Soybean rust

Soybean rust is a disease that affects soybeans and other legumes. It is caused by two types of fungi, Phakopsora pachyrhizi, commonly known as Asian soybean rust, and Phakopsora meibomiae, commonly known as New World soybean rust. P. meibomiae is the weaker pathogen of the two and generally does not cause widespread problems. The disease has been reported across Asia, Australia, Africa, South America and the United States.

Importance Soybean is one of the most important commercial crops around the world. Asian soybean rust is the major disease that affects soybeans. It causes lesions on the leaves of soybean plants and eventually kills the plants. The disease has caused serious yield loss of soybeans. In the areas where this disease is common, the yield losses can be up to 80%. The first reported cases in the United States occurred in 2004. This population originated in northern South America or the Caribbean. The spores were most likely spread by Hurricane Ivan. In 2002, USDA reported 10-60% yield losses in South America and Africa. in countries where this pathogen is established, estimated losses are between 10 and 80% depending on the inoculation and environmental conditions.

Host and symptoms Soybean rust 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. Asian soybean rust also infects yellow sweet lover, vetch, medic, lupine, green and kidney bean, and lima and butter bean. 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 be observed. In Phakopsora pachyrhizi will begin to form small brown and brick red sports on the lower canopy of the leaves that will begin to turn into lesions. Asian Soybean Rust produces two types of lesions. Lesions at the later stage will turn from gray to tan or reddish brown. Mature tan lesions consist of small pustules surrounded by discolored necrotic areas. Tan spores can be found at the necrotic areas on the underside of the leaf. In the case of reddish brown lesions, there are 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 produces. ASR pustules usually do not have the yellow halo which is typical of bacterial pustules. 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. Soybean plants are one of ASR's most known hosts and are susceptible at any stage in the life cycle. However, symptoms are most commonly found during or after flowering. ASR infection will reduce pod production and fill.

Environment Asian Soybean Rust (ASR) was first detected in Asia. It has been found in many countries around the world since then. For example, Australia, China, Korea, India, Japan, Nepal, Taiwan, Thailand, the Philippines, Mozambique, Nigeria, Rwanda, Uganda, Zimbabwe, South Africa, Brazil, Argentina, and Paraguay. This disease was first detected in the Caribbean in Puerto Rico in 1976 and first reported in the continental United States in 2004. Asian Soybean Rust favors environments that are humid and warm. A continuous period of wetness on leaves will aid the growth of this disease since this situation is required for spores to germinate. Therefore, is most likely to appear under conditions of 60 to 85 °F (16 to 29 °C) and a relative humidity of 75% to 80%. Therefore, ASR is a more serious problem in tropical and subtropical areas in Asia, Africa, Australia and South America. It is unable to survive the cold winters of northern habitats. However, this pathogen can be spread easily from wind and storm fronts and can inoculate quickly within a soybean field under favorable environmental conditions. And the Asian Soybean Rust may be favored by the climatic anomalies phenomena that cause extreme rainfall natural disasters that is called El Niño.

… excerpt ends here. Continue reading the full article.