Powdery Mildew on Roses...An Ounce of Prevention
Powdery Mildew (PM) can be a serious disease on roses and it looks like white fuzzy powder that accumulates on leaves and stems of many plants, predominantly in spring. It is actually a fungus that is spread by millions of microscopic spores. By the time the naked eye can see the white 'powder,' it has already invaded the plant tissue and is feeding and reproducing at a rapid pace. As it spreads on the surface, it eventually kills the cells of the leaves, leaving the leaf rippled and curled. Mildew spores are everywhere in the garden - in the air, the soil, on debris and on plant surfaces - ready to sprout when the environment is just right. Warm days (50 - 80F) and cool nights with elevated humidity and resultant dew provide ideal conditions. Though humidity promotes fungal growth, PM grows on DRY plant surfaces, unlike blackspot which requires immersion in water for about seven hours in order for infection to take place. Tender new growth needs a chance to 'harden' and develop its waxy coating that provides somewhat of a barrier to fungal growth. Therefore, we gardeners must provide protection for new spring growth on a weekly basis.
CONTROLLING POWDERY MILDEW
Controlling mildew doesn't mean spraying the planet into oblivion. It includes plant genetics, cultural practices and something as simple as WATER. We're going to focus on Cultural Practices since this is what we can accomplish.
CULTURAL PRACTICES:
Planting with sufficient space between the plants and away from walls and fences will help to provide good air circulation which reduces the chances for mildew. Annual pruning plays a major role in disease prevention. Stripping leaves from the bush at pruning time, and cleaning up debris in the garden contribute to a cleaner environment. Dormant spraying will at least wipe out last year's spores. Keeping the centers of the bush open during the growing season will aid air circulation.
WATER- Many gardeners still subscribe to the belief that you should NEVER get rose foliage wet. On the contrary, a high-pressure spray of water will remove mildew spores that haven't imbedded themselves yet and this will prevent them from germinating. Higher incidence of mildew during periods of rain is caused by the moisture in the air and soil - increasing the humidity that promotes mildew - not by water on the leaves. Similarly, watering early in the day will allow the soil surface to dry out a bit before the cool night temperatures arrive, reducing humidity from moist soil.
PREVENTION IS THE ONLY CURE
Once powdery mildew is apparent to the eye, it can't be eradicated. It simply must be prevented. Prevention is achieved by coating the plant tissue with something that provides a barrier to prevent fungus from gaining a foothold and invading the plant tissue. Growth is so rapid in spring that the leaves unfolding THIS week won't be protected by what you sprayed LAST week. This is the reason you find application schedules of every 7-10 days on most fungicides, and the reason you must follow that schedule. Fungicides are most widely used because they are chemically formulated to specifically combat fungal diseases. Recent reports of non- toxic, environmentally-friendly products such as baking soda and anti-transpirants are proving very encouraging also.
FUNGICIDES: Fungicides must be in place on the plant before the spores arrive and they may have systemic action - meaning they move into the plant tissue - providing protection for a short period. They are mixed at various rates and require application every 7-10 days. Be sure to follow label directions and safety guidelines.
BAKING SODA: New research shows that simple baking soda is a powerful weapon against fungus-caused rose diseases. Sodium bicarbonate (grocery-variety baking soda) was tested originally to determine its effectiveness in preventing blackspot.
Enjoy your roses and protect them as suggested for a great summer of beauty!
Meadow View Growers
Gardening with you since 1984