Are your beans suffering from some strange affliction that you can’t identify? Or are you planning to cultivate legumes this season and would like to prepare for the worst in advance? Then you need to know about bean diseases.
As with all diseases, diligent observation and quick action can determine whether you get a good harvest or a blazing bonfire of broken plants. Inspect your beans daily, remove any plants you think may be affected, and be proactive with fungicides.
Take care of your plant babies, and they’ll reward you with an abundant food crop at the end of the season.
Below are nine of the most common bean diseases you’ll likely encounter and how to deal with them.
Diseases We’ll Discuss
1. Powdery Mildew
This is one of the bean diseases I’m most familiar with, as it has plagued my garden for years. If you live in an area that gets hot, humid summers, your bean plants are also likely at risk for this fungal pathogen.
It’s caused by fungi from the Erysiphales family and manifests as white, floury blotches on your beans’ leaves and pods.
Powdery mildew thrives in shaded and humidy. You can try to avoid it by planting your beans in an area that gets sun for at least six hours a day. Also, keep your bean plants’ foliage pruned to maximize light, and there’s sufficient airflow so spores can’t accumulate.
Inspect your plants daily to look for signs of this pathogen, and remove affected parts immediately. Burn these, then apply organic fungicides such as milk or baking soda spray to the surrounding plants. Copper works well, too.
Furthermore, ensure that low branches are trimmed and that there’s no detritus around the stems for spores to congregate, nor splash upwards during watering times or rainfall.
Check out our article on homemade organic fungicides here for helpful recipes and tips.
2. Bacterial Blight
If your bean plants’ leaves are turning black and necrotic, and if those black marks are also showing up on the twigs and branches, then you’re likely dealing with bacterial blight.
It’s caused by a pathogen named Pseudomonas syringae pathovar (pv.) Syringae, and can affect several different species tree species as well as numerous food crops.
Unlike fungal infections, bean diseases like this one are challenging to combat. This is because they’re spread via insects, which transmit them to the plants they feed off. Once plants are affected by this bacteria, you can try to salvage unaffected parts as best you can, but there is no cure.
As a result, prevention is often your best option. Practice regular crop rotation, and solarize your soil between planting seasons.
Choose disease-resistant cultivars to plant whenever possible, and cover your crops with floating row covers to protect them from insect predation.
When and if you find an afflicted plant, use clean snips to remove damaged parts, and then sterilize the blades with alcohol wipes before trimming any other plants to minimize disease transmission.
3. Anthracnose
Since I live in a cool, wet climate, Anthracnose is another one of the main bean diseases I’ve had to contend with. It thrives in the northeastern USA and eastern Canada, as well as cooler regions in the UK, Scandinavia, Slavic, and Baltic regions in Europe.
You’ll recognize Anthracnose by the small, dark, sunken lesions it causes on your plants’ stems, leaves, twigs, and eventually fruits.
This disease is caused by Colletotrichum fungi, and likes to overwinter in soil debris such as fallen leaves. It only thrives in the aforementioned cool, damp weather, and will retreat when the hot, dry weather arrives.
While there’s no cure for Anthracnose, you can treat affected plants and try to avoid the issue with preventative measures.
For example, always sterilize your tools, and prune off and burn affected plants as soon as you spot them. Keep the soil clear of any debris, spray the area with neem regularly, and rotate your crops annually.
Furthermore, if you do find that any of your plants suffer from bean diseases such as this one, don’t save the seeds: Anthracnose can lurk on them and then repeat the cycle anew when they’re planted.
Read our article on Anthracnose to learn more about it and how to treat it.
4. Crown Gall Disease
There aren’t many bean diseases that cause rough, woody tumors to grow on the plant’s stems and twigs. As such, if these show up on your plants at any point during the growing season, you’ll know that you’re dealing with crown gall.
It’s caused by Agrobacterium tumefacien, a gram-negative, bacilliform bacterium that thrives in and around the roots of several different plant and tree species worldwide.
This bacterial disease doesn’t often affect plants until they get wounded somehow. For example, via open wounds left after pruning, small cuts and scrapes from lawnmowers or strimmers, or puncture wounds from sap-sucking insects.
These open wounds allow the bacteria to get in, at which point they form these unsightly galls.
There’s no cure for this, so the only recourse is to pull up the affected plant and burn it. Then, since you know the bacteria is in the surrounding soil, you can treat the area with copper- or creosote-based solutions so it doesn’t affect future crops.
