are other least-toxic ways to discourage the buildup of aphid infestations on roses. Avoid excessive or unnecessary applications of nitrogen fertilizer. The nitrogen promotes vigorous, succulent growth where aphids like to feed. Using a slow-release or low nitrogen fertilizer can avoid lush early-season growth.
Another non-chemical option for managing aphids is water. A strong stream of water can be used to wash aphids off rose leaves and stems. Spraying roses regularly with water is an easy way to keep aphid populations down.
If these methods fail, there are a number of organic and inorganic pesticides for aphid control. The easiest to use are the systemic insecticide products that are mixed with water and applied to the soil for uptake by the roots. The Bayer Advanced product line includes several soil-drench products containing imidacloprid for use on roses.
Powdery mildew is a fungus disease characterized by a white powdery coating on leaves and buds. You can minimize powdery problems by not encouraging succulent growth, which is most vulnerable to infection by powdery mildew. Also, sprinkling plant leaves with water helps by washing spores off the plant.
One new "organic" spray that gardeners have been reading about for control of powdery mildew is milk -- yes the stuff that comes from a cow. However, while this recommendation has appeared in various gardening publications, Linda Chalker-Scott, WSU Extension Horticulturist, points out that there have been no published scientific studies investigating the use of milk to prevent powdery mildew on roses or other ornamental plants. There have been studies on the effectiveness of milk spray applications for the control of powdery mildew on melons, cucumbers, and squash. These studies indicate that whole milk does provide some control of powdery mildew.
Chalker-Scott notes that only anecdotal evidence, not scientific research, indicates that milk is effective in controlling powdery mildew on roses. She also points out the drawbacks of using milk for powdery mildew prevention include the unpleasant odor of the milk fat as it breaks down, the growth of benign fungal organisms that colonize the leaves as part of the break down process, and that milk may only be effective if it's applied prior to powdery mildew developing.
With those drawbacks, you may prefer to use an organic or inorganic fungicide for control of powdery mildew on roses.
Most of these require frequent (each seven to 10 days) application to protect new growth as it develops.
However, tebuconazol can be applied as a soil-drench for uptake by the roots. Several Bayer Advanced rose care products contain both tebuconazol and imidacloprid, providing aphid and powdery mildew control for "up to six" weeks. These products are a bit pricy, but they avoid the risk of spray drift and don't require spray equipment or frequent re-application.
* Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.