Summer horticultural oil is a lightweight version of the "dormant" oil used in very early spring. It can control infestations of whitefly and some scales and mites during the growing season. Spray on a day under 80o and water well before spraying. Plant injury is possible, so read labels carefully and see CSU Fact sheet 5.569.
Never spray any pest treatment on a plant when it's in full sun or when it's drought-stressed.
Powdery mildew is easy to spot on the tops of leaves--patches of white or gray powdery stuff. It is a fungal disease common during our hot summers, especially where plants are crowded and kept damp. Many different kinds of plants are susceptible, for example, squash and cucumber, phlox, bee balm, and roses. To avoid it, choose powdery mildew-resistant varieties; space susceptible varieties further apart; avoid overhead watering where they grow; and cut down on use of nitrogen fertilizer in late summer. Sulfur may help control powdery mildew. An experimental alternative is a combination of 2-1/2 tablespoons of summer horticultural oil and 1 tablespoon of baking soda in one gallon of water, applied to susceptible plants about every two weeks through the season. CSU Fact sheet 2.902
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888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010