One of the challenges of identifying soybean rust is that the early stage of the disease can look like other leaf diseases of soybean. Below is a photo guide that provides images and descriptions of common soybean leaf diseases in Iowa.

Brown Spot
Septoria glycines

Causes small, irregular-shaped, darkbrown spots scattered on the upper and lower surface of leaves. Adjacent spots frequently join together to form dead blotches. Late in the season, affected leaves may turn yellow or orange-brown and drop prematurely.

This disease is very common, and is usually one of the first to appear on young plants, starting at V2 stage. It begins at the bottom of the plant and moves up the plant if conditions are favorable (warm and wet).

How does this differ from soybean rust?
First symptoms closely resemble rust; spots more angular; no pustules (pimples) on the underside of the leaf.

Bacterial Pustule
Xanthomonas campestris pv. glycinea

Causes small, yellow-green spots with angular reddish-brown centers. The spots may join together to form large, irregular dead areas which rupture and tear away during windy, rainy weather. Pimple-like structures (pustules) may be seen on the underside of the leaf surface.

This disease is seen on the leaves at the top of the plant. Favorable conditions are high temperatures and higher than average rainfall.

How does this differ from soybean rust?
Pustules (pimples) not associated with each lesion; pustules (pimples) do not have spores; pustule (pimple) openings are cracks instead of round pores.

Bacterial Blight
Pseudomonas savastanoi pv. glycinea

Causes small, angular, yellow-to-brown spots surrounded by yellow halos. The angular spots enlarge and join together producing large, irregular dead areas. The centers of old dead areas tear away so infected leaves have a ragged appearance.

This disease is seen on the leaves at the top of the plant. It is very common and usually one of the rst to appear on young plants, starting at V2 stage. It is common after heavy rains and if temperatures remain cool.

How does this differ from soybean rust?
Angular spots; ragged leaves; no pustules (pimples) on underside of the leaf.

Downy Mildew
Peronospora manshurica

Causes light-green to yellow, irregularshaped spots on the upper surface of the topmost leaves. On the underside of the leaves, the fungus may be seen growing out of the center of the spots.

This disease is favored by cool, moist conditions. Therefore, it is usually found on leaves inside a well-developed plant canopy. Occurs in mid to late season.

How does this differ from soybean rust?
Spots are larger; no pustules (pimples) on under-side of leaf; fuzzy fungal growth may be seen.

Frogeye Leaf Spot
Cercospora sojina

Causes small angular spots with gray centers and distinct purple to reddishbrown margins. In older spots, dark fungal structures form in the center of the spot and the spots look like frog eyes.

This disease is found in the mid and upper canopy during warm and humid weather. It occurs in mid to late season, and is not very common in Iowa.

How does this differ from soybean rust?
Spots are larger; spots have dark, de ned edges; no pustules (pimples) on underside of leaf.

Cercospora Leaf Blight
Cercospora kikuchii

Starts as a mottled purple-to-orange discoloration which becomes orange or bronze. The leaves become leathery in texture.

Usually occurs on topmost three to four trifoliate leaves and on the upper surface of the leaf in warm, wet weather. Usually occurs in mid to late season.

How does this differ from soybean rust?
Only upper leaf surface discolored; few spots and no pustules (pimples) on underside of the leaf.