Broccoli (Brassica oleracea var. italica) is a popular vegetable to grow in home gardens. Broccoli is vulnerable to fungal diseases that cause white spots on the leaves, stem, or head. You may be wondering how to identify white spots on broccoli.
White spots are generally treatable if you know what disease you are dealing with. This guide will walk you through identifying the problem your broccoli has and treating it.
Why Does My Broccoli Have White Spots?
Your broccoli has white spots because it has a fungal disease. There are four fungal diseases that cause white spots on broccoli.: powdery mildew, sclerotinia stem rot, white rust, and downey mildew. They can be treated with sulfur or copper fungicides.
Fungal Diseases Causing White Spots on Broccoli
There are four common fungal diseases that cause white spots on broccoli. Match the location and appearance of the white spots to one of them to know how to treat the problem.
Powdery mildew (Erysiphe cruciferarum)
Small white patches on the top and bottom of the leaf grow together until they completely cover the leaf in a thick white layer. If not treated, the leaf will drop off. If enough leaves drop off, the broccoli plant can die. Powdery mildew often occurs when it is dry, with low humidity and moderate temperatures.
Sclerotinia stem rot (also called white mold, Sclerotinia sclerotiorum)
This causes irregular white-gray spots on the stem and irregular grey spots on the leaves. White mold appears when it is cool and the humidity is high.
White rust (Albugo candida)
Ugly white pustules form on the leaf, stem, and flowers of the broccoli plant. The pustules grow together until they cover large areas. The leaves may thicken and curl, too. The spores of this fungus can survive a long time and are spread by the wind.
Downy mildew (Hyaloperonospora parasitic)
Small lesions form on the leaves. These lesions grow together and become orange and yellow. White fluffy spots occur on the underside of the leaves. The leaves wither and die. It is common in cool, moist conditions.
Treating White Spots on Broccoli
Identifying the disease that is causing white spots on broccoli gives you an idea of the treatment you need to use. All of these problems are caused by fungus. The white spots can be treated with a fungicide labeled for use on vegetables.
The label has directions for using the fungicide to treat the white spots. Here are some suggestions. Not all products will be available in all places, so consult your county Extension agent for more information.
Sulfur can be used as a fungicide. It is available in a powder or a liquid. Sulfur treats powdery mildew. It is allowed in organic gardens.
Copper is another good fungicide and is allowed in organic gardening. It treats powdery mildew and downy mildew. It can treat white rust and sclerotinia stem rot as well.
How to Prevent White Spots on Broccoli?
Preventing white spots on broccoli is much easier than treating them. Prevention comes down to how you choose what broccoli to grow and how you take care of it.
Plant Disease Free Seed
Be careful when you buy seeds. Buy from reputable dealers and make sure the seed is guaranteed to be disease-free. If you buy seeds and the seed looks damaged, like it was in the water for a long time, or has mildew or mold on it, do not use the seed.
Plant Resistant Varieties
Older types of broccoli may not be resistant to the diseases that cause white spots. If you have been having problems with your broccoli developing these diseases, plant a newer hybrid variety that has been bred to be resistant to the diseases in your area.
Rotate the crops in your garden each season. This means if you plant cabbage, Brussels sprouts, cauliflower, or any other Cole crop in space, avoid planting broccoli there for at least a year.
Space Broccoli Plants Properly
It is tempting to plant your broccoli as close together as possible in order to grow as much to eat as you can. However, planting the broccoli closer than 18 inches will cause problems.
The air cannot circulate if the plants are too close together. Air circulation helps dry wet parts of the plant when it rains or when there is dew on the plant. The longer the plant is wet, the more likely it is a disease will cause white spots on your broccoli.
In addition, crowded plants are under stress. Stress causes changes in the plants that make them more vulnerable to diseases and pests. Proper spacing causes less stress.
One of the most important ways to reduce white spots on your broccoli is to water it properly. Water your broccoli with a soaker hose or drip irrigation. Not only does this reduce the amount of water you use, but it also keeps the broccoli dry. The diseases that cause white spots need wet leaves to grow, except for powdery mildew.
If you use a sprinkler to water your broccoli, water between 5 a.m. and 10 a.m. This gives the plant time to dry before nightfall. At night the temperatures fall, making it easier for diseases to grow.
Putting a three-inch layer of mulch around your broccoli plants has several benefits. The most important is preventing soil from splashing on the broccoli plant. The fungus that causes white spots on broccoli often lives in the soil. When the soil splashes on the broccoli, the fungus gets on the broccoli too.
Mulch lowers plant stress by keeping the soil warmer in winter and cooler in summer. It also retains water so the plant roots have it available longer. Weeds have a hard time growing through the mulch, too.
Be Careful with Fertilizer
Too much nitrogen fertilizer can predispose your broccoli to powdery mildew. Use a soil test to find out exactly what type of fertilizer you need for your broccoli to grow best.
In the absence of a soil test, use ½ cup per ten feet of row of a 21-0-0 fertilizer when you plant the broccoli. Apply an additional ¼ cup per ten feet of row when the broccoli head is the size of a quarter. When you harvest the head of the broccoli, apply another ¼ cup of fertilizer per ten feet of row to encourage side shoots.
After the broccoli stop producing, it is important to remove the old plants and any plant debris on the ground. Diseases can build up in the debris and infect your next crop.
Keep Weeds Down
Not only do weeds take precious water, nutrients, and sunlight from your broccoli, they retard air circulation. As mentioned previously, poor air circulation can encourage diseases to occur. Weeds also stress the plant, making it more vulnerable to disease.
Clean Tools and Equipment Regularly
Just like you wouldn’t eat with a dirty fork, you should not use dirty tools or equipment on your broccoli. Dirty tools spread disease.
Putting tools that touch your plant in a solution of one part bleach and nine parts water for thirty minutes will sterilize them. Rinse the tools before you put them away. Doing this after you finish gardening for the day will sterilize the tools. That way, they will be clean next time you use them.
Eating Broccoli with White Spots
Most of the problems with white spots happen well before the broccoli develops a head big enough to eat. However, if there are white spots on the broccoli head, it is not safe to eat. Not only will it taste bad it may make you sick.
In conclusion, several fungal diseases cause white spots on broccoli. The diseases can be treated with organic fungicides containing sulfur or copper. If you find white spots on your broccoli head, do not eat the head.