Scouting for Soybean Rust

Soybean rust pustule on bottom of leaf under magnification. How should I scout for Asian soybean rust? Watching weather forecasts or reports from sentinel plots and counties with Asian soybean rust will help producers monitor the movement of soybean rust and make management decisions. Maps showing counties with confirmed cases can be found on DTN or the USDA Soybean Rust Information Site. When scouting fields, producers should look in the lower canopy with a 20X hand lens to detect spores. Dow AgroSciences recommends evaluating 20 locations in a field, examining five plants per location. Determining if you have soybean rust in your field can be difficult because other diseases exhibit similar characteristics. These diseases include bacterial blight, bacterial pustule, downy mildew, Cercospora blight, frogeye leaf spot and brown spot. The table below shows factors that distinguish these diseases from soybean rust and will help you as you scout for the disease.
Disease	Leaf region infected	Lesion properties	Distinctions from soybean rust
Bacterial Blight	mid-to-upper leaves	Small, angular-to-circular lesions. Lesion color progression is from yellow-to-light brown to a dark reddish or blackish brown. Lesions have a translucent or water-soaked yellow halo.	Angular lesions. No raised lesions on leaf underside.
Bacterial Pustule	mid-to-upper leaves	Small, yellow-to-light green lesions, to large irregular shapes. Initial lesion centers may be slightly raised. Little to no water soaking seen with lesions.	Pustules not always with each lesion. Pustules do not have spores in openings. Openings are cracks instead of circular pores.
Downy Mildew	upper leaves	Pale-green to light-yellow spots initially, with areas enlarging to blotches of pale or bright yellow lesions of indefinite size and shape. Fuzzy fungal gray tufts on leaf underside during periods of wet or humid weather.	Lesions larger than rust lesions. No raised pustules on underside. Fuzzy fungal growth on underside.
Cercospora Blight and Frogeye Leaf Spot	Blight – upper leaves Frogeye – lower leaves	Blight starts as light purple areas on upper leaf surface that expand to cover surface. Leaves are leathery and dark reddish-purple on upper surface only. Frogeye lesions start as dark, water-soaked circular-to-somewhat irregular spots that can have light centers. Centers become light-brown to light-gray in color as lesions age, with older lesions having a light center with a darker-red to purple-brown border.	Blight – overall leaf area is discolored on upper surface only. Frogeye – discrete lesions larger than rust with defined lesion margins and no pustules evident on underside of leaf.
Brown Spot	lower leaves	Small, angular-to-somewhat circular reddish-brown to dark-brown spots. Infected leaves quickly yellow and drop.	No raised areas on leaf underside. Angular lesions. If dark lesions, lack of uredia is a key symptom. First symptoms can look like rust. Has same canopy distribution as rust.

Download a copy of the University of Missouri-Columbia Extension’s (392KB PDF) soybean rust guide for images of each disease.