We’ve probably all been standing in our garden at one point or another, thinking to ourselves, “What’s this white powder on my tomato leaves?”.
While it may seem like a harmless eyesore, powdery mildew affects many different crops and can spread quickly, wreaking havoc across your garden plots.
Find out once and for all what causes powdery mildew to appear on tomato plants, and what you can do to prevent it.
What is Powdery Mildew?
Powdery mildew is a widespread and damaging fungal disease caused by Oidium neolycopersici, an obligate parasite that survives as mycelium, living on the nutrients of a host plant. Basically, this fungus lives on or inside the plant matter of a host plant, exploiting the living cells by stealing nutrients and dying when the plant dies.
The fungus travels through wind or rain, lands on a plant host, and quickly germinates asexually by producing conidia on the plant surface. Each time these spores (conidiophores) are dislodged from the plant and travel to a new one, a new infection cycle begins. So not only does the conidia play a role in reproduction, but it is also part of the pathogen that physically travels to a new host plant and continues to spread
Oidium neolycopersici can affect a broad range of host plants and unfortunately, its origin is not completely understood nor is the reason for its extreme worldwide spread that occurred many years ago.
Conidium: a type of asexual reproductive spore of fungi (kingdom Fungi) usually produced at the tip or side of hyphae (filaments that make up the body of a typical fungus) or on special spore-producing structures called conidiophores. The spores detach when they mature. The plural form of conidium is conidia.
Mycelium is the white, hair-like root system of fungi you might see across your soil surface and when you dig beneath your plants. They are responsible for the decomposition of matter, serve as food for many soil creatures, and their growth helps aid in carbon dioxide released into the atmosphere. The plural of mycelium is mycelia.
Crops Vulnerable to Powdery Mildew
Powdery mildew is most often found on cucurbits (cucumbers and squash), nightshades (tomatoes, peppers, and eggplant), and legumes (beans and peas).
It is believed to affect over 60 species in 13 plant families. Unfortunately, since the spores of powdery mildew can travel quickly from one plant to the next and can jump around different crops, they can destroy a plot pretty quickly if conditions are ideal.
Many commercial growers expect powdery mildew to affect certain crops so they tend to counterbalance this by planning successions throughout the season of crops such as basil, cucumbers, and summer squash.
This allows them to remove infected plants, start fresh in a different area of their farm and hopefully break that cycle of spread.
Ideal Powdery Mildew Conditions
While many fungal diseases thrive in wet, humid conditions, powdery mildew prefers warm and dry. Ideal temperatures are between 60°-80° with high humidity around the plants, and it does best on cloudy days.
It slows its spread in rainy periods and extreme heat (above 90°) and does not affect plants in direct sun as much as those in the shade. For these reasons, spring and fall are the best times for powdery mildew to show up.
Pro tip: Find a spot in your garden that offers lots of sun for plant species highly susceptible to powdery mildew.
Because of the spores’ wind-bourne traveling nature, water is not required for powdery mildew to spread, germinate or infect, making it more worrisome than some other fungal diseases.
Identification & Symptoms
Did someone come through your garden and sprinkle your plants with flour? Probably not! It’s more likely that you are seeing early signs of powdery mildew, as that’s how it’s often described. It starts out looking like little dusty circles or spots on leaves, hence the name powdery mildew. Fruit is rarely affected.
Light yellow and green circles will appear, and as they grow, they may turn purple with necrotic centers. Leaves will typically only show white powder on the tops of leaves; symptoms are much less likely on the leaf underside. They will sometimes yellow and dry up, and if the disease is severe enough, they will twist and may even fall off the plant.
Your plants are most susceptible when they are young and vulnerable, but breakouts can occur at any life stage.
If you want to wipe powdery mildew out completely, consider destroying infected plants so powdery mildew does not spread throughout your whole garden. Since this pathogen can travel by wind and rain, as well as from plant to plant, adding the plant to a burn pile or trash is best. Otherwise, the spores may make their way back over to your garden.
To fully and properly kill the late blight pathogen, pull out infected plants on a hot and sunny day, lay them out in the sun, bag them, or cover them with a tarp so they cannot travel and spread. Use a preventative measure on the remaining plants to reduce spreading risks.
Pro tip: If the breakout is not severe, you may be able to simply remove the affected plant parts. Many growers have had success with DIY fungicidal preventative sprays. More on that a bit later.
Preventing Powdery Mildew in Tomatoes
Fungal diseases spread quickly and affect many different crop families, making them very difficult to get rid of once they are present. Prevention is key!
Prune For Adequate Airflow
You must follow proper pruning practices to get good airflow, as overcrowding of plants and dense foliage makes it very easy for diseases of all kinds to spread.
Be sure to provide your plants with adequate spacing. Tomatoes should have no less than 12 inches between one another. Certain varieties might require up to 36 inches on either side. Ideally, leaves should not be touching those of another plant nearby.
