If you picture a vegetable patch in your mind you will likely have an image of a glorious row of succulent green beans clambering up bamboo canes. They are a traditional staple of any vegetable garden, and my children delight in watching the seeds germinate and then picking the fruits of their labour!
But there are many pitfalls to be wary of that can cause your green bean leaves to turn yellow. All can be relatively easily resolved or prevented so I will go through each potential cause to help you to pinpoint and prevent any issues that might crop up across the growing cycle.
Why Your Green Bean Leaves Are Turning Yellow
Green leaves are a solid indication of good health in any plant, but especially with beans. As soon as they start to turn yellow it is a sign something is off-kilter. It might be the watering schedule has gone awry, the soil could be deficient, or it might indicate a more serious issue like pests or disease. With early intervention, most issues can be resolved.
1. Under or Over Watering
By far the most common cause of yellow leaves on bean plants is overwatering. Too much water in the soil can deplete it of oxygen, compacting the soil and causing fungal root rot. The leaves on the bean plant will start to turn yellow as they are no longer able to absorb the nutrients, oxygen and water they need.
To correct this, simply reduce the amount of water you are adding to your bean plant and water every few days. You might find if you live in one of our more tropical states with good regular rainfall bean plants can survive well on this alone.
A top tip to prevent overwatering is to add a 2-inch layer of straw mulch under your beans to help to regulate the amount of water that reaches the roots. Mulch can also help to reduce soil runoff during heavy rainfall and can reduce soil compaction, so it is a simple yet effective idea with multiple benefits!
2. Soil pH
Every vegetable has an ideal pH range and beans prefer a soil pH between 6.0 to 7.0. Anything out of this range can impact nutrient uptake. In very acidic soil aluminium and manganese can become too strong and toxic to the plants while calcium and magnesium are less available. Whereas if your soil is more alkaline manganese will be lacking making chlorosis a potential issue. Plants need manganese to form chlorophyll for them to complete photosynthesis so when it is deficient chlorosis occurs which makes the leaves turn yellow.
The soil in my garden tends to be slightly acidic so I apply lime to my vegetable beds at the start of every year to raise the pH a little to try and prevent this. If your soil pH is higher than 7.0 you could consider applying a soil acidifier to reduce the alkalinity.
It is a fine balance to perfect so is worth testing your soil before you plant your crops and treating them. If you do notice your leaves starting to turn yellow and you can rule out watering issues or other more obvious causes it would be wise to test the pH and act accordingly.
3. Nutrient Deficiencies
Micronutrients or trace elements are often forgotten aspects when cultivating vegetables at home. Micronutrient deficiencies are pretty rare in green beans but there are some soils where symptoms can occur and even just a small deficiency or excess can wreak havoc with yields and plant health.
Micronutrients should ideally be added before planting as they are often not very mobile and adding them to the soil once the plant is established will only have a limited impact. It is always wise to treat your soil with a good dose of fertiliser with a range of micronutrients before you start planting.
If you do detect some deficiency during the growing season you are best to rectify it with a foliar spray directly onto the plant. Word of warning though, they are often expensive, so it is best to do a trial on two or three plants you suspect are deficient and if you note no reasonable improvement within a week it is probably not worth using anymore.
These are some of the more common deficiencies of green beans:
A zinc deficiency will leave a plant with abnormally small leaves. This is because it will cause the internodes to be smaller resulting in stunted growth. If you suspect this is an issue it is best to apply a zinc oxide spray to your plants to alleviate some symptoms but unfortunately, once it has started you will likely have lost 30% of your potential yield. Treating the soil before planting next season will be key.
Plants with a manganese deficiency will show smaller than normal leaves with speckled yellowing between green veins. Brown spots can also occur so the deficiency is often confused with various fungal infections/symptoms. Luckily some soluble manganese foliar sprays can quickly rectify this deficiency.
A boron deficiency is characterised by a slight yellowing of leaves and thickening of stems. Sometimes the stems split open and growth points die. Another symptom is for your plant to form more than the normal amount of early bean nodules which then die off. Borax is the solution and you can apply it during the growing season to try and help maximise what the plant can achieve for you. But you may need to write off this year’s crop of green beans and focus on treating the soil next year.
Copper is a very important trace element that should be noted but deficiencies are luckily rare in green beans as they seem to have a high tolerance to low copper concentrations due to the root hairs which absorb and find copper in the soil very efficiently.
Lack of Sunlight
Green beans are a warm season crop and as such require 6 to 8 hours of sunlight daily to reach their maximum potential so sunlight deficiency is a common reason leaves might start to yellow.
