About Asian Soybean Rust Fungus
Identification of Asian Soybean Rust
(The following material was reprinted from the Iowa Soybean Rust Response Plan)
Accurate and timely identification is the key to determining whether a response will be attempted and, if so, the extent, direction, and magnitude of that response. It will also help determine program changes and failures.
Symptoms of soybean rust appear identically regardless if they are caused by Phakopsora pachyrhizi or Phakospora meibomiae. Host plants infected with soybean rust first exhibit small lesions that gradually increase in size and turn from gray to tan or brown. They become polygonally shaped restricted by leaf veins, and may eventually reach 2 to 3 square millimeters.
Infection begins on the lower first leaves of plants and appears as chlorotic or mosaic-like areas with uredinia observed usually at or after the plant flowering stage. Lesions may appear on most above-ground plant parts, but are most common on the underside of the leaves. As the plant matures and sets pods, infection progresses rapidly under the right environmental conditions (i.e., moisture, high humidity and heat) to cause high rates of infection in the middle and upper leaves of the plant. Clouds of spores have been observed within and above canopies of highly infected plant stands.
Plants show two different lesion reactions to infection by soybean rust. Tan lesions consist of small uredinia surrounded by slightly discolored necrotic areas of leaf surfaces. Early stages show an ostiole, or small hole, where urediniospores emerge. As uredinia become larger, they release masses of tan colored urediniospores that appear as light brown or white raised areas. Uredinial pustules become more numerous with advancing infection and often will coalesce, forming larger pustules that break open and release masses of urediniospores.
The other type of lesion that occurs with soybean rust infection is the reddish-brown lesion. These lesions have larger areas of necrosis that are reddish brown surrounding a limited number of uredinia. A few urediniospores are usually visible on the surface.
Early symptoms of soybean rust are easily confused with bacterial pustules (caused by Xanthomonas campestris pv. phaseoli (Smith) Dye), or bacterial blight (caused by Psuedomonas glycinea Coerper), and brown spot (caused by the fungus Septoria glycines). These diseases also occur often on the underside of soybean leaves, causing a raised light brown blister within a lesion. These leaf lesions vary from small specks to large irregular brown areas that form when small lesions coalesce. A hand lens or dissecting microscope is usually used to distinguish these disease symptoms from ASBR, but early stages of the disease are difficult to distinguish if no spores, conidia, or bacteria are evident.
