Figure 2. Soybean stem tips wilting is a symptom of Fusarium root rot and wilt. Image: D. Mueller

2020 was an exceptional year for many Ontario soybean growers. Although not all regions had good yields, much of the province harvested outstanding soybeans. The overall provincial average will be one of the highest on record at 50.7 bu/ac.  In several cases growers were able to achieve over 80 bu/ac.  The 10-year average for those reported acres is 47 bu/ac. The previous highest yield was in 2018 at 51.4 bu/ac. What also made 2020 exceptional is the high prices that materialized this fall. It’s not often that both high yields and high prices occur at the same time. For those that were able to harvest a good crop and sell at excellent prices, 2020 will be remembered as one of the most profitable soybean growing seasons to date.

Although much of June and July was very dry, for those that received rain in August these dry conditions actually helped the soybean crop. Early season dry conditions drive roots deeper and minimizes diseases that prefer moist conditions, such as pythium and white mould. These deeper roots are then able to pick up more nutrients when the plant needs them the most, in August. Good root development, low disease pressure, rain in August and high solar radiation levels (sunlight) all worked together to achieve good yields in 2020. For those that missed August showers, yields were below average. An open fall meant that most soybeans were harvested early enough for timely winter wheat seeding.

Plant Stand Establishment

Although a perfect plant stand is not as critical for soybeans as it is for corn, higher soybean yields are often associated with more uniform plant stands. There are numerous factors that influence emergence, including planting equipment, seed quality, crusting, temperature, and planting depth, among others. Research trials from 2020 highlight the importance of proper planting depth. Due to the high-water demand of soybean seed to germinate, it’s recommended to plant soybeans at least ½ inch into moisture. This often means a total seeding depth of at least 1.5 inches. Some growers prefer to plant soybeans 2 inches or deeper even if moisture is present. 2020 trials showed that the best plant stands were achieved from planting less than 2.0 inches deep, if adequate moisture is present. See Table 1.

Table 1. Soybean Plant Stands at Various Planting Depths (plants/ac X 1000)

Planting DepthPlanted April 22Planted May 22Planted June 10
1.0 inches132142165
1.5 inches134135159
2.0 inches122111153
2.5 inches95117130

No-till soybeans planted in 15-inch rows at 175 000 seeds/acre

Lower yields were realized when seeding was 2.0 inches or deeper for the first two planting dates. See Table 2. The 2.5-inch planting depth lowered yield in every case compared to a 1.5” depth. On average the 1.5” planting depth provided the most consistent yields.

Table 2. Soybean Yields at Various Planting Depths (bu/ac)

Planting DepthPlanted April 22Planted May 22Planted June 10
1.0 inches62.965.751.6
1.5 inches64.166.350.8
2.0 inches60.860.149.8
2.5 inches59.560.947.1

No-till soybeans planted in 15-inch rows at 175 000 seeds/acre

Spider Mites

Under the dry conditions experienced in July, spider mite populations proliferated quickly this year. Mites feed on individual plant cells on the underside of leaves leaving stipples. Severe stippling causes yellowing, curling and bronzing of leaves. Spider mites usually start on the edge of the field, but wind can carry them to any part of the field. From the road these pockets may look like moisture stress. Fields that are close to neighboring winter wheat stubble, hay fields and no-till fields are more at risk. This pest appears to be on the increase in Ontario, and some fields suffered significant yield losses in 2020, especially where conditions remained dry into August.

Figure 1: Spider mite damage causing yellow, stunted plants along a field edge. Image: H. Bohner
Figure 1: Spider mite damage causing yellow, stunted plants along a field edge. Image: H. Bohner

Fusarium Wilt

Fusarium wilt caused by the soil-borne fungus Fusarium oxysporum was widespread this July. Plant infection occurs under wet conditions, but symptoms often go unnoticed until the soil dries out in the summer. Fusarium oxysporum causes a wilting of the soybean plant which can easily be misdiagnosed as phytophthora root rot or stem canker. Affected plants have a wilting of the stem tips and the upper leaves are scorched. The middle and lower leaves can turn yellow or have pale (dull) yellow spots. In severe cases the leaves will dry up and drop prematurely, leaving the petiole behind.

Unlike Phytophthora, there is no evidence of a stem lesion or external decay that goes above the soil line. Fusarium oxysporum survives in the soil or in plant residue. Plant stress caused by herbicide injury, high soil pH, nematode feeding, and nutrient deficiencies can all predispose plants to infection. Reducing stress factors will help the plant fight off root rots. No resistant varieties are available, but fungicide seed treatments will help protect early seedlings from being infected.

Figure 2. Soybean stem tips wilting is a symptom of Fusarium root rot and wilt. Image: D. Mueller
Figure 2. Soybean stem tips wilting is a symptom of Fusarium root rot and wilt. Image: D. Mueller

Weeds

Farmers and agronomists expressed frustration in controlling several weeds during the 2020 season. August rainfall, although beneficial to the soybean crop, stimulated germination of species like pigweed and crabgrass. Late germination of these weeds made for dirtier fields, and in some cases, requiring preharvest herbicide applications. Lamb’s-quarters, bluegrass, wild carrot, ragweed, Canada fleabane, perennial sow-thistle and waterhemp were often cited as being “difficult to control” or requiring special attention. Herbicide tolerant soybean cultivars like “E3 – Enlist” and “Xtend” provide better options to deal with herbicide resistant broadleaf weeds in the post-emergent application window.

Regardless of soybean cultivar grown, many growers are seeing the value in using a soil applied herbicide for better season long control of weeds like lamb’s-quarters, bluegrass and waterhemp. Soil applied herbicides will remove most early emerging weed “flushes”, causing any second flushes of weeds to be of lower density and more uniformly sized.  Waterhemp was detected in more fields during the past season and is now found in 13 counties across the province. When waterhemp is tested for herbicide resistance, most plants are resistant to multiple herbicides and most commonly to glyphosate, “group 2” herbicides (e.g. Classic, Pursuit) and group 14 herbicides (e.g. Reflex). While some waterhemp populations are also resistant to “group 5” herbicides (e.g. Sencor).

Figure 3. Waterhemp has been found in the following counties: Brant, Bruce, Chatham-Kent, Elgin, Essex, Haldimand, Huron, Lambton, Middlesex, Norfolk, Northumberland, Wellington and Wentworth (Hamilton).  For more information on the management of Waterhemp see: https://fieldcropnews.com/2020/07/waterhemp-biology-and-control/ Image: M. Cowborough
Figure 3. Waterhemp has been found in the following counties: Brant, Bruce, Chatham-Kent, Elgin, Essex, Haldimand, Huron, Lambton, Middlesex, Norfolk, Northumberland, Wellington and Wentworth (Hamilton).  For more information on the management of Waterhemp see: https://fieldcropnews.com/2020/07/waterhemp-biology-and-control/ Image: M. Cowborough