Bean Rust

We’ve all heard that beans cause gas, but did you know beans rust? Well, not rust like the undercarriage of a New England truck, but rust just the same.

Bean rust, like other plant rusts, is a fungal disease. Rust is found worldwide and it can wipe out your bean crop if it takes hold early enough in the growing season.

Bean rust (USDA) CC BY 3.0 US

California’s cool, wet springs are just the conditions needed for rust to thrive. Add overhead watering or a decent breeze and the stage is set for an epidemic. Fungi are so efficient that, under ideal conditions, the disease cycle can be repeated every 10 to 14 days!

There are several strains of bean rust. Two of the most common are Uromyces appendiculatus and Uromyces phaseoli typica, but you don't need to know the Latin to recognize bean rust in the garden.
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Bean rust symptoms

​Similar to other rusts, bean rust prefers moist places and moderate temperatures (65 to 85°F). While it can occur on any aboveground portion of a plant, bean rust is most commonly found on the underside of leaves. Pods can also be affected. At first, it just looks like tiny white or yellow bumps. Then those bumps break open and turn into bright orange, reddish, or yellowish flecks. Those flecks are pustules that are made up of more fungal spores than any of us cares to count. [Okay, some scientists love counting things like that.] A yellow outer ring is sometimes visible. Leaves may begin to curl downward and plants may develop a scorched appearance. These symptoms are easy to see and make identifying the condition simple. Getting rid of it is something else all together.

Bean rust control

The fungi that cause bean rust can be spread by ants, aphids, and gardeners. It can stick to tools, fingers, and clothing. As with many other plant diseases, prevention is far easier than eradication. Use these tips to prevent and control bean rust in your garden:

• Space plants to allow for good air circulation.
• Avoid overhead watering.
• Plant resistant varieties.
• Monitor plants frequently.
• Apply sulfur to make leaves and pods less hospitable to bean rust.
• Remove infected leaves and dispose of them in the trash.
• Avoid excessive use of nitrogen and overfertilizing. 
• Cut spent (uninfected) plants off at soil level and add them to the compost pile.
• Use a 2-year crop rotation with bean plants.
• Infected leaves should be removed and thrown in the trash.

Keep in mind that rust pustules are easily dislodged and can land somewhere else, or on the soil, where they can be bounced back up into your plants by rain, wind, and overly exuberant irrigation. And be sure to disinfect your tools after removing rust-infected leaves, to avoid spreading the fungus to healthy plants.