JULY 3, 2024

• Crop Update
  • Grasshoppers in Soybeans and Pulses →
  • Scouting for Pea Aphids →
  • Root Disease in Field Peas & Sampling →
  • MPSG Crop Staging Guides →
• Assess Soybean Nodulation →
• Assess Field Pea Nodulation →
• Fungicide Decisions in Peas →
• Hail Damage in Soybeans

Crop Update 

• Soybeans range from V2 to V3, some have flower buds.
  • IDC is widespread in many soybean fields due to the high rainfall received and saturated soil conditions.
  • Ensure inoculation strategies were successful ahead of pod development and filling stages by checking up on soybean nodulation. Assess soybean nodulation →
  • Grasshopper nymphs have been found in headlands, defoliation has been found to be below thresholds.
  • Hail has hit several soybean fields. Yield loss depends on the amount of defoliation and stem nodes removed. More information below.
• Field peas range from V9 (nine true leaf nodes) to  R2 (flowers open at one or more nodes).
  • Mycosphaerella (Ascochyta) blight is being found in field peas, it is more common to find in the lower pea canopy and is the disease we target with our fungicide application(s). Fungicide applications have started and are on-going. Fungicide Decisions in Peas →
  • Bacterial blight is wide spread this year due to sandblasting, strong winds/rains and hail damage.
  • Pea aphids are present at low levels in some pea fields. Scouting for Pea Aphids →
  • Yellowing of peas and root rots are being found in low spots or areas holding moisture. Root Diseases in Field Peas & Sampling → 
• Dry beans range from V1 to V3.
  • Many fields are still recovering from wind damage and sandblasting, some root rots are developing.
  • Potato leaf hoppers are a pest of dry beans and something to keep your eye out for. You will see triangular brown areas at the tips of leaves or dwarfed, crinkled leaves are a sign of hopper burn. Examine the undersides of leaves to look for potato leaf hoppers and determine the number per trifoliate on average. At V4, the economic threshold is 1 leaf hopper/trifoliate and at R1 is it 2 leaf hoppers/trifoliate.
• Faba beans range from R1 to R3 (50% bloom).
  • Pea aphid thresholds for faba beans are 24-50 aphids per main branch. This gives a seven-day lead time before populations reach economic injury levels (96-142 aphids/main branch).
• Assessing grasshopper damage in soybeans and pulses →
  • Defoliation thresholds:
    • Soybeans: 30% defoliation pre-bloom; 15% defoliation from R1 (flowering) to R5 (beginning seed).
    • Dry beans: 35% defoliation pre-bloom; 15% defoliation after R1 (flowering).
    • Peas are not a preferred food source for grasshoppers. Infestations of 10/m² (1/ft²) do not cause economic losses in peas.

Assessing Nodulation

SOYBEANS

Assess nodulation in soybeans at flowering (R2). Soybean nitrogen requirements peak during pod fill, so assessing nodulation at flowering will provide enough lead time to come in with a rescue treatment if one is needed.

• Dig up 5-10 plants from 2 or 3 different areas of the field at R1-2 (flowering).
• If soils are heavy-textured and sticking to the roots, soak roots in a pail of water to loosen up the soil. Nodules can easily be stripped from the roots.
• Count the number of nodules per plant. Research has shown that soybeans need 10 nodules per plant, regardless of size, to reach 90% of maximum yield.
• Cut open a few nodules to check that they are pink-ish red inside, meaning they are actively fixing nitrogen.
• Note the location of the nodules – successful seed-applied inoculant will result in nodules clustered around the main taproot near the crown of the plant, while in-furrow applications may result on nodules further from the crown. Naturalized Bradyrhizobia populations may form nodules further down lateral roots.

If insufficient nodulation is found and the crop appears pale green to yellow, a rescue application may be necessary at R2 (full flower) to R3 (early pod). Broadcast granular N or direct liquid N below the crop canopy to avoid leaf burn. Apply at a rate of 50 lbs N/ac.

More Information on Assessing Soybean Nodulation →

PEAS

Assess nodulation in peas at flower bud (R1) to flowering (R2) stages to capture peak nodulation. Peas have indeterminate nodules that branch as they grow, forming globular or cylindrical shapes.

• A scoring card is used to assess nodulation.
• Select 5-10 plants from 2-3 different areas of the field.
• Visually assess above-ground growth and vigour.
• Using a shovel, carefully dig up plant roots. If soils are heavier, roots may need to be soaked before rating.
• Count the number of nodule clusters and note their location (at the crown, near the seed, or on the lateral roots). Break open a few nodules to determine if they are pink inside, indicating active N fixation.

