Powdery mildew is one of the most prevalent plant diseases in the world. If that wasn’t bad enough, squash plants are one of several garden vegetables that are particularly vulnerable to it.
Nothing improves the odds of recovery from this disease like early diagnosis and treatment. Since powdery mildew can wreak havoc on a squash patch, any case of white spots on foliage should be taken very seriously.
If you’ve noticed white spots on squash leaves in your own garden, you’ve come to the right place. In this article, I explain the key signs and symptoms of powdery mildew and how best to prevent and treat infections. I also break down the difference between powdery mildew and a similar disease, downy mildew, also seen on squash plants.
Causes of Squash Plant Leaves with White Spots
In my experience, nearly every case of white spots on squash leaves is actually a case of powdery mildew. This disease is so common among squash plants that I recommend treating for powdery mildew unless you have a specific reason to believe that the white spots are caused by something else.
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What is Powdery Mildew?
Powdery mildew is the common name for a number of fungi species that infect thousands of different plants. According to Utah State University, the specific varieties found on most gourds are Erysiphe cichoracearum and Podosphaera xanthii.
Powdery mildew is a fungal infection that primarily grows on the exterior of plant leaves. However, it can also be found on stems, buds, and fruit. This is in contrast to many other fungal pathogens that infect the interior plant structure.
This disease is aptly named for the powdery appearance it gives afflicted plants. Infected foliage bears a layer of white or gray spores that resemble baby powder or dust.
Take note that powdery mildew spores most often grow on the top of leaves. These fungal spores are easily spread to other vulnerable plants via physical contact, water, or wind.
As the disease progresses, small brown or black spots can appear on the leaves. Infected foliage may also turn yellow or brown, and completely die off in extreme cases.
Powdery Mildew Vs Downy Mildew
Downy mildew (Pseudoperonospora cubensis) is often mistaken for powdery mildew, partially due to its name. However, these diseases differ both in taxonomy and common symptoms.
Unlike powdery mildew, downy mildew is not a true fungus. Technically speaking, it is a type of water mold that is more closely related to algae.
While downy mildew also produces powdery spots on foliage, there are a few key differences. Downy mildew growths may be white, brown, or gray and typically appear on the bottom of leaves.
In addition to these powdery spots, downy mildew often turns foliage yellow. This discoloration typically occurs between leaf veins. You can use this characteristic to distinguish many cases of downy mildew from powdery mildew.
Downy mildew is most active when the weather is mild and humid. The disease flourishes most when temperatures are between 59 and 68°F. It may spread to cooler climates but rarely survives through winter in these areas.
In terms of how this disease spreads, downy mildew requires adequate moisture to germinate and reproduce. So squash plants must be damp for the spores to take hold and cause a new infection.
How Does Powdery Mildew Get On Squash Plants
Powdery mildew naturally spreads via the wind or water. The spores are incredibly lightweight, so it doesn’t take much disturbance of infected plant matter to release them into the environment. Powdery mildew spores may travel for hundreds of miles and are impossible to keep out of your garden altogether.
Within the garden, you can unknowingly spread powdery mildew between plants on contaminated tools, gloves, or your own hands. This is why it’s so important to use clean tools, especially when handling particularly vulnerable plants.
Another way powdery mildew is often introduced to the home garden is on nursery plants or supplies. New plants should be inspected carefully for signs of illness before adding them to your garden.
Preventing Disease on Squash Plant Leaves
Proper prevention for a number of fungal diseases, including powdery and downy mildew, starts with routine maintenance. Good cultural practices will reduce the spread of disease in the garden. Also, healthy plants are far less susceptible to infections in the first place.
Do not crowd squash plants within garden beds. Airflow is crucial to preventing the spread and growth of fungal spores on foliage.
I also recommend tracking the amount of sun exposure your vegetable garden receives each day. Adequate sunlight will enable the foliage of squash plants to dry more quickly following rain, dew, or irrigation.
In addition, providing a regular supply of nutrients with a pumpkin or squash fertilizer will help to keep plants healthy and less prone to disease.
