What Causes Broccoli Leaves to Turn White?

You plant your broccoli seeds, you water and nurture your plants waiting for the day of harvest. After all that work and time you notice a problem, your broccoli leaves are turning white. It’s discouraging but there is hope for your plants.

Broccoli Powdery Mildew

What causes broccoli leaves to turn white? It’s a bit subtle but you’ll notice small white patches that look like dusted flour on the bottom and top of your broccoli plants. This is a disease called powdery mildew caused by a fungus.

The article below describes what the fungus is and how you can manage it with your broccoli plants. 

What Causes Powdery Mildew?

The fungus Erysiphe cruciferarum is what produces the white substance. The fungus is a pathogen that attacks all types of crucifers beyond broccoli. It can show up on cabbage, Brussels sprouts, and cabbage as well as broccoli.

It typically appears on late-maturing plants where the broccoli leaf becomes “frosted.” The lower bases of the larger stems, as well as the exposed head, have fine flecking. Many researchers use gene resistance breeding to eliminate it in crops. 

The fungus podosphaera xanthii produces another type of powdery mildew that infects things like squash, cucumbers, muskmelons, gourds, watermelons, and pumpkins. 

Both types of powdery mildew grow most in warm temperatures of between 68 to 81 degrees Fahrenheit that also have humidity. This environment is what produces spores that easily spread the disease.

What Happens If It Progresses?

The fungus will eventually cover all the leaves and some will start to turn yellow. This prevents the plant from thriving and the heads formed will be much smaller than normal. There may be white powder on the heads too.

This fungal disease doesn’t kill the plant but weakens them to the point that it keeps them from growing or producing much for food. 

What Do I Do?

The first thing to do when you spot the white leaves removes the infected leaves. You will spray the rest of the plant with any number of options to get rid of the disease.

Foliar fungicide helps even when the plant is suffering severely from the disease. One I like to use is Bonide Copper Fungicide as it’s approved and suitable for organic gardening.

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Other options are insecticidal soap, and neem oil. Most people start with the oil or soap as they are less stringent than other options. It’s also best to rotate treatments. 

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Natural Remedies

You can mix a natural remedy to spray on plants with powdery mildew. The recipe is 1 tablespoon of baking soda, 1/2 teaspoon of non-detergent, liquid soap, and a gallon of water. Put the concoction in a sprayer and coat the plant evenly. Be sure to spray on the underside of the leaves and stems. 

The baking soda, which has potassium bicarbonate, is what kills the fungus with the soap helping it cling to the leaf’s surface. 

Be careful because this mix can burn the plant’s leaves if you overdo it. You should water the plants with a good dousing a few days before spraying them and do not spray them in full sun. 

It’s also good to spray one area of the plant first to see how it responds. Discard leftover mix and it no longer works over time.

A new treatment that remains in a testing phase is a milk spray. Research is continuing regarding this as an antifungal measure and it appears to do well in helping squash and cucumbers, although scientists are still trying to figure out why it works. 

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Milk protein, located in the milk fat, seem to produce an antiseptic when its in the sun. Unlike the other fungicides and natural remedies, you should apply the milk spray to plants in bright sunlight. 

All you need for this remedy is milk, water and a sprayer. The mix ratio is 2:3 milk to water. Lightly spray the plants. 

Most believe the milk spray is a better preventive measure than a disease-killing remedy but one of the best advantages of using it is that is is safe for plants. Other remedies can be harsh on plants. 

Preventing Powdery Mildew

The best way to deal with powdery mildew is to prevent it. This involves many types of things from picking good plants to placement in a garden. Understanding how to properly garden will help prevent this disease. 

Pick Good Plants

Healthy plants are the way to go to avoid these types of problems. You might as well start with good plants so you’re not spending all your time trying to save plants susceptible to disease. 

Plants can be grown to be powdery mildew-resistant. Much research has gone into creating plants that don’t easily contract this disease. You will need to ask before you buy your plants but this is especially important if you live in an area that is known to have this disease. 

Plant Correctly 

Don’t plant broccoli or other susceptible plants like cauliflower in the shade. Shaded areas remain damp and that gives the fungal spores a great place to grow so sunshine is best for these plants. 

Long-term planning helps prevent the problem. Garden experts tell home growers to rotate crops where you don’t plant broccoli or other crucifers in the same area more than once every three years.

Keep Up with Maintenance

Plants that are properly fed and well-watered won’t be as likely to get this disease. One key thing to remember is not to give broccoli plants too much nitrogen as that prompts infections of powdery mildew.

Is it safe to eat broccoli that has had powdery mildew?

One of the problems with powdery mildew is that it can get on the vegetable heads. Some may wonder if this produce is still alright to eat. 

Garden experts state it’s not safe to eat food with powdery mildew. This causes some people to have allergic reactions. Most advise to talk to your doctor if you ate vegetables with powdery mildew.

Powdery mildew reduces the flavor in vegetables and makes them taste a little “off.” 

Can you just wipe the powdery substance off?

You can wipe the powdery mildew off the leaves but that won’t resolve your problem over time. The fuzzy patches have airborne spores and they can still move to other plants through the air while you are wiping them off. 

The only way to get rid of them on permanent basis is to use a fungicide.

Does powdery mildew stay in the soil?

The fungal spores live in soil, mulch, compost or plant debris. A problem is they can survive in these areas over winter. They can then continue to spread to plants.

How does powdery mildew spread?

Most of the time they spread by the air and easily move from plant to plant on the wind. They can also spread by water splashes and insects.

How can I prevent powdery mildew from coming back next year?

Those who cleanup their garden area well at the end of the growing season can prevent powdery mildew from returning. Cleanup means collected all diseased leaves, cutting affected stems and getting rid of all plant debris. 

This is important because this fungus can live in the soil, stems, dormant leaves, flower buds and even leaves left on the ground over the winter. Cleaning up all of it and disposing of it will go a long way to preventing powdery mildew next year.

Does overwatering cause powdery mildew?

No, overwatering doesn’t cause powdery mildew. Powdery mildew is caused by fungal spores. However, overwatering can create an environment where these spores grow well and quickly. High humidity is the primary factor in growth of powdery mildew so keeping an area dry and sunny will help. 

The best time to water plants is first thing in the morning because that gives enough time and sunshine in the day for water to evaporate before nightfall. Water left after nightfall creates a dampness and humidity that promote spore growth.

How long does it take to get rid of powdery mildew?

Those who remove damaged leaves and use fungicide faithfully according to the directions will see improvement in about three to four weeks. You should see the powdery substance disappear and your plants should start thriving by then. 

Can I reuse soil after it has a powdery mildew infection?

Reuse the soil once you clear all the infected debris from it. Some experts say you should take the extra step and dispose of the top two inches of soil too and take all of that to the landfill. 

Wash pots that had infected plants in them before using them again. Potting soil from the pots can be used in other parts of the garden once you remove all the debris. 


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    Author, blogger, podcaster, homesteading and permaculture enthusiast. I have a passion for sharing what I learn and helping others on their journey. If you're looking for me, you'll usually find me in the garden.

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