Characterized by a powdery white or gray cotating on leaves and stems, powdery mildew is a fungal disease that affects squash plants. It starts as small, circular spots that quickly spread and cover the entire plant. The fungus thrives in warm, humid conditions and, if left untreated, can cause the leaves to turn yellow and crispy until they eventually die.
To deal with powdery mildew on squash, remove and destroy infected plants, increase air circulation around plants by thinning them out, and water early in the day so the leaves have time to dry before nightfall. You can also dust the plants with sulfur, use neem oil, or make a baking soda spray.
Dealing with powdery mildew early on is an excellent way to ensure healthier, more productive squash plants. Check your plants regularly for signs of disease and take action immediately if you see any telltale powdery mildew symptoms. Read on as I go into more detail about each of these powdery mildew control measures.
1. Check Your Squash Plants Regularly
Powdery mildew is a common fungal disease that most squash farmers have to deal with at some point. The best way to combat it is to catch it early on before it has a chance to take hold and do severe damage to your plants.
With its telltale powdery white or gray coating, powdery mildew is easy to spot. The fungi live on the leaf’s surface and multiply quickly in warm, humid conditions. You’ll first see it as small, circular spots that quickly spread and cover the entire plant.
To inspect your squash plants for powdery mildew, look closely at the leaves and stems. Are there blotchy patches of white or gray powder? Do you see any small, circular spots?
If so, your plant is likely infected with powdery mildew. The leaves can turn yellow, curl, and brown in advanced stages.
Powdery mildew can find its way into your squash plants by blowing in on the wind from infected plants nearby or hitching a ride on your gardening tools or clothing. Purchasing infected squash seedlings from a nursery is another common way powdery mildew makes its way into gardens.
You must take action immediately if you spot powdery mildew on your squash plants. The longer you wait, the more damage the fungus will do and the harder it will be to get rid of.
However, some squash varieties have natural white markings that you might mistake for powdery mildew. Luckily, you can easily distinguish between the two by doing a simple test. Gently rub your finger across the white spots. If the white comes off on your finger, it’s powdery mildew. If they don’t budge, it’s just natural markings.
2. Remove and Destroy Infected Plants
With its quick-spreading nature, powdery mildew can quickly infect all your squash plants within 7-10 days if left untreated. For this reason, the best way to deal with powdery mildew is to nip it in the bud by removing and destroying infected plants as soon as you spot them.
To remove an infected plant, you will need a pair of gloves, sharp pruning shears, and a garbage bag:
- Put on your gloves to protect your hands from the fungus.
- Use the pruning shears to cut the infected plant at the base, as close to the ground as possible.
- Place the entire plant into the garbage bag.
- Tie up the bag and dispose of it in the trash.
- Do not compost infected plants as the spores can persist in the compost and infect other plants.
Once you’ve removed and destroyed infected plants, it’s essential to clean your pruning shears with rubbing alcohol or a 10% bleach solution to kill any remaining spores. You should also disinfect any other gardening tools or clothing that came into contact with the infected plants. This will help prevent the fungus from spreading to healthy plants.
If you have a serious powdery mildew problem, you may need to remove and destroy all your squash plants. The fungus can cover the entire leaf surface, reducing the photosynthetic area and causing the plant to produce fewer and smaller fruits. In severe cases, the spores can overwinter in the soil and infect your plants the following year.
If you have to start from scratch, clean up your garden bed and remove all infected plant debris before planting new squash seedlings. Purchase healthy, disease-free seedlings from a reputable nursery and space out your plants to improve air circulation and reduce humidity.
3. Increase Air Circulation Around Your Squash
Powdery mildew thrives in humid conditions, with crowded plants and dense foliage providing the perfect environment for the fungus to spread. To discourage powdery mildew, increase air circulation around your squash plants by thinning out overcrowded plants and pruning off dead or diseased leaves.
Squash plants tend to sprawl and will give you a bountiful harvest when you allow them to spread out. When planting, space summer squash 24-30 inches (61-76 cm) apart and winter squash 30-48 inches (76-122 cm) apart. Spacing out your plants provides better air circulation, which will help prevent powdery mildew from taking hold.
You can also try growing squash on a trellis system to keep the foliage off the ground and improve air circulation. Trellises also allow more sunlight exposure, create more space in your garden, and make it easier to spot powdery mildew and other problems on the undersides of leaves.
