Plant diseases are costly in the wholesale industry. A single infection can wipe out an entire crop. At home, things are a bit different. Diseases are annoying, sure, but they won’t destroy your garden overnight.

Home gardens are diverse, and diseases tend to infect a select number of plant species within them. Though you may see powdery mildew on your squash and cucumbers, they won’t spread to non-cucurbit crops. Plus, you’ll still have a harvest to pick despite the fungus’s presence. 

Instead of rushing to the fungicide aisle, learn how this disease works so you can halt it in its tracks. Prevention, management, and treatment ensure it doesn’t take over your garden. Let’s protect your squash and cucumbers from powdery mildew!

What is Powdery Mildew?

Powdery mildew (PM) is a fungal disease that spreads through spores. The spores waft in the air, landing on susceptible specimens during the growing season. They germinate and spread, forming large swaths of powdery white patches. 

Conditions for Growth

Crowded gardens invite fast-spreading fungal growth.

All diseases need three things to exist: a host, a pathogen, and environmental conditions conducive for growth. Remove one of these three factors, and the disease will falter and fail. 

PM needs humid conditions, warm temperatures, and ample moisture to thrive. This is why you see it commonly during wet spring and fall seasons. Occasionally, it’ll show up during the growing season in areas with mild summers. 

Because the spores waft through the air, cramped conditions lead to their rapid spread. Overcrowded gardens with plants too close to each other are appealing to the fungus, and you’ll notice it growing rapidly once it appears. 

Crops Affected

Some fungal species specialize in specific plant groups.

Unfortunately, squash and cucumbers aren’t the only crops affected. Different PM species infect various crops. Some infect multiple groups of plants, though most of them are species-specific. 

The species that infects cucumbers and squash, known as Podosphaera xanthii, also attacks other cucurbits, namely melons, gourds, and pumpkins. 

Other species of PM attack grapes, lilacs, roses, azaleas, and rhododendrons, among others. Consider growing resistant plant varieties if you know this fungus is common in your region.

How to Identify Powdery Mildew

Identify powdery mildew on squash and cucumbers using symptoms and signs. Symptoms are responses the plant has to the fungus, while signs are outward growths of the disease itself. Watch for both to determine if your cucumbers and squash have the fungal condition.

Signs

Cooler, damp conditions make leaf infections spread faster.

Signs of powdery mildew on squash and cucumbers include white, splotchy growth on the leaves. Patches start small in the late season, and they spread quickly when the weather grows wet and cooler in the fall. They may appear on the tops or undersides of the leaves, and they’ll spread to the stems if left unchecked.

PM doesn’t spread to squash like it does with grapes. It may occur sporadically on cucumbers and melons, though it tends to stay on the leaves and stems. 

Symptoms

White powder appears first, covering leaves over time.

Symptoms are responses the plant has to the disease. In the case of powdery mildew on squash and cucumbers, you’ll notice the signs first before the symptoms. White powder is a sign, and it spreads to cover the entirety of the leaves. 

After the condition spreads, the plant responds with leaf death and poor growth. Old leaves turn yellow and brown before shriveling off. Vegetables that form may be of a small size or poor quality.

Prevent the Disease

Prevention is the best way to stop PM from taking over your garden. Hold off its return, and prepare the garden for its entrance. Knowing it returns during the late season, you can take measures now to discourage its presence. 

Practice Crop Rotation

Moving crops yearly helps reduce leftover plant debris.

Crop rotation is a great way to prevent this disease in the late-season garden. Crop rotation involves moving where you plant certain crops every few years. It works well because PM overwinters in old crop debris. 

When you move where you plant cucumbers or squash, you avoid exposing your new crops to old debris. There will be fewer spores on the ground, which means there will be fewer spores in the air once they’re airborne. 

Rotate squash with non-cucurbits, like tomatoes, corn, or leafy greens. After three or four years, you may plant the cucurbits in the same spot again for another three to four years. 

Hot Compost Debris

Hot compost kills pathogens hiding in old plant material.

Compost recycles debris, and it kills weed seeds and diseases if it’s hot enough (at least 140°F or 60°C). Use hot composting methods to erase the remnants of powdery mildew on squash and cucumbers from your garden. Gather the infected leaves and throw them on the pile. 

Hot compost requires a proper ratio of greens to browns, daily turning, and frequent watering. It needs more attention than cold compost, also known as lazy compost, but it’s a great method for reducing the pressures from weeds and diseases.

