powdery mildew/fungus

Asked May 21, 2013, 3:45 PM EDT

I am getting ready to plant some zucchini and cucumbers. In the last few years the powdery mildew has gotten really bad. Does this fungus stay in the soil and can I treat the soil with something before planting to eliminate this problem? I would like to get this under control, possibly in an organic way. I do clear out the beds in the fall and I have a drip irrigation system.

Larimer County Colorado fruits and vegetables

3 Responses

Hi Lulu,

Powdery mildew is really more of a cultural problem, caused by higher humidity and poor air circulation. Zucchini and cucumbers get it because of their larger leaves and foliar surface area. Powdery mildew also thrives in shaded, damp locations, which is why many vegetable plants get the disease. In addition, it's common on lilacs, turfgrass and roses.

CSU Extension has a great Fact Sheet on the subject: http://www.ext.colostate.edu/pubs/garden/02902.html

The fungus is host specific and doesn't stay in the soil, but clean-up in the fall of your vegetable garden is always recommended. There are both cultural and chemical controls listed in the above-mentioned Fact Sheet. Other ways to approach it are to prune foliage and allow for spread of plants.

The plants are in direct sun and not planted close together, lots of air movement especially if it's breezy. Are there any other fungi that could live in the soil?
We also get flea beetles, any remedy for those?

Thanks for your response,

Even in the most optimal garden conditions, powdery mildew can still occur. If you suspect it's another fungus, please bring in a sample to our office (1525 Blue Spruce Drive, Fort Collins) when/if it occurs this season. I can't really diagnose without pictures or a sample, but do know that zucchini and cucumbers get this almost always. It should be noted that powdery mildew rarely affects yield of mature plants--it's really only an issue on seedling plants, where it can kill the seedlings. If it happens at the end of the season, simply remove the foliage that is affected. It looks unsightly, but it's really not something to treat for...nor does it affect the fruit of these crops.

Flea beetles are season-dependent...I haven't heard the prediction if it will be a "good" flea beetle year. They are not out yet, but one thing to consider is what crops they are affecting. If it's just foliage (not foliage you're eating), control usually isn't necessary as it's primarily aesthetic. The rule of thumb is that damage should exceed more than 10-20% of total foliage before doing controls...lower than those amounts rarely affects yield. Cultural controls are floating row covers, thick mulch or plant radishes (they will attack radish foliage...these are planted just for the flea beetles to enjoy). Chemical controls are listed in the CSU Fact Sheet below. If you choose to use a chemical, apply according to directions on the label and make sure it's labeled for food crops: http://www.ext.colostate.edu/pubs/insect/05592.html