Regularly prune your squash plants throughout the growing season to keep the air moving and discourage powdery mildew. Dead or diseased leaves are a prime breeding ground for the fungus, so remove them as soon as you spot them. However, be careful when pruning, as you don’t want to cut too much of the plant and reduce its fruiting potential.
4. Water Squash Early in the Day and at Ground Level
Water is essential to the squash-growing process, but wet leaves are also a prime breeding ground for powdery mildew. Plant processes like photosynthesis and nutrient transportation rely on water, so you can’t completely eliminate watering to prevent the fungus. However, you can reduce the spread of powdery mildew by watering early in the day so the leaves have time to dry out before nightfall.
Since the fungus needs humid conditions to spread, it’s also important to water at ground level. Watering from above will wet the leaves and create humid conditions favorable for powdery mildew growth.
Use a soaker hose or drip irrigation system to direct water straight to the roots without getting the leaves wet. If you must water with a hose or sprinkler, do it in the morning so the leaves have time to dry out before evening.
Mulch around your squash plants to help retain moisture and reduce water needs. A 2-3 inch (5-7 cm) layer of organic mulch will help keep the soil moist and reduce evaporation. Be sure to mulch around the base of the plant, but not up against the stem, as this can encourage rot.
If you have grown your squash in containers, ensure you water them deeply and infrequently. Allow the top 1-2 inches (2.5-5 cm) of soil to dry out between waterings, and empty any saucers or trays after watering so the roots aren’t sitting in water. Soggy conditions can lead to root rot, which makes squash plants more susceptible to powdery mildew.
5. Dust Your Plants With Sulfur
Sulfur is a common fungicide used to prevent and treat powdery mildew. You can purchase sulfur dust or wettable sulfur at your local garden center and follow the directions on the back of the package.
Dust the entire plant, including the undersides of the leaves, stems, and fruits. Wear a dust mask, gloves, and eye protection when handling sulfur, as it can irritate the skin and eyes. Sulfur can also be harmful if inhaled, so be sure to work in a well-ventilated area.
You can also use sulfur as a preventive measure, so consider dusting your plants before powdery mildew appears. Monitor your plants closely for signs of the fungus, and dust them at the first sign of trouble.
Reapply sulfur every 7-10 days, or as needed, to keep powdery mildew at bay. You may need to apply it more often in humid or wet conditions.
When dusting the plants, follow the manufacturer’s instructions for amount and frequency, as too much sulfur can damage plants. In high application, sulfur can alter the soil pH, so get your soil tested before using it to make sure the sulfur won’t throw off the balance.
6. Use Neem Oil
One of the most effective ways to deal with powdery mildew in its early stages is to use neem oil. This natural oil is sourced from neem tree seeds. It has fungicidal, antibacterial, and insecticidal properties that kill and repel a range of plant pests, including aphids, whiteflies, and leaf miners.
Neem oil is also effective at keeping powdery mildew in check. It works by creating a barrier on the plant that prevents the fungus from attaching to the leaf’s surface and reproducing. You can purchase neem oil at your local garden center or online.
Since it’s plant-based, it’s naturally safe to use on fruits, vegetables, and ornamentals. However, it can burn squash foliage when used excessively or diluted improperly. Test it on a small area of the plant first, and if there’s no burning or wilting after 24 hours, you can treat the entire plant.
To use neem oil, you will need:
- 2 tablespoons (29.57 ml) of Neem oil
- 1 teaspoon (4.93 ml) of liquid soap
- 1-gallon (3.79-liter) of warm water
- A clean sprayer bottle
- Mix the liquid soap and warm water in the sprayer bottle. Shake thoroughly so the soap is completely dissolved.
- Add the neem oil to the mixture and shake well again. The soap is an emulsifying agent that helps the oil and water mix. Shake well to ensure the mixture is completely combined.
- Spray the solution onto the plants, covering the undersides of the leaves and the tops. Get the stems and fruits as well.
- Reapply every 7-10 days, or as needed, until the powdery mildew is gone.
Neem oil can also be used as a preventative measure. Simply spray the plants every 2 weeks or so during the growing season to keep powdery mildew at bay. Remember, neem oil is ineffective in severe cases of powdery mildew, so it’s crucial to catch it early. Avoid applying it under direct sunlight, as this can cause leaf scorch.
7. Make a Baking Soda Spray
Besides being a common ingredient in baking, did you know that baking soda can also be used to treat powdery mildew? Sodium bicarbonate, the active ingredient in baking soda, is a natural fungicide that inhibits the growth of powdery mildew and other fungal diseases.
