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Most of my cucumber and melon plants have these (cream) white spots of various size.

The white spots grow and some of the older leaves have withered (partly).

All plants come from seed, and have been planted in potting soil. Only common denominator for them all is the water, fertilizer (general fertilizer for "all flowering plants") and location.

Edit based on comment : location is indoor. Started seeds under grow lights in february. They are now behind a south facing window. Temperature is about 19 - 20C. Can't see anything "unwanted" under the leaves. New leaves start out fine, but seems to become "affected" over time.

What can I do to stop this?

  • I've tried to spray the leaves with a mix of water and baking soda. Not successful / makes no difference.
  • I've removed old leaves, but I obviously can't remove all the leaves.

white spots

  • You've said the common denominator is 'water and location' but what is the location? Indoors, greenhouse, outdoors, cold frame? please specify and add any further information about how you''ve grown these (light? temperature?) if necessary, as well as how long they've been growing. Please also check beneath the leaves to see if there's anything there... – Bamboo May 1 '18 at 10:43
  • Updated question based on comment – sbrattla May 1 '18 at 10:55
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This looks like insect damage rather than something fungal - the patches turn cream from the damage the insect causes to the leaf surface. Quite which insect is hard to say, could be spider mite or something else, but spraying with neem (especially the underside of the leaves) should help, although your plants are already quite damaged. See here https://youtu.be/R5ypdzm6kVQ

Your plants look as if they are getting a little large for the pots they're in, so hopefully you'll be able to plant out soon - or give them larger pots.

  • Thanks. I've got tomato plants right next to the cucumbers and melons. Tomatoes are unaffected. Are there cucumber specific mites? – sbrattla May 1 '18 at 13:04
  • Probably/possibly - I'd just spray with neem and see if it helps, but keep an eye on the tomatoes too in case you need to treat those as well a bit later. – Bamboo May 1 '18 at 13:18

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