Tidewater Gardening - July 2007
An Organic Control for Mildew Diseases
K. Marc Teffeau
I noticed this spring that the Red Meidiland rose I have along the fence came up with a case of powdery mildew. Normally this rose is not bothered by this disease but I guess that this late spring had weather conditions that were favorable for its development.
Powdery and downy mildews, caused by a variety of fungi, are common fungal diseases on ornamental plants on the Mid-Shore. These diseases frequently infect rose, phlox, lilac, begonia, zinnia, alpine currant and other annual and perennial flowers and shrubs. Powdery mildew does not usually cause serious damage to its host, but does derive nourishment from the plant, slowing down its growth.
Powdery mildew is most prevalent under conditions of cooler temperatures and high humidity. These conditions occur with poor air circulation or when cool night temperatures follow warm day temperatures. Since high relative humidity is an important factor favoring disease development, certain cultural practices can help prevent the disease or decrease its severity. Increase air circulation and light penetration. Trees and shrubs outside can be pruned and thinned to reduce overcrowding in the landscape.
When planting new materials, select those plants which have resistance to powdery mildew and other foliar diseases and allow for adequate spacing of plants in the landscape. Powdery mildew seldom warrants chemical control in the home landscape.
If you do want to try to control powdery mildew however, before reaching for a synthetic chemical control for mildews, consider using fungicides based on potassium bicarbonate. These products rely on a bicarbonate salt (usually potassium bicarbonate) as the active ingredient. They are promoted for use against powdery mildew diseases.
The use of baking soda (sodium bicarbonate) as a fungicide is not a new idea. In Alfred C. Hottes’ A Little Book of Climbing Plants, published in 1933, mention is made of using one ounce of baking soda per gallon of water to control powdery mildew on climbing roses.
Sold on the market as GreenCure® fungicide, the use of the material was developed by a plant pathologist at Cornell University. GreenCure® has been proven effective against powdery mildew, rose black spot, anthracnose, downy mildew and many other plant diseases. It is used in certified organic crop production and in orchards, vineyards and vegetable farms. The nice characteristic of potassium bicarbonate is that it will actually control and eliminate these diseases.
If you want to use a chemical control, Cleary’s 3336 or chlorothalonil (Daconil 2787) may be used. These materials will prevent infections from occurring. They do not cure existing infections, therefore it is important to treat for powdery mildew as soon as it appears to prevent more infections from occurring. Apply additional sprays as needed, according to the label.
For deciduous shrubs that have repeated, yearly infections, a dormant fungicide such as lime sulfur may be applied (do not use on viburnum). Apply in early spring before the new growth starts.
If you are a vegetable gardener, there are some disease problems that you need to be on the look-out for. Tomatoes are susceptible to a couple of diseases that show up this time of year. Early blight on tomatoes can be seen on the oldest leaves of susceptible varieties. This is a fungus disease that produces large brown spots that typically have alternating light and dark brown concentric ring patterns on the leaves and stems.
Once early blight becomes established it can cause premature defoliation of the plant and spot the fruit. Large dark brown to black spots can develop at the stem end of the fruit. You can control this disease by using Daconil 2787 or mancozeb fungicides.
An organic approach might be to use a copper or wettable sulfur spray or the potassium bicarbonate material - GreenCure®. I would apply these materials when temperatures are below 80° as they can burn the foliage of the tomato plant when sprayed during the high heat of the day.
Septoria leaf spot is another tomato disease that is present in the garden. It produces smaller circular spots with a light brown center. Anthracnose can also spoil your tomato crop. Anthracnose causes sunken dark spots on the fruit as they turn red. This disease is favored by wet weather or frequent overhead watering. The same sprays used for early blight will control Septoria and anthracnose if sprayed on the plants before the symptoms appear.
Diseases may not be the only problem in the vegetable garden. Summer weather conditions such as drought and hot, dry winds can cause pepper and tomato blossoms to drop off. Try misting plants twice a day to cool them and help the blossoms set fruit.
For the best flavor, pick ripe tomatoes as needed; flavor peaks within three minutes of picking. If you must wait to use garden-fresh tomatoes, don’t refrigerate them. Fruit texture and some aroma compounds deteriorate quickly in the cold.
A garden needs one inch of rain or water each week. Early morning is the best time to water. Watering in the evening or at night is not desirable because leaves that remain wet through the night are more susceptible to fungus diseases. Mulch plants with an organic mulch to reduce water loss and improve yields.
For a fall harvest of cabbage, broccoli, cauliflower and Brussels sprouts, set the transplants out in late July or early August. These crops always do better as a fall crop around here. For a fall harvest of lettuce, radishes, carrots, beets, turnips, kale or spinach, sow seeds in late July to early August.
In the summer, dry soil may make working the soil difficult and inhibit seed germination. Plant your fall vegetables when the soil is moist after a rain, or water the area thoroughly the day before you plant. Seeds may be planted in a shallow trench to conserve moisture. Cover the seeds about twice as deeply as you do in the spring. Early maturing varieties of vegetables are best for late plantings.
July is the time when many retail garden outlets use a mid-summer clearance sale to rid their sales yards of plants left over from the spring sale season. In properly managed sales yards where plants have been watered and fertilized and where insects and diseases have been controlled, plants are still in good condition. They will tolerate transplanting at this time of the year provided they are balled and burlaped or container grown. Do not attempt to transplant bare-root plant material now.
Be careful about buying clearance plants from businesses where the sale of plants is a side income source. Most of the time little attention has been paid to the proper care of this material while on the lot. When selecting sale plants under these conditions, make certain that the plants are alive. Regardless of what the sales clerk tells you, horticultural scientists have not discovered a method of reviving dead plants.