Photographer: Reid Frederick Source: www.bugwood.org Copyright: CC BY-NC 3.0
Soybean rust (Phakopsora pachyrhizi) is a foliar disease that effects soybean plants. It's native to Asian countries such as Thailand, Taiwan, and Japan. This disease can appear as holes in plant leaves and is known to effect the leaves of the plant rather than the stem or the root. Early stages of soybean rust are often confused with bacterial pustule (Xanthomonas axonopodis pv. glycines), Septoria Brown Spot, Downy Mildew and Bacterial Leaf Spot.
Symptoms: The initial symptoms of soybean rust appear as small irregularly spots that become brown or green with progression of the disease. The lesions are the size of a leaf hair and confined to the veins of the leaf. Spores form on the underside of leaf and rapidly become abundant. In later stages of infection soybean rust can be distinguished from other diseases by egg shaped pustles that secrete cream colored or tan spores.
Host(s): Soybean plants are preferred, but other legumes can be infected with soybean rust.
Soybean rust greatly decreases vigor of soybean plants by causing premature defoliation resulting in fewer seed pods and an overall reduction in yield of soybean plants. With a rapid dispersal ability in the spore stage large fields are quickly effected by soybean rust greatly affecting agriculture yields and economic turnout.
Phakopsora pachyrhizi and P. meibomiae are the known causes of soybean rust with a large possible host range species of 41 and 34 natural hosts respectively. This disease is well adapted for dispersal with ability to be transferred by wind for several miles allowing for infection of large fields in addition to transfer to new uninfected fields. Spores can also be transferred by water in the form of rain splashes rather than on large bodies of water such as streams. Following ten days of infection spores become infective to the plant with appropriate environmental conditions allowing for damp conditions with temperatures ranging from 59 to 82o F. Spores are continuously released and spread with appropriate moist conditions capable of infecting soybean plants at any stage of development. It is believed that the spores can't overwinter and are unable to survive without the host plant.
Soybean rust was first detected in Puerto Rico in 1976 in the form of P. meibomiae with the more aggressive form of soybean rust (P. pachyrhizi) found in 1995 in Hawaii. It is believed that soybean rust was first transferred to the United States from South America via wind transportation of spores. Further spread was facilitated by wind and human transport.
U.S. Habitat: Soybean rust is able to grow in moist areas with spore production and spread limited by areas that become dry. If temperatures exceed 86o F growth of soybean rust is greatly reduced and it is unable to spread and produce further spores.
U.S. Present: FL, GA, LA, TX
Current management methods are still being developed using biological defenses such as fungus in South America. It is important to detect early onset of the disease to prevent large scale infection. Fungicides can be applied and effective if the disease is caught in the early stages of infection. If an infestation is suspected it is important to take necessary precautions to prevent further spread of the spores. Spreading of the spores is easily facilitated by humans through attachment to clothing and boots. It is important to wash all gear and clothing prior to leaving infected areas. Other management techniques and uses of fungicide can be identified on a case by case basis with further research required.
Garcia, Alexandre, Eberson Sanchez Calvo, Romeu Alfonzo de Souza Kiihl, Eliezer Rodrigues de Souto. 2011. Evidence of a Susceptible Allele Inverting the Dominance of Rust Resistance in soybean. Crop Science 51(1): 32-40.
Narvaez, Dario F., Wayne M. Jurick II, James J. Marois, and David L. Wright. 2010. Effects of Surface Wetness Periods on Development of Soybean Rust Under Field Conditions. Plant disease: an international journal of applied plant pathology 94(2): 258-264.
Ngugi, Henry K., Paul D. Esker, and Harald Scherm. 2011. Meta-Analysis to Determine the Effects of Plant Disease Management Measures: Review and Case Studies on Soybean and Apple. Phytopathology 101(1): 31-41.