Is it some virus attack which I read on the internet. Please refer to the image.

yellow patch on cucumber leaves

Thanks in advance. Muddassar

  • Is this primarily affecting old growth or are fresh leaves like this? Nov 19, 2015 at 10:40
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    These are in fact old leaves at the base of the plant. The newer upper leaves are not affected yet.
    – Muddassar
    Nov 19, 2015 at 11:01

1 Answer 1


I'm not a hundred percent sure, but my guess is downy mildew or maybe cucumber mosaic virus. For cucumber mosaic virus, I don't know if there's any treatment in this generation. Downy mildew tends to cause yellow chlorotic blocky chunks on the leaves of cucumbers like that. Downy mildew prefers moist conditions (unlike powdery mildew, which prefers it dry, according to what I've read).

Keeping your plants strong with proper nutrition, and with plenty of light is probably one of the best things you can do. Keep the leaves dry.

Downy mildew doesn't over-winter where it gets particularly cold (unlike powdery mildew). However, if you don't want to risk problematic infections, you can plant resistant varieties, such as the Little Leaf cucumber (it's also resistant to cucumber mosaic virus).

There are treatments for downy mildew, such as neem oil, but neem oil can hurt your pollinators. I imagine there are lots of other things that might work. My plants, which were indoors in a humid environment, had problems similar to this, although it may have just been edema (or both). Whatever the case, giving them extra potassium sulfate, reducing humidity and not crowding them helped.

If it's downy mildew, it'll probably be affecting mostly old growth. I'm not sure, but I imagine cucumber mosaic virus would affect the new growth more readily.

Anyway, it could be a pest (keep an eye out for small insects on the leaves, or signs of them), but downy mildew or CMV is my current suspicion.

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    Thank You for the analysis. For now, I have removed the affected leaves at the bottom of the plant and applied some vermi-compost. I am also making sure that the soil is not too wet and the leaves remain dry. Well the plants have just given us the third cucumber for this season :-)
    – Muddassar
    Nov 20, 2015 at 4:02
  • Although the beneficial microbes in the worm castings should help, I've heard that worm castings can increase the available nitrogen in the soil. So, you might want to give the cucumbers some extra potassium to balance the nitrogen. Too much nitrogen and too little potassium can make plants more susceptible to infection. Your leaves look like they have lots of nitrogen, whether or not too much. A very small amount of wood ash should help (too much can raise your soil PH, due to the calcium), or else some potassium sulfate. Potassium chloride can hurt your beneficial soil microbes, however. Nov 20, 2015 at 7:59
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    My plants are better now. I think you are right by pointing that the nitrogen content had increased. I remember dropping DAP beads into the container. In a couple of containers accidentally more beads got dropped. And plants in only those containers showed these signs of leaf coloration.
    – Muddassar
    Nov 21, 2015 at 12:46

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