Spots and pimples are rarely pretty. For gardeners, there is no such thing as a beauty spot. Every spot is a sign of something gone wrong.
The color of the spots on plant leaves is how to identify the cause.
In the case of white spots on cucumber leaves, there is only one culprit.
It makes a start on the lower shaded leaves, feeds on the plant nutrients, spreads across the leaf surface, disperses to other leaves, blocks sunlight that prevents photosynthesis, and ultimately starves your cukes to death.
At the earliest sign of white spots on any part of a cucumber plant, intervention is needed.
Read on to discover how to identify, treat, and ultimately… save your cukes from an early demise.
White spots on cucumber leaves are the fungal spores of powdery mildew. Leaves can be treated with neem oil, horticultural oil, or even sprayed with a homemade baking soda solution. Heavy infestations require a chemical fungicide treatment, or the leaves need to be removed and trashed.
Start with disease-resistant varieties
The magical letters you want to see on your seed packets, or the online description of the seeds you are about to order are “PM”. That abbreviation means the cultivar is resistant to powdery mildew.
When to pay more attention to disease resistance is when buying cucumber seedlings.
In nurseries, seed germination is done in commercial greenhouses. The exact environment for powdery mildew to thrive. A warm and dry climate.
Plant with enough space between cucumbers to allow good airflow
Cucumber vining varieties need support to climb. These should be grown with trellis support that is at least six feet tall.
For bush varieties, those need a lot more space. At least three to four feet between rows.
The spacing requirements change between seed planting and seedlings.
When planting cucumber seeds, six-inch spacing is ideal. However, once the seedlings emerge and reach about 4-inches in height, they need thinning to prevent crowding.
Crowding reduces airflow, which encourages powdery mildew disease.
The spacing for cucumber seedlings is one seedling for every sixteen inches.
That ten-inch difference is massive. Once the plants mature and the canopy closes later in the season is when the difference is noticeable.
Pruning has the advantage of contributing to humidity control and that is what is needed to control powdery mildew. The spores germinate in high humidity. But, when the humidity drops, the spores spread.
Selective pruning is how to maintain good air circulation. It prevents humidity from reaching high levels that allow the fungus to thrive.
Direct sunlight keeps it at bay
The first white spots on cucumber leaves will appear on the lower part of the plant. This is because the fungal spores struggle in full sun.
The fungal spores of powdery mildew cannot survive in full sun. In shade plants, the disease is more prevalent.
Cucumbers are considered in the gardening sense to be low-maintenance vegetables (despite being a fruit) only when grown in full sun.
They need an average of 8 hours of full sun. Without it, diseases are quick to invade and difficult to tame.
As the plant matures, the lower leaves become shaded.
When tending to your plants, regularly inspect the lower leaves.
This is where you will see early signs of all types of discoloration. From brown spots on cucumber leaves to the yellow spots caused by mite bites or downy mildew.
They all start on the lower parts of the plant then spread up the vine.
White spots on cucumber leaves are rarely fatal. The damage caused by the fungus is though. The fungal spores feed on nutrients in the plant.
The weaker the plant becomes, the more susceptible it will be to different bacterial and fungal infections, and attractive to garden pests.
The longer white spots remain on the leaf, it will spread until it eventually coats the leaves causing the leaves to be blanketed in white spores.
When that happens, sunlight is blocked. The leaves discolor to the extent it causes defoliation. Without enough leaves for photosynthesis, fruit production becomes hampered.
Watering at the right time
There are two things to pay attention to when watering your cucumber plants. The first is at the soil level.
When the soil is overwatered, humidity is increased. Soil temperatures over 60-Fahrenheit in high humidity are what powdery mildew favors.
Saturated soil at temperatures under 68-Fahrenheit is an even higher risk of more sinister infections as that can set the stage for cucumber stem rot disease.
The second issue is the time you hydrate cucumbers. The best time for watering is early morning. Never in the late afternoon or the evening.
Watering later in the day leaves the soil and leaves wetter for longer, meaning the morning dew on the leaves will be present for longer. More so when the plants are unpruned, and bushy causing reduced airflow.
Treating white spots on cucumber leaves
There are numerous treatment options for powdery mildew disease.
Some are homemade recipes that are as simple as diluting baking soda in water and spraying the leaves. The acidity of baking soda increases the pH on the plants’ leaves, killing the fungus.
Baking soda can be effective as an early treatment.
For advanced stages of powdery mildew, or for treating white leaves on cucumber plants, a stronger chemical fungicide will be needed to treat powdery mildew. Not just one either. Two at least because powdery mildew can quickly develop resistance.
Frequently Asked Questions related to white spots on cucumber leaves?
Can infected leaves be composted?
Powdery mildew is an obligate parasitic fungus, meaning it needs living plant tissue to survive. It cannot survive among decomposing plant material. Infected leaves can be composted, but it will take time to kill it.
How does powdery mildew spread?
Everything spreads powdery mildew. The slightest breeze; the tiniest mite, irrigation, or fallen leaves. The spores can infect soil, overwinter, and germinate when the conditions are right. Once you see the white spots, treatment is needed to kill the spores to prevent them from spreading.
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.