Bean blight, 7 diseases that cause spots and wilting on beans

Beans are a favorite in the veggie patch, and not only for us! Many types of bean blight exist, caused by fungus, viruses, mites and bacteria. They infect bean plants and trigger brown or yellow spots,wilt, and other issues on bean leaves, bean pods and vines.

Here are the main culprits responsible for bean blight.

7 main types of bean blight

Fungus, virus, bacteria and mites trigger the following seven of the many diseases bean plants can catch.

• The “pv.” abbreviation in some names stands for “pathovar”. It’s a sub-group with the ability to infect a specific plant, which other groups can’t. But they’re still the same species.

1. Common Bean blight on leaves

Common bean blight, or common bacterial blight, is the result of infection by either of two related bacteria. One of  these also infects citrus plants, so it’s more likely to appear if you’ve also got a citrus orchard nearby.

• Culprit: Xanthomonas phaseoli pv. phaseoli and/or X. fuscans subsp. fuscans and/or X. citri pv. fuscans
• Type: bacteria
• Dormancy: indefinite (over 15 years when lodged in bean seeds)
• Main hosts: bean, lupine (the flower), pea

2. Phytophthora blight, bean pods rot away

Phytophthora blight is a plant disease where bean pods turn mushy and mold away. Usually this happens when the pods form near the soil. Varieties with very long pods like string beans are more vulnerable since they occasionally reach down to the ground.

• Culprit: Phytophthora capsici and/or Phytophthora phaseoli
• Type: oomycete
• Dormancy: indefinite (survives as long as the soil is moist)
• Main hosts: plants of the Cucurbit family and bell pepper. Bean, tomato and eggplant are minor hosts.

3. White patches on bean leaves: spider mite!

Red spider is a type of mite that feeds on sap. It drains leaf cells of their chlorophyll, turning them pale yellow. Webs appear.

• Culprit: Tetranychidae
• Type: spider mite
• Main cause: hot, dry weather
• Other hosts: nearly all plants, if conditions are met
• Spread: touching plants, wind (on strands of webbing)

4. Halo blight: round spots on leaves and pods

Halo blight results in round spots forming on bean leaves and pods. In extreme cases, vines dry out.

• Culprit: Pseudomonas syringae pv. phaseolicola
• Type: bacteria
• Dormancy: 2 years or until infected plant matter fully decomposes
• Main hosts: bean (all species)

5. Bean bacterial brown spot, tan round blotches

Note the last word in the name to distinguish it from halo blight above: the “pathovar” descriptive is different. This is the same species of bacteria as above, but it’s a different group within the species. Interestingly, symptoms are different because the spots on the leaves don’t look the same. For instance, the color of this infection is a deep brown, hence its other common name: chocolate spot of bean.

• Culprit: Pseudomonas syringae pv. syringae
  
• Type: bacteria
• Dormancy: 2-3 years in infected plant material, numbers dwindle after that.
• Main hosts: lilac (where it was first discovered, all Prunus trees (cherry, plum…) and all types of beans.

6. Bacterial wilt, sap stops flowing in bean vines

The bacteria causing this disease also infects other host plants. Initially, brown spots form that seem swollen with water. As it spreads in the plant, it eventually reaches the root system. It causes the roots to rot, to the point they can’t channel water to the vine and stem anymore. In a few days, leaves show signs of chlorosis, the plant wilts, dries up and dies from tip to root.

• Culprit: Curtobacterium flaccumfaciens pv. flaccumfaciens
• Type: bacteria
• Dormancy: indefinite (over 24 years when lodged in dried beans)
• Main hosts: bean, all families

Another group from this same species, Curtobacterium flaccumfaciens pv. oortii, infects tulip bulbs. It causes bacterial canker in tulip (yellow pustule).

7. Virus mosaic, checkered yellow leaves

Viruses are the cause of dreaded mosaic bean blight. Leaves take on a patchy appearance, portions turning yellow while the rest remains unscathed. These viruses don’t get there on their own: host pests inject them into the plant.

• Culprit: Sobemoviruses
• Type: virus
• Vector hosts: aphids, thrips, whitefly, spider mite, nematode… and humans through tools and tobacco stubs
• Dormancy in soil: undetermined
• Main hosts: bean (all families) and many more: tobacco…

Preventing and controlling bean leaf blight

Crop rotation

The first and most effective manner to avoid having to deal with these diseases every year is crop rotation. Most of these pathogens only stay for 2 or 3 years in high amounts in the soil. This means that you should practice a 4-year crop rotation plan. By the time beans grow on the same plot of soil again, levels of bacteria will have dropped to very low levels. They won’t be able to devastate your crops entirely.

However, it’s nearly impossible to eradicate them completely just by rotating the plants over 4 years. There will always be a few bacteria that will reappear. Nonetheless, they won’t be numerous enough to cause any real damage.

Crop rotation is, by far, the most natural solution to prevent bean leaf blight: no additives and no extra work! All you have to do is avoid planting beans in the same spot, and infection drops to acceptable levels.

Controlling water and wetness

These bacteria and oomycete spread thanks to water. Keeping leaves dry is an excellent way to help protect the plants.

• Only water directly at ground level.
• Space plants far apart so that air can circulate and dry plants off faster.
• Remove lower leaves and fruits to at least 1 foot from the ground (30 cm). Rain splash won’t reach as high, keeping the leaves safe.

Avoid cuts and breaks

• Avoid wounding the plants if possible: most of these survive on the surface of leaves, but can’t get in without a breach.
• Keep your beans free of pests, especially sap-suckers that drill holes along the surface of plants.
• Clean tools between plants to minimize contamination.