Thursday, August 19, 2010

Telling powdery mildew by its spots

Powdery mildew can be a chronic late summer and fall disease in the vegetable garden. I’ve found that some people mistake the normal color variation in leaves for the disease. Many squash have silver-white blotches like spots on a leopard. See photo of the normal blotches on the leaves of the All America Selections winner, ‘Papaya Pear’ squash right.

When squash becomes infected with powdery mildew, the dusty flour appearance of the disease looks more irregular and is on the surface instead of being part of the leaf. See photo left. Note that the disease appears on the upper leaf surface, not leaf undersides. Infected leaves can turn yellow, become distorted and fall prematurely (photo below right).

The severity of the disease depends on many things including plant variety, age of the plant, health status of the plant and weather conditions. Young, succulent growth is more susceptible than older plant tissue. Avoid late summer applications of nitrogen fertilizer.

Powdery mildews tend to be severe in warm, dry climates. High relative humidity is needed for fungal spores to germinate once they land on leaf surfaces. Shaded sites with poor air circulation favor disease. This is another reason to grow in open, full sun locations. These sites promote plant health by having ample sunlight for photosynthesis. They also ensure that humidity around leaves is quickly dispersed by adequate air movement.

Ground applied (drip) watering rather than overhead sprinkling also helps humidity control. Water in the morning rather than evening to take advantage of sunshine that quickly dries leaves.

If these cultural controls are not adequate, supplement with chemical applications of potassium bicarbonate (preventive) and neem oil (eradicant after infection). Read all label instructions and make sure the product you purchase is labeled for use on squash.

Photo credit: Healthy ‘Papaya Pear’ squash leaf, Powdery mildew infestation on squash leaves, Severe infestation and leaf yellowing – all three Carl Wilson


  1. Hi - great pictures and description. I had wondered about my zucchini leaves, and now I know they are fine!

    I poked around a bit on your site but couldn't find anything. Maybe sometime you could address (or point me to a past post) splits in tomatoes. It seems that about every tomato I grow (in TN in the past or now in CO) has ugly deep splits in the sides, radiating from the top. I have read that it's due to uneven watering, but I always have mine on a soaker hose on a timer. I don't know how you could possibly water more regularly!

  2. My zucchini plant is currently infected with this powdery mildew, as is the case every year at this time.

    I have neem oil and plan to spray tomorrow, however 1 question. Is there a remedy to the infected leaves? Should I cut them? What about the zukes that are still growing? Any dangers that I should worry about?

    Love the site. Thanks!

  3. See next post for comments on tomato cracking.

  4. Many people don't spray for powdery mildew and plants produce some fruit in spite of infected leaves. I suspect some people are glad if zucchini production slows as they may be tired of harvesting a zucchini fruit a day after while. If you do spray, observe the cautions on the label including the waiting period before harvest. If the fruit will have to be harvested before the waiting period expires, harvest all fruit before spraying.

  5. I discovered right after this post that I did have the mildew in a big way. The leaves it was on have totally died. The rest of the plant seems ok for now, but I'm definitely disappointed. We water with a soaker hose at the soil level in the morning, so I don't know how it happened.

    Thanks for the article on tomato cracking!

  6. Some of my squash plants have the silvery finish as shown in the top photo. However, other leaves of the same plant do not have the silvery pattern. Does it show up in older leaves or just some leaves?

  7. Normal silvery leaf color is variable with some leaves having more and some hardly any. There seems to be no variation by age of leaf that I've seen.