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First Report of Phakopsora pachyrhizi Causing Rust on Soybean in Malawi

Murithi, H. M., Beed, F. D., Soko, M. M., Haudenshield, J. S., Hartman, G. L.
Plant disease 2015 v.99 no.3 pp. 420
DNA, Glycine max, Phakopsora pachyrhizi, cultivars, genome, leaves, new geographic records, pathogens, quantitative polymerase chain reaction, signs and symptoms (plants), soybean rust, soybeans, urediniospores, Democratic Republic of the Congo, Ghana, Malawi, Tanzania, Uganda
Soybean rust (SBR), caused by Phakopsora pachyrhizi, has rapidly become established in Africa since the first report in Uganda in 1996. The urediniospores, as windborne propagules, have infested new regions of Africa, initiating SBR in many countries, including Ghana and Democratic Republic of the Congo in 2007 and Tanzania in 2014. The typical symptoms and signs of SBR, including leaf yellowing and tan, sporulating uredinia, were observed on soybean in May 2014 during field surveys in the major soybean growing areas of Malawi (Central: Dowa, Mchinji and Kasungu. Southern: Thyolo). SBR symptoms were observed in 9 out of 12 sites surveyed. When microscopically examined, urediniospores were elliptical, echinulate, and hyaline to pale yellowish brown. To confirm the pathogen, symptomatic soybean leaf tissue of approximately 1 cm2 was excised from each of the samples, and DNA was extracted using the FastDNA Spin Kit (MP Biomedicals, Solon-OH), with further purification using the MicroElute DNA Clean-up Kit (Omega Bio-Tek, Norcross-GA). The resulting DNA was analyzed by quantitative PCR using published Taqman assays for P. pachyrhizi and P. meibomiae, with a multiplexed exogenous internal control reaction to validate negative results. P. pachyrhizi DNA was detected in excess of 180,000 genome equivalents /cm2 in all samples, indicating a substantial infection. Urediniospores dislodged from three leaves and inoculated onto susceptible soybean cultivar Williams 82 produced tan lesions after 2 weeks of incubation in a detached-leaf assay. This is the first confirmed report of P. pachyrhizi causing rust on soybean in Malawi.