Finally, you can reduce the spread of downy mildew and other moisture-loving pathogens by watering squash plants at the roots. This will decrease the amount of time that the leaves are wet.
How To Get Rid of White Spots on Squash
Unfortunately, no amount of preventive care will stop all cases of white spots on squash plants.
Early diagnosis is crucial when it comes to treating powdery mildew. By the time your garden shows serious signs of infection, it may be too late for recovery. Periodically inspect vulnerable plants for any signs of fungal growth so you can apply fungicidal treatments as early as possible.
Delayed treatment of downy mildew tends to be more successful than that of powdery mildew. However, I still recommend treating early on to prevent long-term health consequences and lackluster crop production.
Treatment for Powdery Mildew
In confirmed cases of powdery or downy mildew, I highly recommend treating with an appropriate fungicide for the best chance of recovery.
According to Clemson University, powdery and downy mildew are responsive to the fungicide chlorothalonil. Downy mildew may also be treated with mancozeb.
Sulfur- and copper-based fungicides are also very effective against these diseases. Clemson University advises the use of sulfur on powdery mildew and copper on downy mildew.
Natural remedies
Many gardeners successfully treat powdery mildew using baking soda, horticultural oil, or a combination of the two. My preferred horticultural oil is neem oil. These “natural” treatments must be applied at the very first sign of disease to yield good results.
Another common natural remedy for powdery mildew is cow’s milk. The Old Farmer’s Almanac specifically recommends mixing 1 part milk with 10 parts water. Keep in mind, however, that research supporting this treatment was conducted using fresh cow’s milk — results may vary when using store-bought milk.
Removing Squash Leaves With Powdery Mildew
I recommend removing and disposing of infected plant material, including leaves, stems, and entire plants. While this probably won’t eliminate the infection it may slow down its spread.
Even if your squash plants made it through the season, it’s best to remove leftover material to prevent the fungus from overwintering and infecting future crops.
I don’t recommend composting diseased plant matter. It’s possible for powdery mildew and other pathogens to live in the compost and spread to other, healthy plants throughout the area.
Small amounts of plant matter can be thrown away in your household garbage bin. Another option is to burn infected plant material (weather and location permitting). Alternatively, you can bury diseased plant material far away from your garden beds. Do not bury infected plant matter in any location that might be cultivated within the next few years.
Stopping The Spread of White Spots
Because powdery mildew and similar diseases spread so easily, I find it’s best to be on the defense from the get-go. Take a moment to evaluate your current gardening practices and, if necessary, add the following steps to your routine:
- Select disease-resistant squash cultivars for your garden.
- Familiarize yourself with the early signs of powdery mildew.
- Follow best practices for plant spacing, watering, and general care.
- Remove and destroy infected plant matter as soon as possible.
- Sanitize all tools (including gloves and hands) after handling diseased plant material.
- Inspect nursery plants for signs of powdery and downy mildew.
- Rotate crops to reduce the chance of repeat infection in later years.
Can you Eat Squash From Plants With Powdery Mildew?
Squash fruit is generally safe to eat even if the plant itself is suffering from powdery mildew. Personally, I like to wash fruit from infected plants just to prevent the accidental spread of spores to other parts of the garden or even indoor houseplants.
The main issue with powdery mildew in squash plants is that the disease greatly impacts productivity. Your plants’ growth may be stunted and fruit production significantly reduced. If powdery mildew is left untreated, it may kill plants outright.
Note that some people have allergic reactions to powdery mildew and other mold or fungal infections. If you have a history of related allergies, you may want to reconsider eating squash from affected plants.
Verdict: Squash Leaves With White Spots
White spots on squash leaves are often a subtle sign that more serious problems are on the horizon. Treating this symptom as soon as it appears could save your squash patch from a season of disease and lost fruit production.
While fungicidal treatments can reverse mild cases of powdery mildew, prevention is the best strategy. Even if you’ve never encountered powdery mildew in your own garden, I highly suggest adopting some of the practices outlined above for good measure.
