Mildew on cucumber leaves: treatment & prevention

Mildew on cucumber leaves is one of the most common problems gardeners can face when growing cucumbers. Read on to find out how to recognise cucumber mildews and how to prevent and treat them.

Cucumbers or Cucumis sativus as they are botanically named, along with other members of the cucurbit family (Cucurbitaceae) such as squash (Cucurbita), courgettes (Cucurbita pepo) and melons (Cucumis melo), are often affected by mildews. Both powdery and downy mildews are caused by fungi that mark the foliage, making it easy to identify.

Powdery and downy mildew on cucumber leaves

Even though powdery and downy mildew sound similar, they are in fact rather different fungal diseases. Both are among the most serious diseases that can affect cucumbers. Powdery mildew on cucumbers is most often caused by Podosphaera fuliginea and Erysiphe cichoracearum fungal pathogens and is the more common and milder mildew cucumbers can suffer from. Downy mildew, on the other hand, is caused by the Pseudoperonospora cubensis pathogen and, while less prevalent, can result in complete and sudden crop loss.

Powdery mildew symptoms

Powdery mildew on cucumber leaves can be identified by patches of white fungal growth on the leaves and stems, eventually affecting the entire foliage. Thankfully, powdery mildew rarely affects the fruits; nonetheless, the fungus feeds on the plant and if a severe case develops, the plant’s growth can be hindered, resulting in a weakened plant.

Downy mildew symptoms

In contrast to the white round marks of powdery mildew, downy mildew on cucumbers can be identified by yellow and brown angular marks on the leaves that are confined by the veins. In turn, the leaf damage reduces photosynthesis, leading to poor growth, reduced fruit size and the plant ultimately dying.

Can you eat mildew-infected cucumbers?

If you are wondering if you can safely eat cucumbers from a plant with mildew, the answer is it depends. Mildew tends to only affect the foliage of cucumber plants and not the actual fruits. However, the disease may have an impact on the fruit quality, resulting in cucumbers that lack flavour.

If mildew only appears on the foliage, the fruit is typically safe to eat. However, if you are not certain that the disease is mildew or if the fruits show any signs of yellowing or whitening, then do not eat the cucumbers.

How to get rid of mildew on cucumber plants

If you identify mildew on your cucumber leaves, there are some options available to treat the fungal disease, with different controls for both powdery and downy mildew.

Powdery mildew thrives in high humidity environments such as greenhouses, but dislikes direct contact with water, thus wetting any affected leaves can help keep the disease from spreading. Downy mildew is similar in that it likes poorly ventilated and enclosed environments, but it thrives when the foliage is damp and unable to dry out. There are no treatments available for the home gardener against downy mildew, so the only option is to swiftly remove any affected leaves as soon as they are spotted, and if this does not help, to remove and destroy the entire plant.

If powdery mildew is confirmed, then there are chemical controls available. However, these should be used as a last resort and per the manufacturer’s guidelines. Plant invigorators, which contain environmentally friendly pesticides as well as nutrients to feed the plants, can be effective in treating powdery mildew and are widely available, but may need to be applied regularly. If plant invigorators do not control the disease, then the fungicide tebuconazole with trifloxystrobin can be applied as it is approved for use on edible greenhouse crops.

Home remedies have long been used in the battle against mildew. A common natural treatment for powdery mildew, as strange as it sounds, is milk; Simply mix milk with water at a ratio of 1: 10 to 1: 2 and spray onto the affected foliage twice a week. Many trials have concluded that it can work, though the jury is still out on why.

Prevention methods

With limited treatment against powdery mildews and none in the fight against downy mildew, preventing mildews is a key part of keeping your cucumber plants healthy and can be surprisingly effective.

Prevention against powdery mildew on cucumbers can include:

• Growing cucumber cultivars with higher levels of resistance
• Planting cucumbers thinly to increase circulation around plants
• Increasing ventilation to reduce humidity
• Watering at the base of the plant, avoiding the foliage
• Removing all plant material at the end of the growing season

Prevention against downy mildew on cucumbers can include:

• Growing cultivars with higher levels of resistance
• Correctly watering and reducing the time the foliage is wet
• Promoting good ventilation
• Regularly checking and promptly removing affected foliage and plants for destroying
• Practising crop rotation

Tip: Try companion planting with cucumbers as good companions that are less susceptible to mildew can minimise the occurrence and impact of a mildew infestation. Powdery mildew fungi often like very specific host plants and, more often than not, attack entire plant families. So, do not plant any other cucurbits next to your cucumbers. 

Another important care measure to keep cucumber plants strong and healthy is fertilisation. Many diseases can be avoided with a balanced, customised supply of nutrients. Find out when to fertilise, the right fertilisers to use and how to fertilise cucumbers in our article on feeding cucumbers.