What’s that white stuff on my plants?

Powdery mildew is a common plant disease. It appears as splotches of white or gray on leaves and stems, as if the plant was dusted with talcum powder.  It comes as a surprise to some that powdery mildew can happen in the dry Colorado climate, but powdery mildews can be severe in warm, arid climates.  How bad powdery mildew becomes depends on several things, including the type, age and condition of the plant and the weather conditions. Powdery mildews are host plant specific, so cucumber and lilac side by side will be infected by different strains of powdery mildew. They won’t “give” each other the disease. That said, there are you can treat both plants with the same approach.  

Avoid overhead watering and do your watering in the early morning, so the plant has a chance to dry. Improve air circulation by pruning plants that are over-crowded or bushy.  Keep an eye on your plants and prune out any suspicious leaves when you first see them. Use a fungicide.

Fungicides rated for powdery mildew can be used on vegetables and ornamentals. You don’t have to have a specific fungicide for each plant type. Sulfur can be applied as a dust or spray. Note that Safer® Garden Fungicide is OMRI rated. Potassium bicarbonate is a highly effective fungicide. GreenCure® is rated for organic gardening.



Finally, clean up your garden in the fall. Fungus can overwinter in the debris left in the garden, so it’s best to remove all debris and dispose of it. Don’t add to your compost pile.