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I have about 8 zucchini plants, about 6- 8 weeks along, which are starting to show signs of disease on the leaves.

It's definitely not powdery mildew, but the end result is quite similar. Leaves are getting yellowish patches which then develops brown spots and eventually the whole leaf turns yellow.

So far only some of the first leaves have turned yellow, but I can see it has spread to some of the larger leaves.

We've had unseasonal excessive rain over the past few months, which I assume is the cause of disease and helping to keep it spreading.

Does this sound like it can be treated and how? or Should I just remove the plants and plant something more suitable to the current weather?

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If you post a picture, I'll look it up in my plant disease book. –  bstpierre Mar 13 '12 at 14:14
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up vote 2 down vote accepted

Without the benefit of a picture, I can suggest (quoting from "The Organic Gardener's Handbook of Natural Pest and Disease Control") the following, with my most likely guesses first:

  • Downy mildew: Leaves mottled yellow between veins; purple spots on leaf undersides. ... thrives during cool, wet weather. Avoid wetting foliage. Maybe spray with potassium bicarbonate, or plant resistant cultivars.
  • Angular leaf spot, scab, alternaria leaf blight, other fungal and bacterial diseases (all of these are listed alongside powdery mildew symptoms): Leaves with spots, blotches, or brown areas. ... Reduce problems with keeping foliage dry when watering and not working amid wet plants. They suggest removing and destroying infected leaves, and making sure to provide good air circulation (i.e. good general fungus-preventing garden hygiene). You can try spraying with potassium bicarbonate, *Bacillus subtilis, or 0.5% baking soda.
    • Powdery mildew shows up as powdery white spots.
    • Angular leaf spot shows up as water-soaked spots that turn gray, die, and drop out.
    • Scab may also show up as water-soaked spots, it causes sunken brown spots with gummy ooze on fruit.
    • Alternaria leaf blight shows up as dark brown spots with concentric rings, usually appearing on older leaves first.
  • Mosaic: Leaves with yellow patches; older leaves mottled and distorted. ... Remove and destroy diseased plants. Control aphids and cucumber beetles... (I.e. there's only prevention, no cure.)
  • Aphids: Leaves yellow, curled and wilted. Inspect plants and remove aphids if present; they also spread disease, see above.
  • Mites: Leaves yellow and puckered, becoming bronzed. Make a positive diagnosis of mites (tiny red, yellow or green pests), and then spray with weak insecticidal soap.
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+1 for downy mildew. –  jmusser Mar 14 '12 at 13:15
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