Powdery Mildew: Why Your Plants Have White Powder or Spots

Gardening is a never-ending source of education. Most gardeners with decades of experience will tell you that they learn something new each and every season. When it comes to managing things like pests and disease, experience goes a long way. If you don’t haven’t experienced powdery mildew in the past, you may be wondering why you have a white powder, or white spots on your plants this season.

This common fungal disease commonly impacts many different types of garden plants. It can also spread from plant to plant if not properly remedied and eradicated from your garden.

So if you’ve been wondering about the white spots or powder on your plants this season, this article is for you. We’ll discuss what powdery mildew is and the top reasons why your plant may have succumbed to it this season.

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.

Hundreds of species of trees, shrubs, vines, flowers, fruits, vegetables, grasses, and field crops can be affected by different strains of powdery mildew.

Visible Signs

Visible symptoms usually appear later in the growing season when soil moisture is low, but air humidity is high. This disease loves temperature fluctuations between hot and dry to moist and humid.

This disease looks just like it sounds – a whitish dust that settles on the leaves of a plant. It almost looks like your 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 disease prefers young plants and newer growth. You won’t often find it on very mature leaves.

How it Spreads

One of the reasons it’s so difficult to prevent this disease is that fungal spores overwinter in soil and leaf debris.

Once the weather warms up, wind, water, and insects transfer spores all around to nearby plants. It can be spread via airborne particles or through water splashback directly from the soil.

Impact to Your Plants

In your garden, zucchini, beans, cucumbers, squash, pumpkins, gourds, tomatoes, peppers, roses, and zinnia are especially susceptible. While the fungus isn’t likely to kill your plants, it can significantly reduce yield and even change the taste of fruit and vegetables.

Once you spot powdery mildew on your plants, take quick action. The faster you treat it and remove the affected material, the better chance you have of eradicating the problem before it spreads further.

7 Reasons Powdery Mildew Occurs

There are a number of different reasons that this disease can start to impact your plants. With a little garden planning, and careful monitoring of the care you provide, you can minimize the chances that this common fungus infects your plant this season. Let’s look at the most common reasons that this disease can take over your plants.

Your Plants Are Too Close Together

Plant spacing is incredibly important to a garden’s success. Whether you’re growing flowers, vegetables, fruit trees, or herbs, every plant needs a certain amount of space to achieve its full size and natural form. To grow and produce optimally, plants need a competition-free area to access the water and nutrients they need.

Some plants need a LOT of space to spread out, while others can be planted fairly close together. In a vegetable garden, spacing your plants too far apart can cause issues with weeds and dried out soil.

But placing them too closely together can cause major issues with air circulation. And in the context of this article, that is a large contributing factor to this disease.

Think for a moment about a sprawling cucumber vine planted near a large tomato plant. Both of these plants have heavy, dense foliage. If they are planted too close together, you’ve created conditions ripe for fungal diseases, including this one.

The foliage is so dense and close together, air circulation is minimal to none. This causes leaves to always be in a state of ‘dampness’, which is exactly the condition fungal spores thrive in.

Lack of Pruning

Every plant in your garden will have different needs. Pruning is no different.

Pruning can stress out your plants, but for many, it’s essential for keeping them healthy and growing efficiently. Just remember that excessive pruning is a really bad idea. It can cause a number of problems including yellowing or curling leaves, stunted growth, and a decrease in yield.

Since we’re not talking about one plant in particular in this article, our advice will be a bit on the general side. At a minimum, be sure to prune any leaves or branches that are touching the soil line.

This will help prevent fungi from having such easy access to the leaves of your plant. Beyond that, follow the pruning guidance for the particular plants you have chosen for your garden this year.

If you do see instances of this disease on the leaves or stems of your plants, be sure to prune them immediately and dispose of them in the garbage (not your compost).

