Cucumber plants are susceptible to numerous diseases.
The majority are contained in the plants’ ginormous leaves. Few will penetrate to the fruit.
All diseases will reduce fruit growth, though.
Read on to discover the different diseases that yellow spots can indicate.
Yellow Spots on Cucumber Leaves
Yellow spots on cucumber leaves often signify downy mildew, thriving in wet, warm conditions. Next, Cucumber Mosaic Virus causes yellowing and “mosaic” patterns, spread by aphids. Finally, mites create yellow blotches on cucumber leaves by draining leaf sap.
Reasons for Yellow Spots on Cucumber Leaves
Reason #1 for Yellow Spots on Cucumber Leaves – Downy Mildew
Downy mildew is a water mold. It can affect all plants in the cucurbit family.
In its early stages, it causes yellow spots on the top of the leaf. It looks like water blisters. When humidity is high, the underside of the leaf can show purple spores.
However, unlike powdery mildew, which is a fungus infection causing white spots on cucumber leaves, downy mildew is a water mold that is caused by Pseudoperonospora cubensis – an oomycete that is not a true fungus.
As such, it will not wash off.
The distinction between downy mildew and similar diseases is that the yellow spots turn brown as the leaf ages.
It starts as yellow spots on the leaves, ending when all the leaf surface is dead, which is when it turns brown and the leaf drops from the cucumber plant.
Downy mildew needs two conditions to thrive.
Wet leaves and warm weather.
It thrives in high humidity when the temperatures are between 59-Fahrenheit and 68-Fahrenheit.
As it is a water mold, keeping the leaves dry by watering the soil can help prevent germination. Overhead irrigation, such as using garden sprinkler systems can encourage mold growth.
It can be spread by the wind, by water splashing between plants, and by tools that have not been sanitized.
For those reasons, temperature and humidity control is imperative to prevent germination.
Unless you’re growing a resistant variety of cucumber, pre-treatments with a fungicide should be applied once weekly.
After it has been detected in your area (not just your garden) increase the frequency of treatment to every 5-days for cucumbers.
The disease does not need to be present in your garden to impact your crops. It can be spread by the environment.
Reason #2 for Yellow Spots on Cucumber Leaves – Mosaic Virus
The first case of the Cucumber Mosaic Virus was described simultaneously by Jagger in New York and Doolittle in Michigan in 1916 according to Science Direct (F. García-Arenal, P. Palukaitis, in Encyclopedia of Virology (Third Edition), 2008).
This is a soil-borne disease that kills the plants it infects.
As it is in the soil, it will infect new plants so the plants need to be removed from the soil and the soil treated by either solarization (heat treatment to kill soil diseases), or by growing in another area of your garden.
Keep in mind that the best pH for growing vegetables and fruit is under 7.0.
The problem is identifying CMV because the yellowing doesn’t coat the entire leaf surface, but instead creates a distinct “mosaic” pattern.
Signs to look for are yellow veins on the underside of leaves, and a variegated pattern, and/or yellow spots on the top side of the leaf. Infected leaves also curl downward and wrinkle.
Once fruits start to grow, yellow ringspot can emerge on the fruit, which is indicative of the cucumber being infected by the mosaic virus.
Unfortunately, there is no way to treat the mosaic virus. Infected plants need carefully removed, gardening tools, and the soil, sterilized.
Reason #3 for Yellow Spots on Cucumber Leaves – Mites, Aphids, and the Notorious Whitefly
They all pierce holes in the leaves to drink the sap.
Aphids are the largest threat to cucumbers as these insects carry and introduce the mosaic virus to plants.
Where the fluids are drained from the leaves, yellow blotching happens.
When an insect problem isn’t treated at the onset, the damage becomes extensive. To the extent they can drain the plant dry of fluids causing it to die.
Heavy mite infestations can make cucumber plant leaves look like their entirely yellow. Look closely, and you will see that it is numerous minuscule yellow spots merging.
The earliest sign of pest damage is yellow spots on the underside of leaves.
Insects hide and feed under the leaves. That is where you will see the most yellow spotting. When the population increases, the extent of the damage causes a mottled pattern that looks like the spots are merging.
A preventative measure is to dislodge the pests by using a garden hose with a strong enough stream to knock the insects off.
Once these make a start, controlling them is usually required throughout the season. An insecticidal soap or neem oil applied to the leaves bi-weekly helps prevent an infestation.
Another precautionary step you can take is to apply a peppermint spray to open wounds when you prune your plants.
Open wounds on plant stems are fragrant to insects. The scent of the sap can attract garden pests. Mask the scent, and it may prevent them from setting up home on your plants’ leaves.
Frequently Asked Questions About Yellow spots on Cucumber Leaves
What to do with cucumber plants infected with a leaf spot disease?
If mite damage is not the cause of yellow spots, it is likely that a bacterial or fungal disease is and will need to be treated. For cucumbers, fungicides that contain either copper sulfate or copper octanoate are effective at preventing spread.
Will yellow spots on cucumber leaves affect fruit?
The less green leaf surface there is, the lesser the photosynthetic surface becomes. That is what causes stunted growth, reduced fruit size, and once defoliation starts, fruits are at risk of sunscald. Fungicides can slow or stop the spread, but if it continues, it may be best to destroy the plant.
Daniel has been a plant enthusiast for over 20 years. He owns hundreds of houseplants and prepares for the chili growing seasons yearly with great anticipation. His favorite plants are plant species in the Araceae family, such as Monstera, Philodendron, and Anthurium. He also loves gardening and is growing hot peppers, tomatoes, and many more vegetables.