Bean leaves will turn brown if they have developed fungus or disease including rust fugus. Brown leaves can also develop if beans are overwatered, if they are attacked by pests or if they are damaged. Check the underside of the bean leaves to see if there are sap sucking insects present like aphids, mites and thrips and treat them with neem oil.
This article will explore the top reasons why bean leaves turn brown and what you can do to solve each one.
Brown spots on bean leaves
Brown spots on the underside or top of the bean leaf can be a sign of a fungus or disease problem. These spots can start off brown, turning yellow, black or even white in the center.
Treat fungal disease with copper based fungicides if the problem is spreading. You can remove individual bean plants affected by the fugus or remove the affected leaves.
Fungal root rot
Root rot caused by fungus growth on the roots of the bean plant can cause the leaves to turn brown. Overwatering can encourage this fungus to grow on the roots which will stop them from absorbing the water, nutrients and oxygen the plant needs.
To address this problem surround the bean plants with straw mulch to absorb excess water, reducing your watering schedule and make sure the beans are getting plenty of light. Move potted bean plants into a position that gets more light and plant seedlings out into your garden beds.
Anthracnose fungal disease
This fungal disease can affect the stems, leaves and beans. They look like dark brown circles which have sunk in on the leaves. It spreads very quickly especially when there is a lot of rain or if you are watering on the leaves.
It is best to remove the whole bean plant that is affected by this problem. Copper fungicides can be used but it is best to do this sparingly. They can affect the growth of good soil bacteria and worms in your soil.
Brown spot (bacterial disease)
Bacterial brown spot can affect bean plant and shows up as brown spots which are about 3-8mm with a yellow circle on the edge of the spot. The spots will gradually get bigger, the entire leaf can turn brown and will start to tear.
These spots can affect the beans as well, causing them to turn brown and bend over. Prevention is the best way to keep bacteria brown spot out of your bean crops. Choose certified, disease free seeds, plant them in a fresh spot in your yard each year and water at ground level.
Sap sucking insects: Mites, aphids and thrips
Pests can affect bean plants and although they are less common than fungal diseases, they can cause the leaves to turn yellow and then brown over time.
Sap sucking insects such as spider mite, aphids and thrips will all cause damage to bean leaves. These bugs usually hide on the underside of the leaf, sucking the sap from the leaf veins or stems.
Sap sucking insects can be addressed with neem oil. This is an oil extracted from the neem plant that only affects these insects. Dilute the neem oil in water as per the instructions and spray it on the affected area. It usually only takes 1-2 applications to solve the problem.
Rust fungus
Rust fungus can affect bean plants if there is excess water on their leaves. Rust fungus will leaves brown or yellow spots and does look like rust like the name suggests. I like to snip off any affected leaves as soon as I see them to help to prevent the disease spreading.
Water at ground level and spread bean plants so they have at least 5 inches of space between them. This will allow more air movement and help to prevent this fungus.
Brown edges of bean leaves
Brown edges on bean leaves usually have environmental causes. Here are the top causes of brown leaf edges on beans.
1. Lack of water
Beans that are not getting enough water will start to show symptoms of this through their leaves. They will first start to droop and if the problem continues their leaves will turn yellow or brown on the edges. Beans like regular water when they are establishing. Once they have grown and established in the soil, you can reduce watering to every few days or when the soil is dry 2 inches below the surface.
2. Frost damage
Bean leaves that have been damaged by frost will start to turn brown on the edges. If the frost is heavy in a temperature range of 30-33 degrees Fahrenheit the whole plant can die. Mild frosts will cause the plant to develop brown leaves.
Once the leaf has turned brown it will not recover but the bean plant itself can survive. Once the chance of frosts has passed, then pinch off damaged leaves and the bean plant will quickly recover.
3. Overwatering
Overwatering can cause brown leaf edges on bean plants. Too much water can rot the bean seed, the roots and stop the plant from getting the nutrients it needs. Plant beans in moist soil but don’t overwater while the bean in sprouting.
How to prevent fungal disease and brown spots on bean leaves
Here are my top tips for avoiding fungal disease in your bean plants.
1. Buy good quality seeds
Choose good quality bean seeds whenever you are planting at home. Poor quality bean seeds can carry disease from the year before. Never save seed from the plants affected by disease or fungus because it could spread to the next year’s crop.
For the best seed, look out for heirloom seeds. They are the old varieties of beans that can be grown and saved for the next year. They are usually hardy, disease resistance and come in interesting shapes and colors.
2. Rotate bean crops year to year
Rotating your bean crops is a great way to prevent disease spreading to your crops. Working with a 4 bed rotation system means that there will be 4 years between planting. Plant your beans in the area that you just grew brassicas like broccoli and cauliflower to help to replace the nitrogen.
You can follow the beans with heavier feeding greens like lettuce, cucumber or even tomatoes.
3. Water at the ground level
Always water bean plants at their root area to avoid splashing soil onto their leaves. This will help to prevent splashing bacteria or fungal spores onto the leaves. This will also keep the leaves dry which also helps to prevent fungal growth.
Why Bean Leaves Turn Brown | Summary
Bean leaves will turn brown due to environmental reasons, pests, disease or fungus. Preventing fungal disease is the best option by buying good quality, disease free seed, planting them in a new patch each year and keeping water off of the leaves.
Follow these easy steps and you will have healthy beans plants that produce well.
Happy growing.
I am an accredited practicing dietitian, experienced gardener and a dedicated cook. I love writing and sharing my experience so you can learn from my successes and mistakes.