Asian Soybean Rust - Symptoms

Early symptoms of rust appear on lower leaves deep in the canopy.

Pustules on the underside of soybean leaves are diagnostic for Asian soybean rust. Use a hand lens when inspecting the lower side of leaves.

Early symptoms of Asian soybean rust

Infection by Phakopsora pachyrhizi begins in the low to mid-canopy. Initial symptoms of soybean rust include small, gray spots, particularly on the undersides of leaves and along leaf veins.

The spots increase in size over time and change color from gray, to tan, reddish-brown or black. It is important to recognize that these symptoms are not exclusive to rust. Other diseases of soybean including brown spot, bacterial blight, and particularly downy mildew can easily be confused with soybean rust.

If symptoms are observed, look for signs (sporulation) of the rust pathogen

Lesions caused by the rust fungus enlarge, becoming tan or reddish-brown with a raised appearance. These pimple-like structures, called pustules, are diagnostic for Asian soybean rust. When rust pustules are mature, they burst and release masses of spores into the air.

Lesion usually coalesce to form larger lesions, often clustering along the leaf veins.

As the plant matures and begins to set pods, rust symptoms can spread rapidly to the middle and upper parts of the plant. Lesions are found on petioles, pods, and stems but are most abundant on leaves.

Look-alike diseases

In the early stages, it is easy to confuse the symptoms of soybean rust with symptoms of other soybean leaf diseases Familiarize yourself with the symptoms and signs of these diseases:

• Brown spot caused by the fungus, Septoria gycines
• Bacterial pustule, caused by Xanthomonas axonopodis pv. glycines
• Bacterial blight (also called angular leaf spot ), caused by Pseudomonas savastanoi pv. glycinea
• Downy mildew
• Frogeye leaf spot
• Cercospora leaf spot

Images of these foliar diseases can be found in Common Soybean Leaf Diseases and Asian Soybean Rust (pdf) by Alison Robertson and Greg Tylka, Iowa State University

View more images of rust in the Photo Gallery

Information on these pages is from the cooperative effort of researchers throughout the North Central states. Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: NCSRP Plant Health Initiative at www.planthealth.info. Please do not use copyrighted photos without permission.