Late season, The Bean Report

The Bean Report – August 2, 2023

August 2, 2023

Listen: Soybean Cyst Nematode Survey Research with Dr. Mario Tenuta

Crop Update

  • Soybeans range from R4 (full pod) R5 (beginning seed).
    • Moisture limitations are apparent in some fields. Soybeans flip their leaves over to reflect sunlight, reducing moisture demands for the crop. Soybeans that were exposed to drought stress during vegetative growth stages are better equipped to handle drought stress now during reproductive stages. Water limitations in pulse and soybean crops →
    • Hail continues to hit select areas of the province. If your soybeans have been hit by hail, estimate yield loss using the latest research reports from the soybean and pulse agronomy lab. The R5 growth stage is the most susceptible and maturity may be delayed. Yield loss estimates due to defoliation and node breakage from hail →
    • Soybean aphids populations have remained low. A few fields in eastern Manitoba are approaching economic thresholds (250 aphids/plant and increasing). Once soybeans have reached R6 (full seed), they are considered safe from aphids. Managing soybean aphids →
    • Disease pressure has been low across the province. Phytophthora root rot and northern stem canker are showing up more often in fields with tighter soybean rotations.
  • Field peas range from R4 (full pod) to R6 (mid maturity). Several earlier-seeded fields are expected to reach R7 (full maturity) this week.
    • Past R4, pea aphid feeding is not expected to impact pea yield.
    • When staging peas for desiccation timing, check top pods and seeds in the least mature areas of the field. Seeds should split in half when pressure is applied and pods will have an orange peel-like texture. Staging Peas for Desiccation Timing →
  • Dry beans range from R4 (full pod) to R6 (50% seed).
    • White mould and anthracnose presence has been low overall. With none noted in dry bean fungicide trials 14 days after application. Bacterial blight is the most common disease noted this year.
    • Potato leaf hoppers and grasshoppers have been noted in several fields, but defoliation damage has remained low for the most part. From pod fill to maturity, thresholds for defoliation are 25%, unless pod feeding is observed.
  • Faba beans range from R5 (full pod) to R6 (beginning maturity).
    • Lygus bugs cause seed pitting in faba beans from feeding damage. Sweep net fabas during podding stages for lygus bugs. A nominal threshold has been proposed of 5-10 lygus bugs/10 sweeps at early R4 (flat pod) to keep damage below 15%. Lygus bugs in faba beans video (1 min 30 sec) →
Soybeans at R4 on July 26, 2023.
Soybeans at R4 near Gladstone on July 26, 2023.
Drought-stressed soybeans near Cromer on July 25, 2023
Peas maturing near Roblin on July 24, 2023.
Black beans at R6 near Minto on July 26, 2023.
Full length pods collected off of a single black bean plant. In addition to these 39 full length pods there were 47 small to pin-bean sized pods on the plant.
Faba beans near Dauphin with full pods on July 24, 2023.

Scouting for Soybean Cyst Nematode (SCN)

SCN cysts next to root nodules on soybean roots.

There are no distinct above-ground symptoms unique to SCN. Since SCN impacts water and nutrient uptake, stunting, chlorosis and necrosis are common symptoms, however, once above-ground symptoms are apparent, up to 30% yield loss may have already occurred. As a result, dig up roots in August to look for cysts and to catch this destructive soil-borne soybean pest early.

Scouting for SCN:

  1. Gently dig up soybean roots from late July to August and examine them for cysts. Cysts are smaller than root nodules and appear as white, lemon-shaped cysts on the roots.
    • In clay soils, roots may need to be soaked to loosen and remove soil.
    • Magnifying lenses may come in handy to take a closer look at roots in the field.
  2. Target high-risk areas of the field, which includes anywhere that soil containing SCN could have been introduced. This includes field approaches and headlands, low spots and water runs, shelter belts, high pH areas and any generally low-yielding areas of the field.
  3. If you suspect SCN, soil samples can be collected and submitted to Agvise to get an idea of abundance (eggs/100 cc of soil) which can guide management decisions. If samples submitted to Agvise come back positive for SCN, contact MPSG for further testing.

