Why You Have White Powder or Spots on Your Cucumber Leaves

When it comes to white spots and powder on the leaves of your cucumbers, there’s really only one culprit – powdery mildew. Although incredibly common throughout North America, the good news is that this particular fungal disease is rarely fatal to your plants. Especially if you catch it early, treating the problem is fairly straightforward.

While learning how to prevent a problem is always a better option than treating it, know that even if you do everything right, you cannot completely protect your cucumbers from powdery mildew. Fungal spores move pervasively through the air, on pollinators, and in water and it’s unlikely you can prevent a few from finding your plants. This common fungal disease is just a part of the garden landscape that we need to learn to manage.

So, if you’ve been wondering about the white spots and powder on your cucumbers this season, this article is for you. We’ll discuss what powdery mildew is, the top reasons why your cucumbers may have succumbed to it this season, and a few simple ways to treat the disease.

What Is Powdery Mildew?

Powdery mildew is a fungal disease commonly found throughout North America. It is caused by several fungal species including Erysiphe, Microsphaera, Phyllactinia, Podosphaera, Sphaerotheca, and Uncinula. Cucumbers – along with zucchini, beans, gourds, tomatoes, and peppers – are especially susceptible to it.

Visible Signs

Visible symptoms can appear at any time in the growing season but tend to be more pronounced when there are swings in temperature and humidity.

Powdery mildew looks just like it sounds – it appears as a whitish dust that settles on the leaves of a plant. In the early days, it almost looks like a child sprinkled flour on the plant.

As the disease progresses, however, that dusting will turn into larger white blotches, fuzzy stems, and dead leaves. This fungal disease prefers young plants and newer growth. You won’t often find it on very mature leaves.

Once you spot symptoms on your cucumbers, take quick action. The faster you treat it and remove the affected material, the better chance you have at eradicating the problem before it spreads to additional plants.

How it Spreads

The first key to successfully protecting your cucumbers from powdery mildew is to understand the conditions the fungus prefers. Once you’re armed with that information, you can set your plants up for success. You can’t control the weather, but you have control of a lot of things that can make a difference!

Location, Location, Location

Powdery mildew thrives in areas that are shaded. If your sun-loving cukes are planted in an area without enough direct light, they’ll be more susceptible to the disease.

Cucumbers need to be planted in an area that receives full sun, meaning at least 6 hours of direct sun each day. They really prefer 8-10 hours of sun for maximum fruit production. Southern exposure is the gold standard for this sun-loving vegetable where they receive light the majority of the day but are protected from the strongest afternoon sun.

If you’re not sure how much sun your space receives, take a few days to actively measure it. Note when the sun first reaches a particular area, if it’s ever shaded by tall trees or other buildings, and when it leaves for the day.

If you realize that your garden bed isn’t receiving an optimal amount of sun, be sure to relocate your cucumbers next season. It will do wonders for the health of your plant, its yield, and its ability to fight off diseases.

Your Plants Are Too Crowded

Correct plant spacing is incredibly important to a garden’s success. Lack of proper air circulation creates an optimal breeding ground for fungal spores like powdery mildew. Imagine a hot stuffy room with no fan to move the air around. This is the condition you can create if your plants are placed too closely together.

Think of both air and light as cleansing agents for your plant’s health.

The good news is that you can grow your cucumbers in such a way as to facilitate good airflow, rather than block it. Train your cukes to grow vertically on a trellis and you’ve already done a lot to solve this problem. If you’re companion planting other herbs, flowers, or veggies nearby, ensure you space them far enough apart that the air can still move freely between them.

Adjust Your Watering Habits

Incorrect watering habits are one of the most common causes of powdery mildew. Remember that this is a fungal disease found in the soil.

One of the easiest ways to transmit it to a plant is by water splashing back to the undersides of the leaves after it hits the soil. This carries the spores from the soil directly to the leaves. This is especially problematic if you have leaves very close to the soil line.

