Soybean Fungicide and Insecticide Seed Treatments

The majority of soybeans planted in Ontario do not receive a fungicide seed treatment. The rationale is that since soybeans tend to be planted later than corn, soil conditions are generally more favourable for rapid germination and emergence. However, when conditions are wet and cool, soil borne diseases cause considerable seed and seedling damage. The extent of the damage depends on moisture, temperature, overall plant health, and soil type. Cold wet soils, crusting, heavy rains, compaction, and even post-emergent herbicides can all cause plant stresses, that make the seedlings more susceptible to disease.

Early season soybean insect feeding has become more of a problem during the past few years. In the spring of 2007, growers experienced very high over-wintering bean leaf beetle populations and the earliest ever appearance of soybean aphid populations. The introduction of thiamethoxam (Cruiser) by Syngenta Crop Protection offers growers a new tool for controlling early season soybean insects. 

A project was initiated in 2004 by University of Guelph and Ontario Ministry of Agriculture, Food and Rural Affairs to evaluate the efficacy of soybean seed treatments on new and expanding pests such as aphids, bean leaf beetles and early season root rot. 

Methods

Plots were established in 35 fields across southern Ontario from 2004 to 2007. Multiple locations across a wide area increased the potential for fields with varied insect and disease levels. Treatments were in a strip plot design, 10 by 410 feet, with 3 replications per treatment. Check plots were monitored twice a week from soybean emergence to the V2 stage for the presence of root disease and soil pest insects, such as European chafer, wireworm, and seed corn maggot. Plant populations were determined 21 days after emergence. Vigour ratings were determined subjectively on a scale of 0-100%. The plots were monitored weekly from late-June until mid-August for additional insect pests, such as bean leaf beetle, potato leafhoppers, and soybean aphids. When aphids were detected, counts were recorded. Seed yields and harvest moistures were recorded. 

Treatments:

1. UNTREATED CHECK (no fungicide or insecticide seed treatment)
2. MAXIM APRON
3. MAXIM APRON + CRUISER @ 50 g per 100 kg of seed
4. MAXIM APRON + GAUCHO @ 120 g per 100 kg of seed 

Results and Summary

Plant stand counts were taken 21 days after seeding. Averaged across all sites, counts were higher by approximately 3% (4,300 plants/acre) for the Maxim Apron, 7.5% (10,500 plants/acre) for the Maxim Apron + Cruiser and 6.7% (9,400) for the Maxim Apron + Gaucho compared to the untreated check. Refer to Table 1.

 Table 1: Soybean Seed Treatment Effect on Plant Stands

Yields were higher for Maxim-Apron in 21 of 35 trials compared to the untreated check, but this yield increase was “statistically significant” in only 3 of the trials. (p < 0.10). Maxim Apron plus an insecticide seed treatment yielded higher at 28 of 35 sites, but only 6 of these were statistically significant (p=0.10). Refer to Table 2.

 Table 2: Soybean Seed Treatment Effect on Yield

The magnitude of soybean response to seed treatments depended mainly on the presence of root rot diseases, insect pressure, soil type, and weather. The greatest yield response was on clay and clay loam soils. Fields that suffered from soil crusting after planting had a greater response than those with little or no emergence problems. At one site where crusting was evident, Maxim-Apron increased plant stands by 38%. At the sites with a statistically significant yield response, rhizoctonia and pythium root rot were the main disease problems. At two sites where pythium reduced plant stands, yields were increased by an average of 32% or 11 bu/ac. Averaged across sites, Maxim Apron + Cruiser increased plant stands by 10,500 plants/acre and increased yields by 1.9 bu/ac. Maxim-Apron + Gaucho increased plant stands by 9,400 plants/acre and increased yields by 2.1 bu/acre. Seed treatments containing insecticides significantly reduced early populations of bean leaf beetle when they were present (up to a 60% reduction).

Significant aphid populations were only observed in these trials during 2005. Insecticide seed treatments kept aphid levels lower than the untreated check for the first 40-60 days after planting, but they had little affect on aphids after that point.

In this set of experiments, the fungicide Maxim-Apron increased yields by up to 32% when high levels of root rot were present and when fields suffered from crusting. This occurred in 3 out of 35 fields. When conditions were excellent for emergence and early growth, no yield benefit was realized. The use of an insecticide was only beneficial when early season bean leaf beetle, aphids or seed corn maggots were a problem.

Figure 1: Bean leaf beetle - overwintering generation significantly reduced with Crusier Max