Early season, The Bean Report

The Bean Report – June 7, 2023

June 7, 2023

  • Crop Update
  • Aphanomyces Root Rot
  • Iron Deficiency Chlorosis
  • Assessing Plant Stands
  • Early-Season Insect and Disease Scouting
  • Pea Leaf Weevil Survey: Call for Participants
  • Herbicide Options for 2023:

Crop Update

  • Soybeans are emerging (VE) to V1 (first trifoliate).
  • Field peas are at V4 to V7 (4 to 7 true leaf nodes). Nodules are starting to form on pea roots, visible as early as V4. Post-emergent weed control applications have been ongoing. Signs of pea leaf weevil feeding are evident in north western Manitoba.
  • Dry beans are emerging to VC (unifoliate).
  • Faba beans range V3 to V4. Signs of pea leaf weevil feeding are evident in north western Manitoba.
  • Field conditions are quite dry across much of Manitoba, with less than 50% normal rainfall. Combined with excessively warm temperatures and high winds, crop stress is beginning to occur.
  • MPSG’s On-Farm Network has started data collection this season with plant counts to evaluate established plant populations. In pea seed treatment trials, pea leaf weevil predation and root rot severity data will be collected this week to assess seed treatment effects.

Root Rot in Peas

Hear about Aphanomyces root rot research from Dr. Yong Min Kim, AAFC – Brandon:

 

In Manitoba, root rots in peas are commonly caused by Aphanomyces and Fusarium root diseases. Aphanomyces root rot thrives in warm, saturated soils, while Fusariums may infect in dry or saturated soils. 

Annually, pea fields are surveyed across Manitoba and soils collected to determine how many fields are infected with Aphanomyces and Fusarium root rots, among other diseases. In 2022, 98% of fields (43/44) had some level of Aphanomyces oospores detected in their soils and 100% of fields had Fusarium infections. If you would like to have your field included in this survey for 2023, contact Laura at 204-751-0538 or laura@manitobapulse.ca to participate.

With root diseases this prevalent, what can we do to manage them? Crop rotation remains the most successful tool.

Once Aphanomyces root rot has been confirmed in a field, rotate away from peas and other hosts for 7 or 8 years to draw oospore levels down. If growing conditions were wet the last time peas were grown in a field, take a longer rotational break. Other hosts for Aphanomyces to avoid include lentils, some varieties of alfalfa and dry beans, clovers, vetches, shepherds purse and chickweeds. Non-hosts or resistant legumes you can incorporate into the rotation in the meantime include soybeans, faba beans, some varieties of alfalfa, sainfoin and birdsfoot trefoil.

Iron Deficiency Chlorosis in Soybeans

Up to V1, soybean cotyledons supply stored iron to the plant. Once that source is depleted, plants need to acidify their root zone to access iron in a plant-available form. Excess moisture, calcium carbonates, soluble salts and nitrates can all impact this.

If chlorosis is present early on and plants recover by the V5 to V6 stages, yield loss will be minimal. If symptoms persist beyond these stages, expect more significant yield loss. Research conducted in Manitoba has shown that for each 0.1 unit increase in IDC score, soybean yield is estimated to be reduced by 2.2 bu/ac, on average (range: 1.5 – 2.8 bu/ac).

Research: Iron Deficiency Chlorosis – Kristen MacMillan, MPSG’s Agronomist-in-Residence, Soybean and Pulse Agronomy Lab, University of Manitoba (2017-2022).

Assessing Established Plant Stands

Take plant counts each year to assess plant populations, estimate seed survival and inform future seeding rate decisions. Percent establishment equals plant count divided by seeding rate – calculate this and consider areas where you can improve establishment each year to dial in seeding rates next year.

Higher plant populations could mean increased yield potential, but also a greater risk of disease pressure. Lower plant populations mean more attention should be paid towards weed control. When soybean populations are so low that you’re considering replanting or topping up soybean stands, review information on soybean replanting decisions →

MPSG’s Bean App plant stand assessor tool is available to calculate plant populations:

  • Use the fixed area method to calculate plant populations if using a hoop or a square or the row length method if using a measuring tape or stick.
  • Enter your hoop diameter, area of square or length of row measured and the number of plants in that area.
  • Assess multiple spots throughout the field and let the tool calculate plants/acre for you.
    • Check poor areas as well as good areas and investigate reasons for poor emergence.
  • The app feedback is specific to soybeans, but the calculator itself can be useful for other crops.

Early-Season Insect and Disease Scouting

Scouting for early-season insects →

  • Cutworms – scout field areas that are slow to emerge or have wilted, yellow or missing plants. Dig around damaged plants to find cutworms. They move deeper in the soil during the heat of the day. It may be easier to spot them under cool conditions or in the evening when they feed.
    • Nominal thresholds:
      • Soybeans and dry beans: 1 or more cutworms (<2 cm in length) per metre of row, or 20% of plants cut
      • Peas and faba beans: 2-3 cutworms (<2 cm long) per square metre
  • Wireworms – assess crops from planting until V3. Dig into the soil to assess damage to underground parts of young seedlings.
  • Seedcorn maggot – an infrequent pest with no established economic thresholds. Cut open suspect seedling stems to find the white to reddish brown larvae.
  • Pea leaf weevil – assess field peas and faba beans from emergence to V6 (sixth true node stage) looking for signs of leaf notching. A newer pest to Manitoba, populations are generally quite low for most of the province, excluding the northwest where they are moderate and increasing.

Scouting for early-season diseases →

  • Check areas of patchy emergence, yellowing or wilting for signs of:
    • Root rots in all pulses caused by Fusarium, Rhizoctonia and Pythium 
    • Phytophthora sojae in soybeans
    • Aphanomyces in peas

Pea Leaf Weevil Survey: Looking for Participants!

Peas are surveyed at V6 to evaluate the distribution and abundance of pea leaf weevils in western Manitoba. We’re looking for pea fields to survey!

Pea leaf weevil feeding damage is assess by counting the number of leaf notches per plant. This directly translates to the abundance of pea leaf weevils in the area. While this feeding damage is quite visible, it does not cause yield loss. Weevil larvae burrow belowground to feed on root nodules, robbing the plant of its nitrogen fixation. Learn more about this pest here →

To participate in the pea leaf weevil survey, contact Laura at 204-751-0538 or at laura@manitobapulse.ca.

Characteristic leaf notches on pea leaf margins caused by adult pea leaf weevil feeding.