Asian Soybean Rust - Host and Symptoms

Host and Symptoms

Asian Soybean Rust (ASR) is caused by two types of fungi, Phakopsora pachyrhizi and Phakopsora meibomiae. It affects several important commercial plants, however, most notable for soybeans. Asian Soybean Rust can infect and reproduce on 90 known plant species. 20 of which are found in the United States, such as, soybeans, dry beans, kidney beans, peas, leguminous forage crops such as trefoil and sweet clover and weeds such as kudzu.

At the early stage of Asian Soybean Rust, it causes yellow mosaic discoloration on the upper surfaces of older foliage. At this stage, it is usually hard to identify since the symptoms are relatively small and poorly defined.

Later as the disease continues to progress, the leaves will turn yellow and there will be lesions mostly on the undersides of the leaves and sometimes on petioles, stems or pods and premature defoliation can also been observed.

Asian Soybean Rust produces two types of lesions. Lesions at the later stage will turn from gray to tan or reddish brown. Mature tan lesion consists of small pustules which surrounded by discolored necrotic areas. Tan spores can be found at the necrotic areas on the underside of the leaf. For Reddish brown lesion, it has larger reddish brown necrotic areas with few pustules and visible spores on the underside of the leaf. A good way to distinguish Asian Soybean Rust from other diseases is to look at the pustules it procudes. ASR pustules usually do not have the yellow halo which is related to bacterial pustule. Besides, ASR pustules are raised and can be commonly found on the underside of the leaf which makes it different from the lesions caused by spot diseases.

As one of ASR's most known hosts, soybean plants are susceptible at any stage in the life cycle. However, symptoms are most commonly find during or after flowering. Soybean plants infected by Asian Soybean Rust will result in declining of pod production and fill.