Bean blight, 5 diseases causing spots and wilting on beans

Types of blight that infect bean leaves, pods and seeds

Beans are a favorite in the vegetable patch, and not only for us! Many types of sickness-causing fungus and bacteria also love to infect bean plants. They cause yellow spots, wilt, and other issues on bean leaves, bean pods and vines. Here are the main culprits responsible for bean blight.

5 main types of bean blight

Fungus and bacteria trigger the following five of the many diseases bean plants can catch.

  • In some cases, the name has the abbreviation “pv.” : this stands for “pathovar”. It means that within the species of fungus or bacteria, a certain group has the ability to infect the plant, while other groups don’t. But they’re still the same species.

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

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.

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)

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.

Bacterial wilt, sap stops flowing in bean vines

Young beans with spots on the pods signalling some type of bean blightThe 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).

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.

Credits for images shared to Nature & Garden (all edits by Gaspard Lorthiois):
Pod with spots by JIRCAS Library under © CC BY 2.0
Tiny spots by Maggie McCain under © CC BY-SA 2.0