When used early on, baking soda can effectively control powdery mildew. As the fungus grows and spreads, it becomes more resistant to treatment. Baking soda is most effective as a preventative measure, so spray your plants every 2 weeks or so during the growing season.
To make a baking soda spray, you will need:
- 1 tablespoon (14.79 ml) of baking soda
- 1 teaspoon (4.93 ml) of dish soap (not detergent)
- 1 tablespoon (14.79 ml) of vegetable oil
- 1-gallon (3.79-liter) of water
- A clean garden spray bottle
- Dissolve the baking soda in water.
- Add the dish soap and vegetable oil and mix well to combine.
- Pour the mixture into a clean garden spray bottle.
- Spray the plants, covering the undersides of the leaves and the tops.
- Reapply every 2 weeks, or as needed, during the growing season.
Spray your squash early in the morning when the sun is not directly overhead to prevent leaf scorch. Repeat bi-weekly or as needed until the powdery mildew is gone. Unfortunately, a build-up of sodium can harm your plants, so it’s best to rotate your treatments.
8. Spray Your Plants With Milk
Milk makes for another inexpensive and effective way to control powdery mildew, according to a 2011 study by the University of Connecticut. The study found that a milk spray was just as effective at controlling powdery mildew as chemical fungicide treatments.
There are several ways milk helps deal with powdery mildew:
- Creates a physical barrier: The proteins and fats in milk form a thin film on the plant’s surface that prevents the fungus from attaching and growing. This works best when used as a preventative measure, so start spraying your plants early in the season before powdery mildew has a chance to take hold.
- Alters pH on the leaf surface: Milk has a slightly alkaline pH, making it inhospitable for powdery mildew. The slightly higher pH will actually kill off the encroaching spores.
- Contains lactoferrin: This natural antimicrobial agent smothers the powdery mildew spores and prevents them from germinating.
- Produce free radicals: When exposed to sunlight, milk produces free radicals that kill the powdery mildew fungi.
To make a milk spray, you will need:
- 40 parts milk (whey, whole, or powdered)
- 60 parts water
- A mixing container
- A clean garden spray bottle
- Mix the milk and water together in a clean container.
- Pour the mixture into a clean garden spray bottle.
- Douse the plants thoroughly, covering the fruits, stems, and undersides of the leaves and the tops.
- Reapply every 7-10 days and after rainfall for the best results.
While the science behind this method is unclear, it has proven to be an effective treatment for powdery mildew. Milk compounds also help boost the plant’s immune system, making it more resistant to other diseases and pests.
9. Avoid Overfertilization During the Growing Period
While fertilizing provides your squash with the nutrients it needs to grow healthy and strong, too much of it can do more harm than good. Overfertilization can promote excessive leaf growth, creating a favorable environment for powdery mildew. The spores love a warm, moist, shady place to grow and multiply.
Stick to a light fertilizer application during the growing period to keep your plants healthy without encouraging too much leaf growth. Avoid fertilizers with high nitrogen levels, as this can exacerbate the problem. Additionally, ensure you water the plants at the base and not on the leaves to prevent powdery mildew spores from splashing onto the foliage.
10. Plant Varieties Resistant to Powdery Mildew
While there are many ways to deal with powdery mildew once it’s taken hold of your plants, the best defense is a good offense. Planting squash varieties that are resistant to powdery mildew will help reduce the chances of your plants becoming infected in the first place.
These varieties have been bred to resist powdery mildew and other diseases, providing you with a bountiful harvest come harvest time. They also save you time and money as you won’t have to constantly be on the lookout for powdery mildew and treat it.
Some of the best squash varieties resistant to powdery mildew include:
- Yellow (straight or crookneck): Delta, Smooth Operator, Sunray
- Zucchini: Dunja, Yellowfin, Payroll, Green Machine, Sebring
- Butternut: Butterbaby, Autumn Frost, Metro PMR
- Acorn: Royal Ace, Taybelle, Table Star
- Specialty: Delicata, Sugaretti, Winter Sweet
These varieties are just a starting point, so talk to your local garden center or agricultural extension office for more specific recommendations based on your area.
Powdery mildew is a common problem for squash growers, but it doesn’t have to be. You can keep powdery mildew at bay by regularly checking your squash plants, increasing air circulation, and removing and destroying infected plants.
Dusting your plants with sulfur, spraying them with neem oil, and using a milk spray are also effective treatments. You can also save yourself a lot of trouble by planting squash varieties resistant to powdery mildew. These varieties have been bred to resist the fungus and will provide you with a healthier, more bountiful harvest.