Not Enough Direct Sunlight

Just like with space, every plant in your garden requires a set amount of direct sunlight each day to thrive. Many summer vegetables and flowers require full sun, meaning at least 6 hours of direct sun each day. Ideally, sun-loving veggies prefer 8-10 hours of sun for maximum fruit production.

There are some herbs, flowers, and vegetables that grow fine in partial shade conditions as well. This means they need to receive between 3-6 hours of direct sun to grow.

So what happens if you plant a sun-loving veggie in a space that only receives 5 hours of direct sun? The most important thing is that your plant won’t grow well and any fruit/veggie production will be seriously limited.

But it also creates conditions that make a plant more susceptible to fungal disease. Light and air are pretty important to the foliar health of your plants. If you significantly reduce the amount of light your plant receives, it won’t be as healthy and able to fight off disease.

Now that you know how important the right amount of light is, take the time to map out your garden. Understand which areas receive full sun, which are shaded during certain parts of the day, and which are always shady.

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.

Poor Watering Habits

Poor watering habits are probably the most common cause of this fungal disease after poor air circulation. 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.

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

Water Slowly and Deeply

Most plants don’t appreciate a deluge of water every few days. Think about the difference between taking a nice sip of water from a glass vs someone opening a fire hydrant for you. In an ideal situation, you can set up a drip system before planting.

This will deliver water to your plants in a consistent and manageable way. If you don’t have a drip installed, don’t worry. You can use a soaker hose or turn your regular hose on the lowest setting and let it slowly stream into the soil.

Watering this way will not only allow your plants to take up more water, it will prevent the kind of splash back 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 plants, 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!

Lack of Mulch

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 splash 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 you may have laying around in your yard. Straw, wood chips, crushed leaves, and even crushed eggshells can offer the soil that extra layer of protection.

Your local nursery or garden center will also have plenty of mulch options available. Personally, I love using coconut coir in my garden because its sustainably produced (it’s a waste product of coconuts), renewable, and a great soil amendment as it decomposes.

No matter what type of mulch you choose, aim to apply a layer 2-3” thick for maximum benefit.

Using mulch along with proper watering habits will go a long way in protecting your plants from powdery mildew and other soil-borne diseases. This is especially true with heavy rain that would definitely splash soil onto the plant if mulch wasn’t present.

You Composted Diseased Material

I love composting and firmly believe that every gardener would benefit from some form of the practice. In our garden, we practice vermiculture, which is worm composting. But we have also done more traditional compost piles in the past. Either way, the material you produce from kitchen and yard waste is pure gardening gold.

You can negate all that good, however, if you compost material that has been infected with disease. Although the organic material will break down in the compost, the spores will remain unless you have a perfectly tuned composting system in place. Once you put that compost back into your garden, you’re starting the cycle all over again.

So what does that perfectly tuned system look like you ask? It all comes down to heat and the proper ratio of browns to greens.

To kill fungal spores, your compost pile needs to maintain an internal temperature between 140 – 180 degrees. It needs to be regularly aerated when temperatures reach around 165 degrees to allow oxygen in and allow the organic matter to be mixed up. This balance must be maintained for at least 10 days to kill fungal spores.

Your compost pile also needs the proper ratio of browns (carbon rich materials like paper, leaves, straw, cardboard, etc) and greens (nitrogen rich materials like kitchen scraps, coffee grounds, grass clippings, etc). A general ratio is about 25 parts brown to 1 part green.

If you’re not actively managing your compost heap or you’re not sure you can keep things in that particular balance, we highly recommend NOT composting your diseased material. I’m a pretty risk averse person and I’m not putting all that beautiful compost at risk for a few affected branches!

Over Fertilizing

There is a tricky balance between providing your plants with the nutrients they need to thrive and overdoing it. Add to that equation that every plant in your garden has slightly different needs at different phases of their growth cycle.

Powdery mildew tends to attack new growth over mature foliage. So if your plant grows too quickly because of excessive fertilizer, it may be more susceptible to the disease.

The best way to avoid this is to only fertilize when your plant needs it for optimal growth and fruit production.