A lack of sunlight will mean leaves are unable to synthesize the chlorophyll required for photosynthesis. A lack of chlorophyll in the plant’s leaves will mean they begin to turn yellow and will eventually die.
When arranging your vegetable patch it is important to ensure that your beans are suitably spaced to prevent overcrowding and planted in a location that has access to direct sunlight and not in a shady spot.
If your beans are in pots or bags and you believe lack of sunlight may be the cause of your yellowing leaves then a simple but careful move to a warmer spot should help. However, if they have been planted directly into the ground or a vegetable bed and the plant is well established, they will unlikely survive the transplanting process, especially if they have started climbing.
4. Temperature Range
In line with sunlight requirements, your green beans need to be exposed to the right temperature too. They thrive in temperatures between 70 and 80 degrees Fahrenheit. Anything lower than this can cause leaves to yellow and can also prevent further growth. On the flip side, it is also possible for it to get too hot and regular days above 90 degrees Fahrenheit can cause wilting.
Before deciding to plant a bean crop it is worth considering whether you live in a climatic zone that is going to provide that ideal temperature range consistently. The odd heatwave day or cold spell will hopefully not do long-term damage but if temperatures could fall regularly out of that range you should probably question if they are the right crop to grow.
The other option if you live in a colder zone is to use a greenhouse. This will also allow you to extend your bean season to start earlier and still be harvesting beans once the traditional season (the end of the summer and the start of Fall) has passed. Just be sure that the temperature of your greenhouse does not start to push out of range on a hot day either! A great compromise is to start your green bean seeds in the greenhouse and then transplant them to the garden once the ideal external temperature range has been reached. If you chose to do this you will need to harden off the seedlings first.
5. Green Bean Diseases & Pests
There are a number of diseases and pests that can plague green beans. Here are the most common ones to look out for:
Bacterial blight and halo blight are both infections that will result in brown greasy looking spots on the leaves surrounded by a bright yellow halo. Not only does it look a bit unsightly but it will also affect yield because the pods develop brown lesions.
In serious infestations, new growth can turn yellow and die. These diseases are typically introduced through infected seeds and when the weather is favourable, take hold in the garden.
Ideal conditions for bacterial blights are warm temperatures and high humidity. The only advice I can offer is never to save seeds from infected plants and remove any infected leaves and plant debris at the end of the season.
White mould can be another common issue. It is a fungal disease that spreads quickly in cool, wet weather. It infects many species of crops including tomatoes and cucumbers as well as beans so can quickly spread through a vegetable garden. It affects leaves, stems, and fruits.
The first signs of white mould fungus are pale-coloured lesions. Cotton-like fungal strands are then produced and the plants turn yellow and wilt. Rotating crops from season to season, spacing plants at the proper distance, and watering early in the morning will act as preventative measures.
Common garden pests are seemingly drawn to green beans in higher numbers than other plants making them especially susceptible to damage.
An infestation of the common aphid will cause the leaves on your plant to wrinkle and turn yellow.
Mexican bean beetles, cutworms and vegetable leaf miners are also responsible for the yellowing of bean plants.
Potato leaf hoppers, thrips and two-spotted spider mites are all sap-sucking insects that cause deformation of the plant and yellowing of the leaves.
Ladybugs can be a great natural way to keep them at bay or organic products like neem oil could be used if you want to immediately deal with an infestation. Another option is to spray on some insecticidal soap.
Black marks or fuzzy grey growth may indicate the presence of fungus. This can happen when the weather is particularly wet with low air movement around the leaves.
To prevent fungus, it is best to surround the beans with mulch to keep the soil off the leaves when watering and to only water at the root zone.
You can treat fungal issues with copper fungicides but I prefer to pinch off any damaged and affected leaves so it doesn’t spread. If you plant plenty of bean seeds you will then easily be able to remove any plants that do appear to be infected.
6. Damaged Roots
Root damage will cause the leaves to yellow. This is because the roots are unable to supply the plant with the nutrients and water it needs to thrive.
Compacted roots may be to blame for yellowing beans if you have grown them in a container. The roots may lack space and suffocate causing leaves to yellow and fall off.
Green beans really need to be planted in the ground or in a large container to allow the root system to develop freely.
When repotting beans or planting out, a quick root inspection will help you gauge the health of the plant. A healthy root system will be a yellow-ish white colour. If they appear darker with a strong odour it may indicate rot has taken hold in the pot. It is best to just discard any affected plants.