As tomato plants grow, you should prune off any old and unnecessary leaves toward the bottom, as this opens up a lot of breathing space for the plant. Many growers remove any shoots that come off the main stem on the bottom foot of the plant.
Be sure to do a weekly “suckering” of the plants. Tomato suckers are those pesky growths that form between a leaf and the main stem. They would become full-size stems and set fruit if you let them, but this just crowds your garden and creates competition for resources.
Remember, a plant only needs about ⅔ of its foliage to remain healthy and viable, so you can remove quite a bit when pruning.
Pro tip: Be sure to rid your garden of any infected leaves and suckers, as they may be harboring fungal spores. Take them straight to the burn pile!
Keep Gardens Clean and Tools Sanitized
Any time you are in your garden to prune, be sure to practice proper sanitizing of your tools afterward to avoid any potential disease spread.
You might even consider wiping shears down after pruning each plant, as the disease will transfer very easily at a wound site. Simply grab a Clorox® wipe or spray the shears down with a 70% isopropyl alcohol solution and wipe clean.
Making sanitization part of your pruning routine helps you get used to doing it each time. This way, you know that each time you grab your shears, they will be clean and safe!
Grow Powdery Mildew Resistant Cultivars
You can select tomato cultivars that are more resistant or tolerant of powdery mildew than others. On many seed company websites, you can filter for specific disease resistance (look for the code PM if you don’t see a powdery mildew option). Below are a few examples of these cultivars bred for powdery mildew resistance.
*Good option for greenhouse growers
Remember that you may still experience powdery mildew even when growing a so-called resistant cultivar because the resistance bred into these cultivars makes the plants more resilient, but will not 100% prevent the pathogen.
Practice Crop Rotation
Typically powdery mildew only lives and thrives on living plants, but sometimes it can survive in overwintering debris or plant matter. If you live in a warmer region where frost does not occur, you may need to be concerned about the fungus living in your soil. Otherwise, it is not likely that the fungus will reappear in the spring from the soil.
That being said, it is always a good idea to practice crop rotation so pests and diseases have a more difficult time finding crops they tend to infect when it’s time to wake up in the spring.
Your crops will most likely become infected with powdery mildew through wind or rain transport or other infected plants, possibly brought in from an outside source.
Growing in a Greenhouse or Tunnel
Due to the nature of growing in a greenhouse, fungal diseases such as powdery mildew are easily spread and very common. Airflow can be improved by proper pruning techniques and by keeping fans lightly blowing at all times. Remember, spores of these fungi travel easily in water and wind, so the fans may also speed up the spread of any disease already present.
Just follow all the techniques suggested for creating good airflow, keep your tunnel tidy, and scout early for disease, and you should have a successful season.
Treatments & Organic Sprays
Prevention before symptoms is key with most fungal diseases, including powdery mildew. Do your research and have a plan in place before the season begins.
Copper-based treatments, neem oil, and bicarbonates are organic prevention treatment options. Sulfur dust has also been shown to kill powdery mildew spores that have yet to infect plants and has been widely used in vineyards since the 1850s. It is an inexpensive option and has a low risk of resistance. However, it may have negative effects on gardening friends, such as beneficial pests and fungi, so use it with caution.
One thing to note is that it’s important not to mix and match treatments without providing a gap between them. Blending neem oil and sulfur at the same time can increase the plant’s risk of sunscalding; similarly, blending oil with copper will do the same thing. Leave a two-week window between different treatment types to allow the prior method to break down in the sunlight.
And as with any treatment plans, be sure to read all labels, follow instructions and keep out of the reach of kids and animals.
Homemade DIY Treatments
It is advised to test the following treatments on a single test plant as treatment results can vary and depend upon external factors such as sunlight, temperature, and stage of disease.
Mix 1 part milk to 2 parts water and spray on plants weekly. While this method is still being studied to gauge why it’s so effective and to determine the ideal ratios to use it in, cow’s milk is a reliable DIY powdery mildew treatment. Full-fat milk is more effective than skim milk or whey, but all forms are effective.
A test from the University of Minnesota mixed 1 part mouthwash and 3 parts water and sprayed leaves had good success rates. New foliage can be damaged, so be careful with younger, less established plants.
Mix 3 tablespoons of sodium bicarbonate (baking soda) in 1 gallon of water. Lightly mist your plants with this solution. But use a light hand for this treatment; you should not overuse baking soda as it can accumulate in the soil. For this method, a little goes a long way!
Powdery mildew can be a devastating fungal disease in backyard and commercial gardens, but it is preventable and treatable in the early stages. Just know what to look out for and scout your plants often.
If you haven’t already, make friends with your local Extension Office agent, as they are happy to help and a great resource to local gardeners and farmers!