Powdery mildew infects a wide range of plants. It is easily recognized by the white to gray talcum powder like substance all over the leaves of the plant. This substance blocks the sunlight to the leaves and can prevent the plant from producing enough food to feed itself. In effect, the plant starves to death.
All the strains of powdery mildew produce this fine talc covering. They appear white at first, then turn black as spores are produced. Leaves may curl, have yellow patches, and fall prematurely. Infected buds may not bloom.
Warm, dry climates favor powdery mildew. So does lack of air circulation. It is often seen in plants that are crowded in a bed and have stagnant air circulation. This is one reason it is important to divide and replant bulbs and other plants that reproduce into thick masses.
There are several things you can do to combat powdery mildew. All effected plants should be removed from the bed. If only one part of the plant has powdery mildew, you can prune the effected part off. Do not compost these sick plants. Toss them in the trash can instead. You do not want to spread this disease and most home compost piles are not hot enough to kill it.
Thinning the beds of plant material so that air circulates is important. Pruning shrubs, trees, and woody perennials to allow circulation around the plants will help. Powdery mildew likes warm, dry climates but requires a high relative humidity in the area it infests. Lower the relative humidity, lower the risk of a problem.
Refrain from fertilizing plants with lots of nitrogen late in the summer. The tender succulent growth that results is very vulnerable to powdery mildew. Fertilize in the spring and fall instead, when the temperature is less to the fungus’ liking.
Sometimes, cultural controls fail to control powdery mildew. Then chemical controls are necessary. These controls include fungicides such as sulphur, neem oil, triforine, or potassium bicarbonate. Be sure that the product you are using is labeled for the plant you want to use it on. Some products, such as those containing triforine, are for ornamentals only.
Chemical control is most effective when combined with biological control. The visibly effected plant material should be removed. The fungicide should then be applied according to the label directions. This process will have to be repeated at seven to fourteen day intervals throughout the growing season to really lick the problem, according to the product label. Powdery mildew is persistent.
If this all sounds like a lot of trouble, you should pick plants that are resistant to powdery mildew when planning your landscape. There are varieties of roses, vegetables and grasses that have been developed to resist powdery mildew. You will need to inquire at your nursery for these varieties if you have a problem with this disease. If you plant varieties that are not resistant, avoid planting them in low, shady spots as this is a favorite location for powdery mildew.
Powdery mildew is a persistent and pervasive pest. Cultural and chemical controls must be used in combination to eliminate it from your garden.