Image 1. Frogeye leaf spot lesion on the underside of the leaf. Note the grey-silver spore mass in the center of the lesion.
Control of yield limiting diseases in soybeans is crucial to maximize profitability. Prior the availability of broad-spectrum fungicides, disease control was focused on variety selection and crop rotation. These are still important parts of integrated pest management, but fungicides give the ability to control important diseases at a much higher level.
Disease Management with Fungicides
Frogeye Leaf Spot
- Frogeye leaf spot overwinters in residue
- Frogeye leaf spot is favored by extended periods of warm (77-86F), wet and humid weather (>90%HR) and frequent rains.
- Depending on environmental conditions, it can take 7-14 days for lesions to appear after infection.
- Frogeye leaf spot is polycyclic meaning it can have multiple generations per season.
- New leaves are more susceptible to infection than older leaves
- Frogeye leaf spot symptoms can be confused with those of herbicide injury or those caused by other pathogens.
- Yield losses range from 10-60%
- Foliar fungicide applications require multiple modes of action. Frogeye leaf spot has developed resistance to the QoI fungicide group.
- Fungicide application timing R2-R4.
Septoria Brown Spot
- Most common soybean disease
- Septoria brown spot overwinters in residue
- Infection can occur throughout the growing season
- Favored by warm (60-85F), wet and humid weather.
- Starts at the bottom of the plant goes upward. Yield losses occur when it affects upper canopy during grain fill.
- Fungicide application timing R3-R5.
Image 2. Septoria Brown Spot, Lindsey Thiessen, NC State
Soybean Rust
- Overwinters on kudzu in the South and spore movement north each year poses risk to infection
- Favors temperature range of 60-82F and high humidity (75%)
- Infection can occur throughout the growing season
- Pustules are reddish-brown in color and form on the underside of leaves.
- Pustules are small and require hand lens for identification
- Pustules turn into lesions that produce spores in 7-14 days
- Fungicide application timing R3-R5.
Image 3. Soybean Rust. LSU, Trey Price.
Fungicide FRAC (Fungicide Resistance Action Committee) Groups – there are 3 main groups of fungicides labeled for soybeans and there are premixes that have one from each group.
- FRAC Group 3. DMI’s – Di-Methylation Inhibitors also known as triazoles. DMI’s are xylem mobile and generally have short residual activity, 7-14 days.
- FRAC Group 11. QoI’s – Quinone Outside Inhibitors also known as strobilurins. QoI’s range in mobility from translaminar to xylem mobile. Long residual activity 21+ days.
- FRAC Group 7. SDHI’s – Succinate-Dehydrogenase Inhibitors. Xylem mobile and long residual activity 21+ days.
For information on fungicide efficacy ratings, here is a link to the Crop Protection Network. https://cropprotectionnetwork.s3.amazonaws.com/soybean-foliar-efficacy-2025.pdf This link provides control ratings for fungicide products as well as a breakdown of active ingredients and FRAC Groups for each product. Talk to your local Crop Specialist about the product and timing recommendations.
Sources:
https://cropprotectionnetwork.org/encyclopedia/septoria-brown-spot-of-soybean
https://cropprotectionnetwork.org/publications/an-overview-of-frogeye-leaf-spot
https://content.ces.ncsu.edu/asian-soybean-rust
Related Categories: General Agronomy; Diseases; Fungicides; Application Timing