Avoid Overhead Watering

Wetting soil, not foliage, prevents rapid disease spread.

Overhead watering is easy to do, but it may cause PM to spread! Water knocks the spores off the leaves, but the humidity it creates causes the disease to grow quickly. It’s better to wet the soil than the leaves. 

Once symptoms of powdery mildew appear on squash and cucumbers, consider transitioning from overhead watering to another irrigation method. Use drip irrigation, or try adding olla pots to the soil. Olla pots consist of terra cotta, and the material slowly drips moisture into the soil when the plants need it most. 

If you are using overhead watering, apply the moisture at a time when it’ll evaporate quickly. Do so in the early morning; the moisture will seep into the soil, and it’ll evaporate from the leaves as the sun rises. 

Plant In the Right Spot

Six hours of sun strengthens delicate climbing vines.

PM loves the shade! Humidity levels tend to be higher in shady sites than sunny ones, where moisture sits in the air for long periods of time. The worst thing you can do is plant a squash or cucumber vine in partial shade instead of full sun. 

If the vines are already growing in the shade, consider pruning tall trees and shrubs to let more light through. If the cucurbit vines are growing in pots, move them to an exposed location with more than six hours of daily direct sunlight.

Sow Resistant Varieties

Resistant varieties handle garden challenges without extra fuss.

If powdery mildew on squash and cucumbers is particularly common in your garden, consider growing resistant varieties that grow well despite the fungus’s presence. Many squash varieties offer resistance.

‘Triumph’ is a resistant variety. It’s a type of green zucchini summer squash that performs well in home gardens. ‘Honeynut’ is a delicious winter squash that’s resistant.

As for PM-resistant cucumbers, try ‘Muncher.’ It’s a Persian variety with snack-sized cucumbers. Eat them fresh, ferment them, or pickle them in vinegar with spices. 

Managing Powdery Mildew

So, what is there to do when powdery mildew infects your cucumbers and squash? Fear not—try these four different strategies to manage the disease once it appears. 

Remove Some Leaves

Dispose of affected foliage far away from plants.

Leaves act as vectors for this fungus. When they have white patches all over, they emit spores onto nearby healthy leaves. The spores germinate and grow, causing more leaves to fall victim to the white patches. 

If the infection is early and small, try removing the infected leaves. Dispose of them far away from the site, or hot compost them in a well-tended pile. 

Be careful not to remove too many leaves. Without leaves, the squash and cucumbers cannot photosynthesize! They’ll suffer more, and they’ll be less likely to resist the infection. 

Prevent the Spread

Avoid overfertilizing to limit excessive leafy growth.

Airflow is crucial in the home garden! Cramped, crowded conditions lead to PM’s rapid spread. The closer a leaf is to another one, the easier it is for the spores to spread. 

Follow the spacing guidelines on the seed packets at planting. If you have a small garden, try growing dwarf or bushy crops that easily tuck into tight spaces. 

Also, avoid overfertilizing. Too much nitrogen causes excessive leafy growth at the expense of flowers and fruits. With more leaves, the vines act as hotbeds for the fungus’s spread. Use a well-balanced fertilizer, and consider testing the soil first so you avoid adding too many nutrients.  

Deal With It

Fungicides slow the disease but don’t fully eliminate it.

This is what I have to do in my home garden! No matter how resistant a cucumber is to PM, the spores waft from nearby sources and infect my crops. The most I can do is prevent the disease and discourage it from spreading. 

You may wonder, why not use fungicides? Many of them lead to disease resistance. PM species are now resistant to many old antifungals, and new ones must be invented every few years to keep up. 

After spraying the PM, resistant patches remain. They spread resistant spores, which remain after you spray the antifungal. In this way, spraying fungicides breeds resistant PM strains. 

Most sprays don’t work like you think they would. They prevent PM from spreading, but they don’t eradicate existing patches. They’ll help slow the spread; they won’t remove the disease. 

Apply an Antifungal 

Light afternoon sprays minimize risk to visiting pollinators.

If you think an antifungal will help you control powdery mildew on squash and cucumbers, you may apply it to infected leaves and stems. Use an organic spray that’s unlikely to hurt pollinators and insects, and apply it in the early morning or evening when insects are less active.

Effective options include potassium bicarbonate solutions, milk sprays, and sulfur products. Apply them according to the instructions on the label, and use them as needed. Don’t overdo it! You’ll cause resistance over time. 

Frequently Asked Questions