Green beans are easy to grow legumes that, in a perfect world, always have green and lush leaves. But this is far from being a perfect world, and even low-maintenance plants like green beans suffer from diseases and attacks. One of the questions we receive is, why are the leaves on my green beans turning white?
Green beans turning white is mostly a sign that fungi or bacteria have attacked the plant. The three culprits that mostly attack green beans include powdery mildew, Pythium blight, and bean rust. However, the leaves could also turn white because of sun scorch or cold damage.
The rest of this article will go deeper into the causes of green bean plants turning white. I’ll also explain what to do about it.
Powdery Mildew
Powdery mildew is among the most common causes of green bean leaves turning white. It starts as a tiny white spot on a leaf and then spreads to cover the entire leaf with a white powdery substance. This is more common in older green beans than young ones.
Powdery mildew quickly spreads through all the leaves and eventually starts affecting all the plant’s above-ground parts. In time, the leaves turn yellow and the pods turn purple before the entire thing withers and dies.
Solution
Since the condition is encouraged by planting the beans too close together and stifling air circulation around them, you can control it by thinning out the leaves or getting rid of the affected plants. You can also avoid this monster by using resistant bean varieties and ensuring nutritionally balanced soil.
As for treatment, applying neem oil to the affected plants seems to help. But when all else fails, use a biological fungicide, Bacillus subtilis, to kill the fungus if it’s not too late for the plants.
Although powdery mildew doesn’t cause significant damage to the plant, it results in serious yield losses. The trick is to identify the disease early enough, prevent the spread, and kill the fungus.
Pythium Blight
Pythium Blight is another type of fungus that causes green beans to turn white at first. The fungus occurs mostly during hot, humid weather, especially if you often water your beans. The combination of a hot climate and moist soil makes a perfect environment for the production of Pythium Blight.
Pythium Blight can attack the green bean seeds even before they sprout, unlike powdery mildew. It starts with a white, mold-type growth in the soil and progresses up the stem and eventually the leaves and pods.
Solution
Unfortunately, once Pythium Blight has started attacking your plants, there isn’t much you can do to stop it, and your green beans are a lost cause.
However, you can prevent this by planting green beans during cooler months and treating your seeds with a fungicide containing sulfur before planting.
Bean Rust
As you may have suspected, given its name, this ugly fungus turns your leaves into the color of rust. Yes, that metal rust. However, it starts as white spots that slowly grow larger and larger before eventually turning into rust.
Bean rust is more common in older plants, and you’ll notice it on the underside of the bean leaves in late winter or spring. If the color of the leaves doesn’t give it away, you can touch the leaf to confirm. Your finger will come off with a rust color on it, and it also has a slight odor.
As the fungus spreads, it will eventually cover the upper part of the leaves with that rust color and then move to the pods. The leaves will collapse and dry up, and eventually, your entire plant will wither away.
Solution
Thankfully, you can treat bean rust with a fungicide if you catch it early enough.
Although, it’s advisable to practice crop rotation to help prevent it altogether. Don’t plant green beans or similar plants affected by this fungus in that area for a while; Skip two growing seasons and do crop rotation, instead. In the future, use resistant bean varieties and fungicides during the planting phase to prevent bean rust.
White Mold
White mold attacks more than 300 plant species, including green beans. It’s caused by a fungus known as Sclerotinia sclerotiorum, which presents as a prominent white, cotton-like covering on various parts of the plant. This disease occurs during cool and wet temperatures because mold needs moisture to thrive.
Sclerotinia sclerotiorum has soil-borne survival structures called Sclerotia, and they can survive in the soil for 5-8 years. However, if your plant is mature, they don’t affect your plants until the rain starts. The disease happens when sclerotia on the soil touches the bean and forms apothecia – mushroom-like structures that produce millions of ascospores to infect the plant.
As the mold grows into the pods, the beans become inedible. The plant may continue to grow normally, but the beans inside the pods are rotten and poisonous.
For more information on getting rid of white mold in your soil, you can check out this article: How to Completely Get Rid of White Mold in Your Soil
Solution
You can prevent white mold by ensuring the plants get good air circulation and plenty of sun. This means you have to space out the plants and keep weeds under control to allow prompt drying when it rains.
You can also use mulch to keep the bean pods from touching the soil. Also, avoid using overhead irrigation methods and sprinklers once your bean pods have formed.
For a permanent solution, rotate your crops for five or more years and don’t plant anything susceptible to white mold.
Sunburn
Now, if you had initially planted your green bean seedlings in indoor pots and then decided to transfer them later, your plants may just be going through environmental stress.
Green beans are a warm-weather plant, but they also don’t like the scorching sun, especially if they aren’t used to it. Like people, plants that aren’t acclimated to outdoor conditions like sun, wind, and cool temperatures suffer from exposure.
Solution
The only solution to this is to harden the plants before exposing them to harsh elements.
Hardening off plants is where you gradually get them used to a change of environment. This can take 5-7 days, depending on how old the plant is but the longer, the better.
For most plants, here are the steps to follow to harden them:
- For the first two days: Start by putting your plants in a spot outdoors where there’s dappled shade and out of the wind for a few hours, then bring them back inside.
- For the next seven days: Move the plants to a slightly more exposed area and then back inside.
- On the last day: Leave the plants out. By now, they should be able to withstand the conditions unless there’s frost.
That said, green beans mature quickly, so you don’t have to start them indoors. They do much better if you plant them right in the garden from the word go. Transplanting and changing the atmosphere sets them back a few days, which is unnecessary.
Chlorosis
Chlorosis is when the plant isn’t making enough chlorophyll to make the leaves green. This could result from a genetic mutation that causes the amount of chlorophyll to go down.
It could also be that your plants aren’t getting enough sunlight, either because they’re too crowded, planted under a shade, or it’s fall.
If the leaves on your green beans turn white because of chlorosis, it means they’re in distress. Chlorophyll is responsible for photosynthesis, which makes plants change sunlight into carbohydrates. Without it, plants run out of nutrients and slowly deteriorate.
Spider Mites
Last but not least, your green beans could be turning white because of a spider mite infestation. Spider mites are tiny arachnids that spin webs on the underside of your ban leaves and then suck all the nutrients out of them.
They reproduce quickly, making them a serious threat to your green beans.
While green beans can usually tolerate some spider mite damage, you’ll still notice small pale spots where the mites are feeding on, and eventually, the leaves turn silvery or white and then dry up.
As the mites colony grows in numbers, webbing increases, causing a white, thin web on the leaf’s surface. They also continue to attack your plant until the leaves drop and your pods become unyielding.
Solution
Luckily, you can sort this problem with some pesticide if it’s out of control. But before then, spray some neem oil on the surface of the leaves to eradicate those pesky spiders.
You can physically remove them using a high-pressure water spray, but this exposes your plants to other problems such as fungi.
Final Thoughts
As you can see, the list of reasons why your green beans could be turning white is long. Like powdery mildew and chlorosis, some are easy to spot, but others aren’t. You’ll only know what’s going on if you take the plant for some testing.
After diagnosis, take the recommended measures to deal with that problem as early as possible to prevent spread. It’s also very important to do crop rotation every year so the fungi and other issues in the soil can have time to die.
