What is happening to my cucumbers?
Yesterday they had beautiful, dark green, huge leaves and healthy blooms. Now it looks like a jealous person snuck into my garden last night and sprinkled flour on the leaves of my pumpkins, melons and squash.
What can I do? Will my plants survive? Will they produce?
Often in mid- to late summer, we see signs of powdery mildew, a fungus that attacks many types of plants. It becomes evident when we see white or gray splotches on the leaf surface, but it has already been busy on undersides of leaves, stems and fruit. Damage to leaves reduces photosynthesis, decreasing crop size and quality. Plants do not usually die but are weakened.
Susceptible plants are often growing in damp, crowded situations with poor air circulation. Some have been developed to be resistant to powdery mildew. Healthy plants (the right plant in the right place) are more resistant than those that are struggling. However, it is mid-summer in Colorado, so we have to make the most of what we have to work with in the limited growing season that we have left. It is too late to restart most plants.
Once you discover mildew, you need to take action. Affected leaves and plant parts should be removed and destroyed, overcrowded plants thinned or pruned, fertilization restricted to limit new growth, and water kept off leaves and stems. Common ingredients that have been reported to minimize the spread of the fungus are potassium bicarbonate, neem oil, copper and sulfur.
Follow instructions carefully before using any commercial products because they can be harmful to plants under certain circumstances, such as in bright sunlight. Careful treatment and handling of powdery mildew should keep it under control long enough for you to harvest a reasonable crop.
Carolyn Brown has been a Colorado Master Gardener since 2009. She lives in La Plata County.