What causes my zucchini leaves to become light yellow?

Question

Please have a look at these images of my zucchini plant:

What disease is this and what is causing this? What are the solutions to counter this?

Location: India. Weather In May-June: Hot! > 95°F days.

Answer 1

That looks like downy mildew. From "The Organic Gardener's Handbook of Natural Pest & Disease Control" (Bradley), p194:

• Remove and destroy badly infected leaves.
• Disease is worst in cool, wet weather; don't water in this weather. (Avoid wetting foliage in general when watering.)
• Spraying with potassium bicarbonate may help control it.

See also Managing Downy Mildew in Cucurbits (Cornell University).

Answer 2

While I think I also see some mildew close to the ground, most of the damage looks like leaf miners - but much, much worse than I've ever had experience with in our cooler climate. I don't find much good guidance for what to do at this point, for this year. When done with the plant, be sure to dispose of it - get rid of the pupae and larvae. Also, clear out nearby weeds that may be hosts.

Answer 3

I would suspect a potassium imbalance (either too much or too little). Maybe even a phosphorus imbalance. There may have been some leaf miners, but there were other problems, too (although it doesn't look like powdery mildew to me, except maybe on the left-most leaf on the bottom picture). Potassium-deficient plants are weaker against insects and are more disease-prone. If you can, I would get a soil test. That way you'll know better what the soil needs. Maybe make sure the soil has enough available calcium and silica, too (a deficiency of either may make a plant more disease-prone). Cucurbits enjoy silica more than say tomatoes in my experience.

If you have diatomaceous earth or rockdust of some kind, it would add extra silica to the soil. Rockdust would probably be better, as it would offer other benefits besides silica, and it shouldn't add as much aluminum (I would suggest basalt rockdust most). Don't add much diatomaceous earth if you use that, though. I'm just guessing, but I suppose a tablespoonful of diatomaceous earth mixed in with your next watering (or into the soil) should be more than fine (no need to repeat).

For calcium, if needed, I would just use eggshells from boiled eggs (boiled to sterilize them). Don't powder the shells. Just break them up into pieces.

Anyway, the main problem seems to be the coloring of the leaves and susceptibility to disease, which seems to indicate a nutrient problem. Even if powdery mildew was the problem, a soil test wouldn't hurt, and a better balance should make future plants stronger to prevent recurrence.