Mid-season, The Bean Report

The Bean Report – July 19, 2023

July 19, 2023

Crop Update

  • Soybeans range from R2 (full flower) to R5 (beginning seed).
    • Many fields are showing signs of drought stress, where beans have flipped over their leaves to reflect more sunlight and conserve moisture. Drought conditions can also results in plants moving through development stages more quickly and result in shorter plants with smaller leaves. Nodule growth and development may also be impacted, with fewer larger nodules observed on drought-stressed plants. These larger nodules are more resilient to drought stress, but N fixation may be reduced. Peak water uptake for soybeans is at flowering (R1-2), but the most critical time for water uptake is at pod initiation (R3-4) and seed fill (R5-6) stages. Water limitations may also impact nutrient movement into the plant, resulting in deficiency symptoms. Water limitations in pulse and soybean crops →
    • Several fields are also re-growing from previous hail damage. Now that the crop is at the R3-4 growth stages, hail damage at this time is expected to have a greater impact on yield and maturity than earlier storms. Yield loss estimates due to defoliation and node breakage from hail →
    • Soybean aphids have blown in on southern winds. Populations remain low in most fields and natural enemies like ladybeetles and green lacewing larvae are able to manage populations. More information on managing soybean aphids →
    • Two-spotted spider mites have been found at low levels in some fields in central Manitoba. As a pest that prefers hot, dry weather, their populations may increase with the lack of rainfall in some areas. Look for white speckles on the tops of soybean leaves. When you turn the leaves over you may see the tiny mites crawling around underneath. Populations are typically the greatest at field edges. More information on two-spotted spider mites →
    • Disease pressure has been low across the province. Bacterial blight symptoms are apparent in areas where high winds or storms created wounds for the disease to infect. Septoria brown spot is showing up in the lower crop canopy as small, brown lesions. Phytophthora root rot and northern stem canker may be found in some fields that have received more moisture.
  • Field peas range from R3 (flat pod) to R5 (beginning maturity).
    • Once peas have reached the full pod stages where seeds fill the pod to capacity (R4), fungicide applications are no longer recommended since peas are within the 30-day pre-harvest interval of several products and because disease is no longer expected to influence yield. Also, once peas have reached this stage they are not as sensitive to pea aphid feeding and control is no longer warranted.
    • If considering a second application of fungicide in peas, scout to see if disease symptoms have progressed up the plant into the mid or upper canopy and if it is humid below the crop canopy. If these conditions are occurring, and rain is in the forecast a second application may be warranted. Use the fungicide decision worksheet → In on-farm trials, when conditions have been right for disease development, a second application improved pea yield 44% of the time, improving yield by 5.1 bu/ac on average. On-farm pea fungicide results poster →
  • Dry beans range from R2 (pin bean, beginning podding) to R4 (full pod).
  • Faba beans range from R4 (flat pod) to R5 (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) →
    • Pea aphid thresholds in faba beans are higher than in peas. The action threshold of 34-50 aphids/main branch gives a 7-day lead time before injury levels are reached (96-142/main branch). Include natural enemies such as lady beetles, hoverfly larvae and others when considering management decisions. Faba bean yields are improved by pollinators in the field, so use practices that minimize effects to pollinator species.
    • Chocolate spot in faba beans →

Developing Improved Diagnostic Tools for Soybean Diseases

Two new rapid, cost-effective diagnostic tools were developed by Dr. Bryan Cassone and his lab at Brandon University. These tools simultaneously diagnose and quantify up to four stem or foliar diseases infecting soybean plants.

Phytophthora root rot and northern stem canker can be difficult to visually tell apart in the field.

Scouting for soybean diseases has typically relied on a farmer or agronomist’s ability to visually identify symptoms accurately. This can be troublesome with common look-alike diseases like Phytophthora root rot and northern stem canker. Over the last three years, Dr. Cassone’s lab has developed new molecular diagnostic tools than can simultaneously detect up to four stem diseases (Phytophthora root and stem rot, northern stem canker, white mould and Fusarium root and stem rot) or up to four foliar diseases (Frogeye leaf spot, cercospora leaf blight, alternaria leaf spot and bacterial blight).

These tools have cut their teeth on diseased soybean plant samples from fields across Manitoba in 2020 and 2021 and are now ready for deployment. They were confirmed to be sensitive, highly accurate and cost-effective, estimated to cost less than $10 per plant sample tested. The turn-around time for results is expected to be as little as a few hours. Using these molecular methods, they were also able to diagnose diseases infecting soybeans before visible lesions occurred on the plant.

Now that these tools are developed, the molecular infrastructure and expertise needed to run these diagnostics are widely available in laboratories in Manitoba. Next steps for this research include the need to develop a lab service that farmers and agronomists may submit field samples to for disease diagnostics using these rapid tools.

