It’s the most common problem I notice every year with cucumber plants—white spots appearing on the leaves. Most of the gardeners usually ask, "What is this and how to treat the spots?"
But we need to know why the white spots appear on cucumber leaves.
Those who don't have enough time to read this entire article, the line below is for them:
It's White powdery mildew. Try to keep the leaves dry, and trim the affected leaves. Remember to throw the leaves far away. Apply both side of the leaves with Neem Oil once or twice a week (depending on damage). When you water, only water the soil, not the plant.
This is powdery mildew, and it's very common on cucumber plant leaves.
Check the leaves with your fingers. If you can easily remove the spots by rubbing or if some residue is on your fingers, it's definitely powdery mildew.
How to treat white spots on cucumber leaves - Powdery mildew treatment
Remove the affected leaves as early as possible. It will reduce the infection speed.
Note: Throw the affected leaves away from your garden. Do not use the leaves for composting.
If the plant is heavily infected, then mix 2–3 ml of neem oil per liter of water, and spray both sides of the leaves every other day. If the plant is not heavily infected, spray once a week.
Remember, your first step to treat plant diseases like this should always be in an organic way.
If you leave the plant untreated, the leaves may turn yellowish, but the white spots will remain there, just like the picture below:
You are lucky if you can identify the white spots on your cucumber leaves at an early stage. In this case, milk spray will do the work.
Mix 1 part milk: 9 parts water (works best early) and spray once a week.
Make sure the plant is not in an overcrowded area. If your plant is not getting good airflow, powdery mildew can appear again quickly.
I planted a cucumber plant this year in a grow bag, and the soil and the lower leaves were not getting enough sunlight. This is why my plant is infected by the fungus that causes powdery mildew.
It's important to have a good amount of sunlight on the soil as well. Do not plant in a crowded place.
I tried another organic method last year, which worked fine for me.
½ tsp baking soda + a few drops of liquid soap + 1 liter water. Mix it well and spray in the early morning (once a week will be more than enough for the early stage). You can do this even if the plant is not infected. This will prevent future problems.
If your plant is highly infected with those white spots, use Sulfur-based fungicides (very effective, but avoid spraying in hot weather).
Those who are growing cucumbers commercially usually use Potassium bicarbonate products. This is the fastest way to knock out powdery mildew.
