Cucumbers (and of course, cucamelons) are one of my favorite plants to grow every summer, so it’s definitely a little bit troubling to see my cucumbers develop odd symptoms and not thrive. White spots on cucumber leaves are one well-known problem among cucumber growers. It is primarily a cosmetic issue, but it can hinder the growth of your cucumbers and reduce your harvest, which is definitely not what we want. So let’s dig into why our cucumber plants develop lesions on their leaves and what to do about it.
What causes white spots on cucumber leaves?
If you’ve noticed white spots beginning to develop on your cucumber leaves, there is probably only one culprit: a bacteria called Pseudomonas syringae pv. lachrymans. Pseudomonas syringae is an extremely common bacterial infection that can damage plants but is not known to cause any harm or danger to human health.
Pseudomonas syringae loves to grow and spread on cucumber plants and can also infect zucchini and honeydew melons. Pseudomonas syringae, sometimes called cucumber leaf spot, bacterial leaf spot, or angular leaf spot, start as small white spots but can eventually turn into large, dark, dead sections of leaves. They tend to grow until they reach the vein of a leaf, at which point they stop. This gives the spots a somewhat angular appearance, thus the name ‘angular leaf spot.’
Treating cucumber leaf spots
Pseudomonas syringae thrives in moist environments. You’ll often notice that the lesions on your cucumber leaves are very wet before they turn more into a white crust when the weather dries. White spots on cucumber leaves that occur in moist conditions will likely resolve on their own when conditions dry out. If wet weather is likely to persist, you can remove infected leaves, but try never to remove more than one-third of the cucumber’s leaves so not to accidentally hinder or kill the plant. Try to keep new foliage whenever possible.
If your watering system gets the leaves of your cucumber wet, it could perpetuate the Pseudomonas syringae bacterial infection, leading to further damage and possibly death. The key to reducing the presence of this bacterial infection is keeping the leaves, fruits, and stems of your cucumber plants as dry as you can. If it’s a persistent problem year after year, it may be wise to begin planting cucumbers in another part of the garden, as the bacteria may be infecting the soil your cucumbers grow in.