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Australasian soybean rust. Causal Agent

Phakopsora pachyrhizi 



P. pachyrhizi is capable of infecting more than 90 species of legumes; however, the number of legumes infected in nature is unknown. Kudzu is widespread in the United States and could serve as a reservoir for the soybean rust pathogen. 



Leaf with soybean rust. Soybean rust symptoms are similar for P. pachyrhizi and P. meibomiae species. Symptoms begin on the lower leaves of the plant as small lesions that increase in size and change from gray to tan or reddish brown on the undersides of the leaves. Lesions are most common on leaves but may occur on petioles, stems, and pods. Soybean rust produces two types of lesions, tan and reddish brown. Tan lesions, when mature, consist of small pustules (uredinia) surrounded by slightly discolored necrotic area with masses of tan spores (urediniospores) on the lower leaf surface. Reddish brown lesions have a larger reddish brown necrotic area, with a limited number of pustules (uredinia) and few visible spores (urediniospores) on the lower leaf surface. Once pod set begins on soybean, infection can spread rapidly to the middle and upper leaves of the plant. Environmental conditions impact the incidence and severity of soybean rust. Prolonged leaf wetness combined with temperatures between 59 and 86ºF and humidity of 75–80% is required for spore germination and infection. Under these conditions, pustules form within 5–10 days and spores are produced within 10–21 days. High levels of infection in soybean fields result in a distinct yellowing and browning of fields and commonly, premature senescence in plants. 



Close up of australasian soybean rust. Early symptoms of soybean rust resemble bacterial pustule (Xanthomonas axonopodis pv. glycines) and brown spot (Septoria glycines). Soybean rust can be distinguished from bacterial pustule and brown spot by examining the lesions under a hand lens (20×) or dissecting microscope. The mature soybean rust lesion contains cone-shaped pustules with a pore on the top with spores inside or on top of the cone. 



All commercial varieties currently available are highly susceptible. Current research includes screening germplasm for resistance and evaluating fungicide efficacy. Early detection is required for the most effective management of soybean rust. Monitoring soybean fields and adjacent areas is recommended throughout the growing season. Fungicide applications may reduce yield loss, depending on the plant developmental stage, time when soybean rust is detected, and fungicide application method. For efficacy information on fungicides labeled for use on soybean, please contact your local county extension office. 

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