Soybean Diseases, Soybean Insects

Scouting for Early Season Disease & Insect Pests

Crop pests such as diseases and insects become more prevalent over time with frequent production of a particular crop. With the continuing rise of soybean production in Manitoba, a gradual increase in soybean pest pressure will follow suit, similar to other crops in the past.

Soybean and pulse crops are now entering the early V-stages and advanced node stages (as with field peas), but it is important to continue scouting for seedling diseases and root rots—some of which can appear throughout the growing season. These diseases can be difficult to diagnose in the field, so lab testing is needed for confirmation. Suspected disease or insect samples can be sent to the Manitoba Agriculture Crop Diagnostic Centre.

Scouting & Diagnosis of Seedling Diseases & Root Rots

  1. Check several areas of the field in a “W-pattern” visually inspecting plants for abnormalities and symptoms.
  2. If root rot is suspected and you want to submit a sample to the lab, dig around the plant to obtain the full root system. Leave a small amount of soil around the root.
  3. Wrap roots in foil to the soil line and crimp the foil to keep soil from moving onto the cotyledons or leaves. Place the foil-wrapped plant in a plastic bag.
  4. Ship or drop off samples the day they are collected. Shipped samples should be in a container or cardboard box to avoid crushing. Avoid sending samples immediately before the weekend.
  5. Do not collect or ship dead plants. Do not wash seedlings prior to submitting.

Adapted from the Crop Protection Network.

Seedling Disease & Root Rot Symptoms

General symptoms include poor emergence and root development, yellowing, root discolouration and lesions on the root or stem tissue near the soil line. Crop rotation is the best way to manage seedling diseases and root rots. See images of the following diseases here.

Pythium spp. is considered a “water mould”. It can affect soybeans, pulses, cereals, canola and alfalfa. Symptoms include water-soaked lesions on the hypocotyl or cotyledons. Diseased plants pull easily from the soil.

Rhizoctonia solani can also affect soybeans, pulses, cereals, canola and alfalfa. Characteristic symptoms include reddish-brown lesions that remain firm and dry on the hypocotyl at the soil line, or on the roots.

Fusarium spp. hosts include soybeans, pulses, cereals, canola and alfalfa. Symptoms include discoloured (brown or pink) root tissue. Severe infection will result in wilting and leaf death.

Phytophthora sojae also known as Phytophthora root rot (PRR), only affects soybeans. All soybean growth stages are susceptible. Infected seedlings will exhibit water-soaked stems or dark brown lesions low on the stem. This disease is characteristic of wilted leaves remaining on the plant. Selecting PRR-resistant varieties is one method of control.

Aphanoymyces euteiches is also considered a “water mould.” Hosts include field peas, dry beans and lentils (soybeans are resistant, faba beans and chickpeas have partial resistance). Caramel coloured roots are characteristic of this disease.

Scouting for Early Season Insects

Insects to scout for at this time include cutworms, wireworms and seedcorn maggot. Inspect fields in a “W-pattern” for signs of poor emergence or feeding damage on above and below-ground portions of the plant. Seed treatments only provide early-season protection against wireworms and seedcorn maggot.

Cutworms should be identified at a species level to determine feeding behaviours and economic thresholds. The most common species in Manitoba are the dingy, redbacked and darksided cutworm. Symptoms include clipped plants, holes in leaves or gouged edges of cotyledons or leaves. Cutworms have a medium economic impact in Manitoba and insecticide control is available if necessary. Click here for more information on cutworms.

Wireworms can affect a wide range of crops in Manitoba, including soybeans and pulses. Due to a four to seven year life cycle, they may persist in a field for several years. Beans should be scouted from planting to V3. Wireworms bore holes into plants or feed on roots. Insecticides do not provide rescue control.

Seedcorn maggot is an infrequent pest of soybeans, dry beans and corn in Manitoba. Seedcorn maggot may be found in manured or heavy residue fields. Scouting should take place from planting to VE. Symptoms include hollowed out seeds or cotyledons. This insect has a low economic impact in Manitoba. No rescue insecticides are available.