To avoid this kind of splash back, practice the following good watering habits in your garden.

Water Slowly and Deeply

Cucumbers don’t appreciate a deluge of water. Think about the difference between taking a nice sip of water from glass vs someone opening a fire hydrant for you.

A drip system or soaker hose is the ideal way to water your plants, but if you don’t have access to them, you can turn your hose to a low setting and let the water slowly stream into the soil.

Watering this way will not only allow your plants to take up more water, but it will also prevent the kind of splashback you find when using a harder spray nozzle and a fast stream of water.

Water at the Base of the Plant

No matter what kind of system you use to water your cukes, be sure to do so at the base. One of the biggest mistakes new gardeners make is to water the leaves. This can cause several problems including pest attraction, leaf burn, and – you guessed it – disease.

Wet leaves are a prime breeding ground for fungal diseases. Airborne spores will happily land on those moist leaves and set up shop immediately.

Remember, leaves need sunlight and roots need water. Put that water where the plant needs it – the roots!

You Didn’t Mulch This Season

Mulch is incredibly important in your garden but is often overlooked. Mulch provides protection against excessive heat, aids with moisture retention in the soil, prevents water splashing back (which as we know, can spread fungal disease from the soil), and discourages weeds.

You can use a lot of things as mulch including organic material found in your yard, commercially produced mulch from your local garden center, or coconut coir (my personal favorite). No matter what type of mulch you choose, aim to apply a layer 2-3 inches thick for maximum benefit.

This layer of mulch will not only protect your cucumbers from splash back from regular watering, it will protect them during heavy rain as well.

Heavy rain can provide a number of conditions ideal for the spread of powdery mildew: cool dampness, wet leaves, a lack of direct sun, and water splashing back from the soil line. Mulch can’t protect against all of those elements, but it can really help by covering up that soil!

Other Potential Issues

While less common, other causes of powdery mildew spread can include overfertilization and infected compost or mulching material.

Overfertilization can sometimes encourage the spread of the disease simply because the plant grows too quickly. All that new growth is what this fungal disease prefers to settle on.

Homemade compost and mulch can also sometimes be a culprit. If you composted infected material last year, you could be unwittingly spreading fungal spores directly to your plants. The same can be true of using organic material in your yard for mulch.

Fungal spores are likely overwintering in that leaf pile you crushed up for mulch this year! While these ways are less common than those we already discussed, it’s something to keep an eye on.

Treating Powdery Mildew in Cukes

The good news is that treating cases of powdery mildew is fairly straightforward. There are a few different steps you can take once you’ve diagnosed the issue. Let’s look at your next steps.

Prune Off Affected Material

The first thing you should do is prune off any leaves or branches you see. Put that affected material into a trash bag (not your compost) and then disinfect your pruning tools. If you skip that step, you’ll just spread fungal spores to the next plant you prune.

Treat the Plant With Neem Oil

If you’re dealing with a large-scale fungal infestation, you may want some help.

Neem oil can be a great option when treating your cucumbers for powdery mildew. You can dilute neem into water to create a foliar spray for your plant or carefully wipe diluted amounts directly on leaves.

If you spray, be sure to do so when there are no pollinators around since they can be harmed if hit directly. Also note that neem will potentially kill predatory insects like lacewings.

Bio-fungicides (look for OMRI approved) can also be a useful tool in killing existing fungal spores and protecting the rest of your cucumbers.

Plant Disease Resistant Varieties

If you know your area is prone to cases of this fungal disease, you may want to opt for a disease-resistant cucumber variety. This is probably the easiest way to prevent the disease from taking hold of your plants.

These varieties were bred specifically to be resistant to diseases like powdery mildew. Look for a ‘PM’ on the back of a seed packet.

There are quite a few options available out there (growers know these varieties are in demand). But if you’re looking for some examples to get you started, take a look at Ashley, Diamondback or Bodega. For pickling cukes, try Adam Gherkin and Picklebush.