Management strategies include growing SCN-resistant varieties, reducing tillage and controlling host weed species (of which there are more than 110 species). Other crop hosts include dry beans, peas, clovers, and vetches, among others. Once introduced to a field, SCN cannot be eradicated, only managed.

Since SCN is just starting to get established in Manitoba, prevention is still a valuable management tool. SCN is soil-borne, so anything moving soil has the potential to move SCN. Ensure new equipment is free of soil before entering your fields. Clean soil sampling equipment and field tools between farms. It is likely that SCN is lurking undetected in several fields due to previous flooding along the Red River.

Distribution of SCN populations in Manitoba and North Dakota. The coloured overlay represents different shared watersheds. Orange is the Red River basing and purple is the Assiniboine.

Staging Peas for Desiccation Timing → 

Peas are ready for desiccation, preharvest weed control of swathing when at least 80% of the field is at R7 (full maturity). Scout the least mature areas of the field and check pods and seeds.

At the correct staging, seeds in the bottom pods will be ripe and rattle in the pod. In the middle pods, seeds will have changed colour. In the uppermost pods of the plants, colour change will have started in the seeds and pods will have an orange peel-like texture (wrinkled). When pressure is applied to these seeds (squishing them between your fingers) seeds will split evenly in half rather than squash. If seeds squash, then it is too early.

Seeds in the uppermost pods will split evenly when pressure is applied when they have reached R7 (full maturity).
If seeds squish when pressure is applied, then it is too early for desiccation or preharvest weed control. There is a chance of locking in the green colour and for residue accumulation in the seed.

Watch out for Waterhemp!

Be on the look-out for smooth-stemmed pigweeds

Waterhemp that has been found in Manitoba has been resistant to combinations of groups 2, 5, 9 (glyphosate) and 14. With soybeans and dry beans, it is easy to spot pigweeds poking up above the crop canopy at this time of year.

Check up on those pigweeds you’re seeing in the field. If their stems are rough and hairy, it’s likely red root pigweed. If stems are smooth, it’s time for more testing. You can collect plant samples and have them tested at the PSI lab in Winnipeg or contact Manitoba Agriculture for next steps.

Waterhemp:

  • Emerges throughout the growing season, typically after most other summer annual weeds have been sprayed.
  • Seeds may move with water, so scout along drains, creeks and in previously flooded areas.
  • Flourishes in row crops due to more sunshine in the inter-row spaces.
  • Produces 250,000 seeds per plant, and individual plants may produce more than one million seeds.
  • Is a Tier 1 Noxious Weed, meaning all of the plants must be destroyed.

For more information, watch the August 2nd Crop Talk for a Waterhemp Update in Manitoba →

Lygus Bugs in Faba Beans

Fabas are late maturing, resulting in movement of lygus bugs into fabas once alfalfa is cut or canola is swathed. Hot, dry weather promotes lygus bug development and damage can be worse under these conditions.

Yield losses are not generally a concern, but quality can quickly be impacted. Lygus pierce pods to suck plant sap. Visible damage is found on the seeds due to enzymes in their saliva. Fabas need to have less than 4% perforated damage to be graded No. 3 Canada or better.

Fabas are susceptible to lygus damage until the pods and seeds become firm and most feeding occurs at the top of the plant. Monitor fields during pod development. A nominal threshold to prevent 15% damage is five lygus bugs per 10 sweeps. At research plots in Alberta in 2015 and 2016, a single lygus bug per ten sweeps resulted in 10–12% damage.

While control may be warranted, pollinator insects are important for seed set in fabas and should be taken into consideration when making spray decisions. Having pollinators in your fabas has been shown to increase yield by 17%, on average, so use practices or products that minimize the impact to pollinators.