Dry bean diseases can be caused by fungi and bacteria, but Dr. Sarah Pethybridge had a reminder about plant diseases in general.

“It’s not just the presence of our pathogen. We need three factors to occur all at the same time to get disease in our field,” said Pethybridge of Cornell AgriTech. She was one of several presenters at the “Beans for Lunch” webinar series put on by the University of Vermont Extension Northwest Crops & Soils Program.

“We need the pathogen, we need the plant and we need environmental conditions,” Pethybridge continued.

Fungal Root Rots

In dry beans, fungal root rots caused by Rhizoctonia solani and Fusarium spp. can cause pre- and post-emergent seedling death resulting in poor crop stands. Plants that survive will often exhibit above-ground symptoms including stunted growth, yellowing of leaves and general non-thriftiness.

“When you pull those plants up, you might have these quite dramatic brown necrotic lesions at the soil line. This is caused by a fungal root rot,” Pethybridge said.

Fungal root rots are soil borne and not seed borne. They are generally exacerbated by abiotic stressors – wet, waterlogged soil, soil temperatures under 60º and compacted soil.

If there have been early issues in the field with R. solani, the disease can cause more problems later in the season; notably, a higher risk of foliar blight characterized by dramatic bleached-type lesions scattered throughout the foliage.

Pods can also be impacted. Pods that come into contact with the ground because of the variety, lodging or the impacts of a storm are more susceptible to foliar blight.

Because fungal root rots are soil borne, long-term crop rotation to non-susceptible crops such as grasses may help to reduce soil inoculum. Avoiding excess irrigation or long drought stress may also help manage root rots in dry beans. Seed treatments for Rhizoctonia and Fusarium root rot pathogens are available.

Bacterial Diseases

Bacterial diseases of dry beans can be seed or soil borne. Brown spot (caused by Pseudomonas syringae pv. Syringae) presents itself as circular necrotic lesions that are generally less than an eighth of an inch in diameter. The lesions always have a yellow ring (or halo) around them. They often occur in the veins of the leaf where wetness accumulates.

When the lesions dry out, the center may fall out, leaving a visible hole. The pods can also be impacted by brown rot. Brown rot is exacerbated by warm temperatures (80º – 85º).

Brown spot on dry beans. Photo by Sarah Pethybridge

Another bacterial disease, halo blight, caused by P. syringae pv. Phaseolicola, presents itself differently than brown spot. “It’s a lot more dramatic in its appearance. Rather than just small necrotic lesions, the lesions are generally a lot larger,” Pethybridge said. The lesions are also surrounded by greenish-yellow halos, but the centers of the lesions are often more of a reddish color. Like brown spot, both leaves and pods can be affected. Halo blight is more likely to occur in cooler temperatures (70º – 75º).

A third bacterial disease, common blight (caused by Xanthomonas axonopodis pv. Phaseoli) is more damaging than brown spot and halo blight. Again, the symptoms occur on the foliage and pods, but they usually begin as small water-soaked lesions on the undersides of the leaves that then become brown and brittle. Like brown spot, common blight prefers warmer conditions.

Controlling bacterial diseases in dry beans requires a multifaceted approach: start the season with certified disease-free seed, scout frequently, rotate to non-host crops for two to four years, promptly incorporate bean debris post-harvest to promote decomposition, prioritize drip irrigation to prevent secondary spread of bacteria and avoid traffic through fields when the foliage is wet. Fungicidal protectant sprays can be an effective control for bacterial diseases.

Anthracnose

Anthracnose (caused by the fungus Colletotrichum lindemuthianum) results in poor pod quality and seed quality. “It’s got a very wide temperature range. It’s happy either way, right down to 55º and up to 80º. But it really likes high relative humidity – over 92%,” said Pethybridge.

Lesions caused by anthracnose are rust brown to black mostly on the petioles of the leaves and possibly the leaf veins. The lesions are more round but sunken eye-spot type lesions. They can be particularly aggressive and spread quite rapidly, so if it’s early in the season it can kill individual plants, but usually it causes general defoliation and lack of thriftiness.

Anthracnose can also get inside the bean pods, creating dramatic black and brown lesions. Those lesions are usually associated with an orangey color around the lesions; these are the spores of the fungus, which will be dispersed and infect other parts of the plant.

A fungicide (used as a protectant) can be an effective anthracnose control, as can rotating away from all bean crops for at least three years.

White Mold

White mold is also caused by a fungus (Sclerotinia sclerotiorum) and poses a significant challenge for dry bean growers. It has a very broad host range, so crop rotation is not always effective in breaking the inoculum carryover cycle between seasons.

White mold is first observed as wet, soft lesions on infected leaves, branches, stems and pods. These lesions enlarge into watery, rotten masses of tissue covered by a white moldy growth.

With this white moldy growth comes the production of sclerotia (the resting bodies of the fungus that fall back to the soil surface). The sclerotia can survive between seasons to cause disease in subsequent crops. Environmental conditions, especially cool wet weather, drive whether white mold is present during a particular growing season.

“Often there’s a mystery about where it’s going to occur, trying to predict whether you’re going to get white mold or not,” Pethybridge said.

The recommendation is to use a three-year cropping cycle that does not include dry beans, soybeans, sunflowers or other susceptible hosts. Fungicides, used as protectants, are commonly used as a control.

When the sclerotia in the soil germinate, they produce spores which fly off, dispersed by the wind, and infect other plants through the leaves. Because of this, Pethybridge recommended spraying when 1% of the plants have at least one open flower in order to get the most use of fungicides. Since dry beans continue to blossom, there may be value in a second application about 10 days after the first.

Scouting is also important. “When you’re scouting, it can be difficult to see those symptoms because they’re way down in the canopy. But sometimes you have a flag leaf – a leaf that’s sort of wilting singly on a stem indicates that there’s something wrong lower down in the canopy. When you part the canopy, then you can see the white mold symptoms,” Pethybridge said.

by Sonja Heyck-Merlin