Diagnosing Nutrient Deficiencies

Suspected nutrient deficiency symptoms may confirmed using plant tissue analysis paired with soil tests. To investigate further, sample both good and bad areas of a field to distinguish which nutrient, if any, are limited. Pair plant tissue samples with soil samples. Taking the extra step to gather soil samples is useful to determine if there are underlying reasons for the nutrient deficiency. It may be that there’s enough nutrient in the soil, but something like moisture availability, salinity, compaction, pH, etc., is impeding its uptake into the plant.

Avoid sampling plants that have been showing visible symptoms for more than 10 days, since at that point uptake of other nutrients may be impacted as well and lead to misleading results. For soybeans and dry beans after flowering, pluck the first fully developed trifoliate leaf from the top of the plant. Collect leaves from at least 25 plants. Keep samples cool and dry until they may be shipped for analysis.

Nitrogen-deficient soybeans (left) are lighter green in colour and shorter than healthier soybeans to the right. N deficiency was confirmed using a paired tissue and soil test, comparing ‘good’ areas of the field and ‘bad’ areas.

Phytophthora Root Rot in Soybeans

  • Phytophthora root rot (PRR) is a root and stem disease specific to soybeans that can infect at any growth stage.
  • Symptoms appear as brown lesions that extend upwards from the soil line, and roots are discolored and unhealthy. Plants will be wilted and leaves remain attached to the plant. Infection occurs in patches or as randomly-infected plants, often in headlands.
    • One look-alike to PRR is Northern Stem Canker where plants will appear wilted and there will be a brown lesion along the stem. But with stem canker, roots will appear healthy and the lesion won’t extend upwards from the soil line. Stem canker often also infects following stem damage from hail or storms.
  • The best strategy to manage Phytophthora is crop rotation. A four-year break between soybean crops will help break up this pathogen’s life cycle.
  • The next best strategy is variety resistance, of which there are two types:
    • Race-specific or major gene resistance: a resistance gene is completely resistant to specific pathotypes or races of PRR in the field. Multiple races of PRR can occur in a field, so knowing which races of PRR are present is crucial to use this variety resistance.
      • There is now a soil test available to identify the races of PRR in a field from AYOS technologies. Last year, 11 soybean fields were sampled as part of a pilot program to use this soil test to identify PRR races in individual fields. Of the 11 fields, 100% of them had races of Phytophthora that could overcome resistance genes 1a and 1c. 55% and 64% of PRR in those fields were also able to overcome rps genes 1k and 3a, and at 3 fields (27%), rps gene 6 was also overcome.
    • Field tolerance or partial resistance: several genes resist infection within the plant and the plant is better able to tolerate the PRR infection without dying. This resistance is only expressed after the first true leaves are open, so it needs to be used in combination with an effective seed treatment. New this year as part of the variety trials, field tolerance for individual varieties will be assessed to provide independent information on each soybean variety’s level of field tolerance to PRR. Stay tuned for results in the fall.
  • More information on Phytophthora Root Rot →
Wilted soybeans plants infected with Phytophthora root rot.

Two-Spotted Spider Mites

Spider mites love hot, dry conditions and have been moving into soybean fields around the province recently. They may also move in after previous pyrethroid applications that have killed off mite predators. An early infestation appears as white speckling on the leaves, then progresses to a more webbed-over, chalky appearance. The actual mites can be found on the undersides of leaves, and you may need a magnifying glass to see them.

Consider insecticidal control if 20% of the leaves are infested. The mites move into field edges, progressing inward, and form patches within the field, so spot spraying is often the best option.

On-Farm Network Soybean Inoculant Trials Update

Nodulation ratings recently wrapped up in on-farm double vs. single inoculant and single vs. no inoculant trials.

Four trials in central Manitoba are comparing a single application of liquid on-seed inoculant to no inoculant at all. In these fields, there have been at least three previous well nodulated soybean crops. Nodulation ratings show no significant differences in nodulation between inoculated soybeans and uninoculated soybeans in these trials, which is consistent with the previous seven years of this trial type. With these four trials in 2023, there will have been a total of 42 trials comparing single inoculant vs. none.

Another seven trials across Manitoba are comparing a double application of granular in-furrow and liquid on-seed inoculant vs. a single application of liquid inoculant on-seed. In these trials, fields have had at least two previous well nodulated soybean crops. Nodulation ratings for this trial type also indicate no significant differences between treatments. This trial type started in 2013 as fields gained more soybean history. So far, it has been infrequent (6% of the time) to see a yield response to a double inoculant strategy if a field has sufficient soybean history (more than two previous soybean crops and the last crop within the last four years). With these seven trials in 2023, there will have been a total of 56 trials comparing double vs. single inoculant trials.

Watch the On-Farm Network Results Summary Video on Soybean Inoculant Strategies (13 minutes) →