More Information on Assessing Nodulation in Peas →

Foliar Fungicide Decisions in Peas →

• Mycosphaerella (ascochyta) blight is the main disease target of foliar fungicides in peas.
• This disease infects most pea crops at some severity level each year. The impact on yield is determined by how early the disease sets in and how quickly it progresses into the upper canopy. Early infections during R2 to R3 cause the most damage.
  • Fungicides may also provide control of suppression of white mould, downy mildew, anthracnose and other diseases depending on the product. Foliar Fungicide Options in Peas (2024) →
• Scout for symptoms from V10 (tenth true leaf node) to R2 (beginning bloom). Use the Fungicide Decision Worksheet for Managing Mycosphaerella Blight in Field Peas to assess if a fungicide application may be beneficial in your field.
• Fungicide applications are typically applied at R2 when one flower is open on most plants across the field. If warm, humid weather persists and the disease progresses up the canopy, consider a second fungicide application 10 to 14 days later using a different mode of action.
  • It is likely that the Mycosphaerella pathogen is resistant to group 11 strobilurin fungicides. If applying a foliar fungicide to manage Mycosphaerella blight, avoid applying a group 11 product alone.
• Due to recent hail and wind storms, bacterial blight is showing up in pea fields as well. This disease is commonly confused with Mycosphaerella blight and bacterial blight will not be controlled by a fungicide. Find photos and descriptions on how to tell Mycosphaerella Blight vs. Bacterial Blight →

Evaluating Hail Damage and Recovery in Soybeans

How much yield loss can you expect once your soybeans have been hit by hail?

From 2015 to 2018, research was conducted to evaluate the effects of simulated hail damage on soybean yield and days to maturity at Portage and Minto, MB by Kristen MacMillan, UM-MPSG Agronomist-in-Residence. Soybeans at the full pod (R4) and early seed fill (R5) growth stages were the most sensitive to leaf loss and stem breakage.

To estimate yield loss following hail:

1. Assess plant stand loss, if any. Plants can regrow from axillary buds on remaining stem nodes.
2. Estimate percent stem breakage. At R1, plants often have 5 nodes (4 trifoliate nodes and one unifoliate node). Stem breakage = nodes removed / total nodes the crop had before hail.
3. Estimate percent defoliation of remaining plant tissue. Is it closer to one-third, two-thirds or 100% of leaf material lost?
4. Using the chart below, add the loss percentages of defoliation and stem breakage together to estimate yield loss.

Days to maturity may also be impacted by hail damage. At R1, 100% defoliation caused a 4-day delay in maturity, while other defoliation severity levels did not impact maturity. At R1-R2, 60-80% node removal resulted in a 4 to 6-day delay in maturity, while 20-40% node removal matured 2 to 3 days later than plants with no damage.

Soybean Yield Response to Defoliation and Node Removal – Research Reports from the Soybeans and Pulse Agronomy Lab →

Case Study from the Field: Soybean Regrowth Following Hail Damage

Last summer, many fields were caught in the crosshairs of various hailstorms. We followed one field through the rest of the summer to observe regrowth potential.

Hail occurred on June 28 and estimates were made the following day. Soybeans were flowering at the time (R1-2) and were left with one, two or no trifoliate leaves remaining per plant and one to two nodes were broken off at the top of the plant. Damage was fairly even throughout the field. Comparing to healthier plants near the shelterbelt, 66-100% defoliation and 20-40% stem breakage occurred, on average. Luckily, very little plant stand loss was observed in the field and plant stand remained at roughly 160,000 plants/ac.

Looking to the research results to estimate yield loss, losing two-thirds of the leaves at R1 resulted in 16% yield loss, while complete defoliation caused 40% yield loss in the small-plot research. For stem breakage at R1-2, losing one node (20% breakage) resulted in 2% yield loss and 8% yield loss when two nodes were broken (40% breakage). Adding those numbers together, and we were anticipating yield losses of 18-24% in the least damaged areas and losses as great as 48% for those beans hit with the worst of the storm.

Revisiting the field throughout the growing season showed the impressive capacity soybeans have to compensate in their growth through branching and increasing number of pods per plant and the number of seeds within those pods. By mid-August, the crop canopy was thick with branches and leaves at R5-6. The only downside was these beans were 12-14” tall and late-emerging weeds had taken advantage of those mid-season openings in the damaged canopy.

In terms of maturity, it looked like these hail-damaged soybeans were about a week to 10 days behind other beans in the area. And as the season progressed the maturity delay become more apparent. Using the small-plot research results, based on the timing of hail and severity of damage, roughly an 8 to 10 day maturity delay would have been expected.

Come early September, plants were at R6.5 with 37 pods per plant and 2.7 seeds per pod, on average. Previous research by Kristen MacMillan has also outlined average yield components for Manitoba soybeans, finding 29 pods per plant and 2.3 seeds per pod on average. These hail-damaged soybeans compensated for the hail damage, in part by increasing these two yield components.

Pre-harvest weed control was necessary to manage green weed material and the field was harvested in mid-October after some weather delays.

How did yields compare at the end of the season to the yield loss estimates?

The average soybean yield for the last five years in that region has been 34 bu/ac, according to MASC data. Estimating a whole-field average yield loss of 18-24%, we’d expect 26-28 bu/ac out of this field. On the whole, the crop averaged roughly 42 bu/ac. Oddly enough, that works out to 24% above the area’s yield average.

Now, we wouldn’t expect this scenario in every field or year. Stars aligned for excellent soybean yield potential despite the hail damage. But this does showcase an impressive ability for soybeans to recover from damage at flowering.

Two other on-farm trial fields also received moderate to severe hail damage in early August near Elm Creek and St. Pierre when soybeans were at R5. We didn’t follow those fields as closely throughout the season, but hail assessments were made. Near Elm Creek, 30% defoliation was noted, along with some stem breakage. That trial averaged 40 bu/ac at harvest. And at St. Pierre, 75% defoliation was noted and the field averaged 19 bu/ac. At that site, moisture was limiting for most of the season, impacting yield potential to begin with, as well as plant recovery after hail.