When you’ve worked hard to keep your summer squash happy throughout the season, watching previously green leaves slowly turn white is understandably distressing. But white leaves on squash are surprisingly common and not as scary as you may think.
If you take a closer look at the leaves, you’ll likely notice the leaves themselves have not actually changed color. Instead, they appear white due to a powdery film on top of the foliage. This is a fungal disease aptly named powdery mildew.
Crops with powdery mildew will have foliage that appear dusted with a white flour. These patches start out small (and possibly out of sight on the undersides of leaves), slowly spreading to the whole leaf and around your garden. Fungal spores are picked up by the wind and spread into soil, carrying the disease further.
The best way to tackle this problem is to prevent it, but there are also management techniques you can use to save your squash crop.
What is Powdery Mildew?
Powdery mildew is a common garden disease affecting a wide range of plants, including cucurbits. In fact, it is one of the more common cucurbit diseases.
There are several fungi responsible for this white sheen on foliage. In cucurbit plants, the two prevalent species are Podosphaera xanthii and Erysiphe cichoracearum. P. xanthii is more widespread today and more aggressive, spreading quickly in warmer temperatures. E. cichoracearum is common in the cooler months of spring.
While most fungal diseases appear in wet and cold weather, these pathogens generally don’t mind heat or dry air. They infect affects squash plants toward the end of summer when fruits are maturing, but can spread at any time when conditions are right.
Spores spread through the air, so this disease doesn’t always start in your garden. It often moves in from neighboring properties. It’s also possible to import the disease from purchased seedlings. You may even battle for successive years with overwintering spores in plant debris.
How Does It Impact Squash?
Once spores spread to your squash, symptoms will develop within a few days to a week. Small white spots appear on the underside of squash leaves and shaded sections of the plant, slowly spreading to cover the whole leaf over time. You’ll likely notice older leaves succumbing first.
The infection may not visibly impact growth at first. However, this fungal layer impacts the amount of light the leaves absorb, therefore inhibiting photosynthesis. Growth will slow over time, impacting yield. Severely impacted leaves may wilt and die off, leaving fruits exposed to possible sunburn.
Limited energy also impacts the quality of fruits. If your squash is covered in this white fungal coating, you may see reduced yield or smaller fruits overall. Your squash may also ripen incompletely, impacting your harvest.
This disease does not instantly kill squash plants or cause as much damage as some of the more serious fungal diseases. However, it does negatively impact growth and needs to be controlled early on to prevent spread around your garden.
How to Treat It
Once you’ve identified that powdery mildew is the cause for white squash leaves, it’s essential to act quickly. This problem spreads rapidly to other susceptible crops in the garden. Although it’s not highly dangerous, fast action will stop the problem from becoming a bigger headache later on.
If you have a minor problem, the easiest solution is to remove the affected foliage. This won’t completely remove spores on foliage that haven’t developed symptoms yet, but it will stop the issue from spreading.
Never add diseased foliage to your compost or around your garden. Discard any leaves you remove to stop them from causing further damage. It’s also best to wash your hands and clean your garden tools before working with any other plants. You don’t want to spend time tackling this disease only to realize you are the one spreading it.
If your squash is completely covered, there are a few sprays you can use to target it. However, these are typically used as preventative rather than treatment measures and may not get rid of it completely. Neem oil is a common choice, along with any fungicide containing Bacillus subtilis.
How to Prevent White Squash Leaves
Like many diseases, it’s far better to prevent this issue than struggle to treat it. Follow these steps at the start of the season to reduce your risk.
Plant Resistant Varieties
The simplest prevention method is to plant mildew-resistant squash varieties. This doesn’t guarantee you won’t encounter any problems, but does make infection far less likely.
There are many resistant cultivars to choose from, like ‘Emerald Delight’, which is also resistant to yellow mosaic virus. Also, look out for ‘Delta’ and ‘Smooth Operator,’ a few of several cultivars unlikely to encounter disease problems.
Avoid Overcrowding
Fungal diseases spread best in areas with limited airflow. To prevent infection, provide plenty of air circulation around your squash plants.
This starts at planting time. Ensure you space your plants correctly based on their mature size, giving them room to fill out later. Planting too close together will reduce air circulation as the vines develop, increasing your chances of powdery mildew.
If you notice excessive growth and early signs of mildew, prune the vines to increase airflow. You can also remove infected plants completely if you have several planted together, sacrificing one for the benefit of the rest.
Water Regularly at Soil Level
Stressed plants are far more susceptible to pest and disease problems. Lack of water can lead to drought stress which invites problems like powdery mildew. The type of watering is important too: use soil-level irrigation to keep the leaves of your plants dry. Soaker hoses and drip lines are the easiest irrigation methods.
Maintain a consistent watering routine, keeping the soil lightly moist but never soggy. Adjust your watering as temperatures increase to stop the soil from drying out too quickly. Add a layer of mulch where possible to retain moisture and watch for signs of stress. This also prevents splashing of spores onto the plant.
If you struggle to water at the right time, it may be better to install an irrigation system. This also keeps water off the foliage and avoids other growing problems caused by lack of water.
Avoid Overfertilizing
When you encounter growing problems in your squash, fertilizer is not always the answer. Adding too much can actually do more harm than good, stressing the plant and making it more susceptible to powdery mildew. If you’re not sure how to fertilize, it’s better to feed too little than too much. Conduct a soil test to determine whether your soil is deficient in any nutrients before looking to fertilizers to solve your problems.
