RESUMO
Five hundred thirty soybean accessions from maturity groups (MG) III through IX were evaluated for resistance to Phakopsora pachyrhizi in a replicated field trial at Centro Regional de Investigación Agrícola in Capitán Miranda, Itapúa, Paraguay during the 2005-06 season. Soybean rust severities of individual accessions ranged from 0% (resistant) to 30.0% (susceptible). In MG III and IV, the most resistant accessions were PI 506863, PI 567341, and PI 567351B, with severities less than 1.2%. In MG V, the most resistant accessions were PI 181456, PI 398288, PI 404134B, and PI 507305, with severities less than 0.3%. In MG VI, the most resistant accessions were PI 587886, PI 587880A, and PI 587880B, with severities less than 0.3%. In MG VII and VIII, the most resistant were PI 587905 and PI 605779E, with severities less than 1.0%. In MG IX, the most resistant accessions were PI 594754, PI 605833, PI 576102B, and PI 567104B, with severities less than 1.0%. The resistance in 10 selected accessions from MG VI, VII, VIII, and XI was confirmed in subsequent greenhouse and field experiments where severities of 0.4% or less and reddish-brown lesions with sporulation levels less than 3.0 were observed. These accessions, with low severities in the adult plant field evaluation, may be new sources of resistance to P. pachyrhizi.
RESUMO
Phakopsora pachyrhizi Syd. & P. Syd., the cause of soybean rust, was first observed on soybean (Glycine max (L.) Merr.) in South America in the district of Itapúa in Paraguay during March, 2001 (2). The disease is now widespread in soybean-production areas in South America on soybean and kudzu (Pueraria lobata (Willd.) Ohwi). On 15 March 2006, leaves of the perennial legume Neonotonia wightii (Graham ex Wight & Arn.) Lackey with lesions and rust sori were observed in the Reserva Biológica de Itabó, Departamento Alto Paraná. Lesions were scattered, most contained a single uredinium, mostly hypophyllous, and appeared to be new infections. Lesions with several uredinia, which are indicative of older infections on soybean, were also observed. Sori (Malupa-type) contained hyaline, peripheral, cylindric to clavate paraphyses measuring 24 to 45 × 6 to 13 µm and urediniospores that were hyaline, ovoid to globose, and measuring 20 to 40 × 14 to 25 µm with an echinulate spore wall, characteristics typical of a Phakopsora sp. DNA extracted from sori from leaves of N. wightii was amplified in a real-time fluorescent polymerase chain reaction with the P. pachyrhizi-specific primers Ppm1 and Ppa2 (1). Sequence alignment of the internal transcribed spacer region 2 further confirmed the identification as P. pachyrhizi (1). The host identification was confirmed by J. Kirkbride, USDA/ARS/SBML, using the Smithsonian Institution Department of Botany, U.S. National Herbarium. To our knowledge, this is the first confirmed report of natural infection of P. pachyrhizi on a host other than soybean or kudzu in South America. Voucher specimens were deposited in the herbarium of the Facultad Ciencias Químicas of the Universidad Nacional de Asunción of Paraguay (FCQ) and the National Fungus Collection (Accession No. BPI 875340). References: (1) R. D. Frederick et al. Phytopathology 92:217, 2002. (2) W. Morel and J. Yorinori. Bol. Divulg. No. 44. Ministerio de Agricultura y Ganadería, Centro Regional de Investigación Agrícola, Capitán Miranda, Paraguay, 2002.