Afzelia xylocarpa is a leguminous tree (Caesalpiniaceae) native to the south-east Asian deciduous forests in Thailand, Vietnam, Cambodia, Laos and Myanmar. In Jan. 2011, leaves of A. xylocarpa infected with rust were found and collected from the National Corn and Sorghum Research Centre, Rai Suwan, Nakhon Ratchasima, Thailand.
Dry herbarium specimens of A. xylocarpa infected with rust were deposited at the Plant Pathology Mycological Herbarium, Department of Agriculture, Thailand (TTPH) as TTPH 002449. (Fig. 1a) The morphology of the rust was examined by cutting thin sections of uredinia and mounting in lactic acid on microscope slides that were gently heated to expel air bubbles. The sections were examined with an Olympus BX60 and images taken with a Nikon DS-Fi1 camera.
Examination of rust infected leaves showed that uredinia developed more frequently on the lower surface of the leaf than on the upper surface. Uredinia were subepidermal, erumpent, dome shaped, ostiolate, pale brown, scattered on necrotic spots up to 2 mm diam. (Fig. 1b), enclosed by paraphyses that were cylindrical to clavate, 16–29 μm [av. = 22.7, SD = 4.7, n = 10] × 6–9 μm [av. = 7.0, SD = 0.8, n = 10], capitate apex, hyaline to subhyaline (Fig. 1d). Urediniospores were single-celled, subglobose, ellipsoidal, obovoid, subhyaline to pale brown, 22–39 [av. = 26.6, SD = 4.7, n = 20] × 18–28 [av. = 21.9, SD = 2.3, n = 20] μm, wall 1.5–3.0 [av. = 1.9, SD = 0.2, n = 20] μm thick, echinulate, spines 0.5–0.1 μm high and 2–3 μm apart, germpores inconspicuous (Fig. 1c).
Rust sori were excised from fresh host material, placed in 2 ml Bead Solution tubes of the UltraClean Microbial DNA Isolate Kit, and extracted according to the manufacturer’s instructions (Mo Bio Laboratories). The small subunit (SSU) ribosomal RNA was amplified using the primers and PCR conditions recommended in Aime (2006), with the exception that Phusion High-Fidelity PCR Master Mix (Finnzymes) was used instead. The PCR products were purified using the QIAquick PCR Purification Kit (QIAGEN), and the purified products were sequenced using the amplifying primers and additional primers recommended by Aime (2006) by Macrogen Incorporated (Korea) using the AB 3730xl DNA Analyser (Applied Biosystems). The sequence obtained was compared to the nucleotide sequence database (GenBank). The SSU sequence (GenBank accession JF990846) was a 99 % match to Phakopsora pachyrizhi BPI 871755 GenBank accession DQ354536.
On the basis of morphology and DNA sequence data the rust was identified as Phakopsora pachyrhizi Sydow and Sydow (1914), which causes Asian soybean rust. The Asian soybean rust pathogen was first identified in 1902 in Japan (Bromfield 1984). P. pachyrhizi is found throughout Asia and has spread to many soybean growing regions around the world, including North and South America (Ivancovich 2005; Schneider et al. 2005) and Africa (Levy 2005). Soybean rust is one of the most important diseases of soybean causing significant yield losses (Goellner et al. 2010).
Phakopsora pachyrhizi has a wide host range in the Fabaceae, with hosts in several genera including Alysicarpus, Cajanus, Calopogonium, Canavalia, Centrosema, Clitoria, Coronilla, Crotalaria, Glycine, Lablab, Lespedeza, Lupinus, Lotus, Macroptilium, Melilotus, Pachyrhizus, Phaseolus, Pisum, Psophocarpus, Pueraria, Senna, Sesbania, Trifoliuum, Trigonella, Vicia and Vigna (Ono et al. 1992). There is a report of an unidentified rust on A. xylocarpa from a seedling nursery in northern Thailand (Kawabe et al. 1998.). Ours is the first confirmed record of P. pachyrhizi on A. xylocarpa in Thailand as well as the first report of P. pachyrhizi on a host in the genus Afzelia.