2021 Considerations: Mind your Peas and Beans

UNIVERSITY OF MINNESOTA EXTENSION - www.extension.umn.edu

Our series prepping you for the imminent growing season continues.  This week we want to inoculate you with information on common issues and considerations in peas and beans.

Inoculating peas and beans

Peas and beans are legumes. They form symbiotic relationships with rhizobium bacteria to fix nitrogen from the atmosphere. In some soils, these bacteria are naturally abundant, while in others they need to be added. How do you know whether your soil contains native rhizobium populations?

• If peas, beans, and other legumes are a regular part of your rotation, you will likely have some rhizobium present in your soil.

• If your soil is acidic (pH <6), rhizobium are less likely to survive naturally in your soil.

• Ultimately, it is difficult to know whether you have existing rhizobium populations in your soil. 

There are a few important factors to consider when deciding whether to add an inoculant to your peas and beans. First, not all legumes are the same. Each species forms relationships with different types of rhizobia, and so even if you’ve been growing beans for years, you may not have the right rhizobia in your soil for peas, and vice versa. If you do choose to inoculate, make sure you’re purchasing an inoculant that is specifically meant for your particular crop. Second, even if you do inoculate, the rhizobia may not thrive in your soil. Maintaining an adequate pH and sufficient molybdenum, potassium, and phosphorus levels seems to help improve the success of inoculations (most MN soils have sufficient molybdenum). 

It costs around $7 to treat 50 pounds of pea or bean seed, so inoculating is a relatively cheap way to increase your chances of nitrogen fixation. One approach that growers can try is to inoculate a few strips within fields, and compare the yields and overall vigor of the inoculated areas vs. the non-inoculated areas. Additionally, dig up the roots of a few plants this summer to check for nodules. Healthy root nodules (the structures formed when rhizobia enter the plant roots) are pink / tan. If you do not see any nodules (see photo for reference), or they are greenish gray in color, consider inoculating in future seasons. If you’re seeing healthy pink nodules in areas that have not been inoculated, you can assume that you have a healthy native population in your soil. 

Healthy pink nodules on the roots of a pea plant. Photo: Natalie Hoidal

For a thorough overview of inoculants, see this publication from Colorado State University. 

Feeding nitrogen to nitrogen fixers

Given that legumes fix nitrogen from the atmosphere, it may seem counterintuitive to provide nitrogen fertilizer at planting. Generally speaking, if there is ample nitrogen present in the soil, legumes will use the nitrogen present rather than fixing it from the atmosphere. In many cases, it is not necessary to add nitrogen to the soil when you plan to plant a legume: 

• If you used a cover crop prior to planting, the cover crop will likely provide sufficient nitrogen to get your peas and beans started (for instructions on calculating an N credit from your cover crop, see this video). 

• If you planted a legume (peas, beans, alfalfa) in the field you plan to plant in this year,, you do not need to add additional nitrogen. 

• If you’re growing your peas and beans in a marginal field (e.g. sandy and unirrigated) and anticipate low yields, extra nitrogen will likely not boost yields. 

That said, in some cases, a little bit of nitrogen can help peas and beans to establish. Depending on your soil organic matter, peas and beans can generally benefit from anywhere from 10-60 pounds of nitrogen per acre. The following tables come from the Minnesota Nutrient Management Guide for Commercial Fruit and Vegetable Crops. Most growers should pay attention to the numbers within the red boxes, indicating high yield goals in a field that was previously used to grow vegetables. 

Pea nitrogen rates: 

Snap bean nitrogen rates: 

Most fresh market vegetable growers should select the highest yield goal when deciphering these tables; lower yield goals are meant for producers growing on marginal lands with limited inputs. Since most fresh market growers are raising vegetables intensively with irrigation and careful management, the highest yield goal is appropriate. Note that soils with higher organic matter require less nitrogen to achieve the same yields.

Aphids: They Suck!

Small, soft-bodied, sucking aphids can cause major headaches in peas.  Pea aphids reproduce quickly, and their numbers can explode during hot, dry weather.  Pea plants are most vulnerable to damage as they flower and pods start to develop, as feeding can cause blossoms to drop and pods to be deformed.  Aphids can also vector plant viruses.

Luckily, Minnesota is home to many natural enemies, and actions you may already be taking on your farm to support pollinators will support aphid predators.  This includes ladybeetle larvae and adults, minute pirate bugs, syrphid flies, and others.  Their importance can be seen in how aphid populations often explode after application of broad-spectrum insecticides, which wipe out both the pest and predator insects.  If considering spraying, visit the Midwest Vegetable Production Guide for a rundown of options.

Syrphid flies are a common sight on flowers, though their larvae look quite different and are voracious aphid predators. Photo: Top: Whitney Cranshaw, Colorado State University, Bugwood.org, Bottom: David Cappaert, Bugwood.org.

More Pea Root Rots than you can Shake a Stick at 

If we were to list all the soilborne diseases affecting peas, we would have a paragraph only plant pathologists could decipher.  The same cultural practices work against multiple diseases.  As with all soilborne diseases, issues causing large problems should be sent to the diagnostic lab.  Some diseases will last as long as it takes for crop residue to break down, while others can survive in the soil for 10+ years. 

• Rotate away from legumes for more than 3 years

  • More than 6 years is needed for some diseases (Fusarium, some damping off pathogens)

  • Avoid double cropping

  • Think about what legumes might be in your cover crop

• There are varieties with resistance, but a correct diagnosis of the root rot will be necessary

• Remove dirt from tools and equipment between fields

• Plant when soils are warm and dry to promote quick germination and emergence

Powdery Mildew in Peas

There are many species of powdery mildew that affect all kinds of plants.  The pea-specific powdery mildew is most commonly seen in later-season peas. This disease starts out as small,  powdery white areas on the surface of the leaves deepest in the canopy.  As time passes (and weather is conducive), the powdery areas spread up the stem and eventually onto pods.  Conducive weather for this disease is warm, dry days and cool, dewy nights.  Powdery mildew will survive in the soil with plant debris.  There are many resistant varieties available, and are good to have on deck when planting peas later in the season.

White Mold in Beans

White mold is a long-lived, soilborne disease that can infect all parts of the bean plant.  Instead of starting in the roots, this disease often first infects flowers, then spread to nearby stems, leaves, and pods.  The first symptom of infection in these plant parts is pale, water-soaked areas that will eventually become covered with a fuzzy white mold. 

White Mold causing wilting, discoloration, and mold to form on bean plants and pods. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org.

White mold is long-lived in the soil.  If you have seen white mold on beans (or cabbage or one of the other vegetables white mold attacks), you may have noticed what looked like rat droppings on infected plants.  These structures are like a spore bunker, surviving in the soil for long periods of time until a period of prolonged moisture triggers it to open and start another round of white mold infection.

Moisture management is the basis of white mold management.  This starts at the time of field selection and planting layout (keeping susceptible crops out of fields with poor wind flow, and orienting rows for ease of drying), and continues when making decisions about varieties and planting density (less dense canopy = more drying).  There are some varieties of beans with some level of white mold resistance.