Let that area stay fallow for a year, solarize the soil well, and then plant gall-resistant cultivars in the future.
Read our article on crown gall to learn more about this disease.
5. Pythium Root Rot
Pythium root rot is one of the primary water mold bean diseases you’re likely to come across if you live in a cool, damp region. It’s caused by Pythium spp. pathogens, which lurk in the soil and destroy plants from the root upwards.
While they often destroy seeds before they can germinate, they can also manifest as damping-off disease in germinated seedlings (which we discuss later), as well as stunted growth/failure to thrive in older plants.
If your bean plants are wilting and falling over, pull them up. Check their roots, and if they look wet, brown, and sludgy, you’re likely dealing with Pythium.
There’s no cure for it, so you’ll need to destroy affected plants and take preventive measures for next season. Avoid planting in low-lying areas, and buy disease-resistant seeds to plant.
Dig up and either solarize your soil, or treat it with a fungicide that includes either metalaxyl or mefenoxam.
6. Bean Yellow Mosaic Virus (BYMV)
Although this potyvirus usually affects pea plants, it’s also common in pole and bush beans, as well as peanuts, fava beans, vetch, clover, lupines, and other members of the legume family.
Like other mosaic viruses that may affect cucumbers, squashes, and so on, this disease causes yellow, mosaic-like patterns on the bean plants’ leaves.
Since this disease is transmitted by aphids, the best way to contend with it is with preventive measures such as floating row covers and spraying plants regularly with neem oil.
As with many other bean diseases, there is no treatment for BYMV, so all affected plants must be pulled up and destroyed.
Keep the soil in the area well maintained by removing debris, and do not plant any susceptible species there for at least five years. When you do, choose virus-resistant cultivars and inspect them daily for any sign of infection.
Furthermore, plant species that attract ladybugs, lacewings, and other insects that prey upon aphids to reduce the chance of vector transmission. You can also purchase critters like ladybugs and lacewings to help your garden along.
7. Bean Common Mosaic Necrosis Virus (BCMNV)
Although this virus causes similar symptoms to the one mentioned above, they’re different bean diseases. BCMNV initially presents as mosaic-like patterns on leaves but then develops into systemic necrosis.
There is no cure for this either, so destroy any affected plants. Then, dig the soil down to a foot, dispose of it safely away from the garden, and replace it with clean, healthy soil. Don’t plant legumes there again for a year or two.
When and if you do plant legume crops there again, choose virus-resistant cultivars and rotate your crops annually.
8. Fusarium Wilt
Although yellowed, dropping leaves can imply several different issues—including but not limited to over- or under-watering, nitrogen depletion, and insect infestations—they can also be a surefire sign of Fusarium wilt.
This disease is caused by the Fusarium oxysporum pathogen, which is untreatable and rather insidious. It’s spread via contaminated soil and thrives in warm, humid weather, as do many other bean diseases.
If you find that your plants are affected by this wilt, pull them up and burn them immediately. Clear debris off the soil’s surface, and sterilize your garden tools with diluted bleach and boiling water after every use.
At the end of the season, solarize your garden soil and treat it with a fungicide, then rotate crops and don’t plant susceptible species there for at least three years.
9. Damping-Off Bean Diseases
Rather than being one specific ailment that may affect your plants, damping-off can be caused by a few different bean diseases. If your bean plant seedlings seem to be doing okay but then keel over and die for no apparent reason, check their roots.
If their roots are brown and rotten-looking, causing the stems to keel over, then damping off is the likely culprit.
It’s often a sign of the aforementioned Fusarium wilt but may also be caused by Botrytis, Diplodia, Alternaria, Phytophthora, Aphanomyces cochlioides, Pythium, Cylindrocladium, or Rhizoctonia solani.
These pathogens thrive in cool, moist soil, which is why damping-off tends to happen most often in early spring and early autumn plantings.
You can try to avoid these bean diseases by starting your seeds in sterile seed-starting mediums instead and then transplanting them outside once the soil has warmed and dried out. Additionally, try to buy seed cultivars resistant to various fungal pathogens.
If your crops succumb to fungal issues more than a couple of years in a row, you’ll need to either treat or replace your garden soil. Turn it over to a depth of a foot and solarize it well during the hottest, sunniest days of the year.
Then treat it with an organic fungicide like copper, mix it with well-aged compost and beneficial microbes, and replace it.
Alternatively, dispose of this contaminated soil in a manner approved by your local bylaws, treat the area with a fungicide, let it stay fallow for a year or two, and then replace the soil completely.
