Courtesy Howard F. Schwartz/Colorado State University
The benefits of raising food-yielding legumes are many—it's unfortunate that legume diseases are, too. It's too-often a common scene: One minute, you have a strong stand of beans, and the next, it's covered with powdery mildew. Here are seven diseases that could damage legumes, including beans and peas, as well as their symptoms and what you can do to treat and prevent them.
1. Alternaria Leaf and Pod Spot
Caused by: Alternaria leaf and pod spot is a disease affecting beans and is caused by Alternaria fungi.
Look for: Bean pods develop small, dark-brown or black flecks that are slightly raised and cone-like. Leaves first appear to have small, water-soaked flecks. These become spots with pale-brown centers and reddish-brown borders and eventually develop concentric rings and can crumble from the plant. Leaves fall off the plant as the disease progresses. Older leaves and pods are more susceptible.
Stop it: Alternaria fungi develop in high relative humidity, rainfall and cool temperatures. Once spores appear, they're easily spread by insects, wind and rain. Crowded bean plants and those lacking nutrition—in particular nitrogen and potassium—are at the highest risk for ALPS. Application of a foliar fungicide can aid in controlling the disease. Use disease-free seed and practice crop rotation to prevent ALPS introduction.
Caused by: Anthracnose is a legume disease caused by the fungus Colletotrichum lindemuthianum.
Look for: Cotyledons develop dark-brown or black lesions, and seedling stems get rust-colored flecks and lesions. On developed leaves, lesions start on the underside near the veins and progress to the upper surface.
Red or purple leaf lesions typically appear first on lower leaf surfaces near the veins and, as the disease progresses, appear on upper leaf surfaces, turning brown or black. On legume pods, tan to rust-colored lesions with brown or purple borders appear, and inside, the seed develops brown or black lesions. Lesions become filled with brown or pink fungal spores.
Stop it: Anthracnose is spread through infected seeds, crop debris, wind, air, insects and shared tools. Cool, wet, windy weather is ripe for anthracnose outbreaks. Prevent infection by planting disease-free seed, rotating crops, disposing of infected plants and waiting until conditions are right in the spring to plant. Spray foliar fungicides to aid in control.
3. Bacterial Brown Spot
Caused by: Pseudomonas syringae pv. syringae is the bacterium causing bacterial brown spot in legumes, most commonly seen in warm weather.
Look for: You'll see small, brown circles outlined in yellow. These progress into streaks along leaf veins. The centers of lesions eventually fall out, causing the plant to look tattered. Pods show water-soaked circles that turn brown. They might develop in a bent shape. Infected seeds may shrivel and discolor. In humid climates, cream or silver bacteria might ooze from infected plant parts.
Stop it: Plant disease-free seeds, rotate crop families, remove or incorporate plant stands after production, do not reuse irrigation water, avoid working in fields in wet conditions, and remove volunteer legume plants throughout the year. Copper-based sprays can be effective against bacterial brown spot.
4. Bean Yellow Mosaic Virus
Caused by: Bean yellow mosaic virus is caused by the virus of the same name.
Look for: Leaves of plants infected by BYMV will show bright-yellow to green mottle—or mosaic pattern—plus leaf distortion, down cupping and wrinkling. Plants may also be stunted and pods distorted. (Differentiate between BYMV and its relative, bean common mosaic virus, by the color of the mottling. BCMV causes light and dark green, not yellow, patterns.)
Stop it: Aphids quickly transmit the virus through entire fields, so aphid control is paramount to preventing BYMV infection.
5. Common Bacterial Blight
Caused by: The bacterium Xanthomonas campestris pv. phaseoli is the culprit behind common bacterial blight, a disease of snap beans and dry beans, especially in warm, humid, wet climates.
Look for: The underside of leaves show small, water-soaked spots that become larger, dry, brown spots with yellow halos. The leaves continue to yellow as the disease progresses. Legume pods develop large, unevenly shaped, sunken, water-soaked areas that turn brick red. They can ooze a yellowish bacteria that dries into a crust.
Stop it: Help prevent common bacterial blight by planting disease-free seed, avoiding work in fields when plants are wet, rotating crop families, removing crop debris immediately, not using overhead irrigation and practicing good weed management. Copper bactericides are an option for control.
6. Powdery Mildew
Caused by: The fungus of powdery mildew (pictured above), Erysiphe polygoni, causes damage in the form of water and nutrient loss and stunted plant growth. It can be found in all climates and is most severe in warm, dry climates.
Look for: Powdery mildew first appears as small white, gray or brown powdery patches on leaves and pods that rapidly spread to cover the surfaces.
Stop it: Prevent powdery mildew from forming by not planting in shady, humid spots; pruning for air circulation; and not over-applying nitrogen fertilizers. Prune and destroy plants at the first sign of infection—do not compost. Chemical and organic sprays are available to control an infection.
7. White Mold
Caused by: Sclerotinia sclerotiorum, the fungus that causes white mold, is a big problem for legumes in moderately warm, humid areas.
Look for: Damage first appears as watery spots and dead plant tissue. Next, mold infects and becomes visible on stems, leaves and pods, eventually killing the plant.
Stop it: Prevent white mold by planting legumes less densely and rotating with other crop families. Topical fungicides are effective against white mold, as well.
About the Author: Freelance writer Lisa Munniksma had a big battle with powdery mildew on peas while working on a farm in Belgium. Follow her as she learns about sustainable living, agriculture and food systems everywhere at www.freelancefarmerchick.com.