M. R. Miles

Breeding for resistance to soybean rust

Soybean rust occurs in all major soybean-growing regions of the world including the North American mainland. Soybean rust, caused by Phakopsora pachyrhizi, is the most destructive foliar disease of soybean, and yield losses of over 50% are common when environmental conditions are conducive for disease development. Heavily infected plants defoliate and mature more rapidly than plants not infected with rust. P. pachyrhizi has a broad host range and can infect many other legumes including some native to Australia. A number of physiological races of the fungus have been reported on these native legumes from Australia and on soybean. In addition, four single genes for rust resistance were previously identified in four different soybean plant introductions. These sources of resistance also have been reported to be susceptible in some field locations and when challenged with certain isolates of P. pachyrhizi. Partial resistance, expressed as reduced pustule number and increased length of latent period, has also been reported but has not been widely used in breeding programs. Yield stability has been used in the past and compares percentage of yields in fungicide and nonfungicide plots. Cultivars or lines with a higher percentage of yield have greater yield stability in the presence of rust. Although soybean rust only recently was found in the continental United States, a proactive project to evaluate the USDA soybean germ plasm collection for rust resistance was initiated in 2002 at the Fort Detrick plant biocontainment facility and at six international locations. Part of this project is to discover soybean lines with greater yield stability, and additional single and partial resistance. To help minimize the impact of soybean rust, the first line of defense will be fungicides, with host resistance and yield stability augmenting the long-term management of soybean rust.

Evaluation of virulence of Phakopsora pachyrhizi and P. meibomiae isolates.

Asian soybean rust (ASR), caused by Phakopsora pachyrhizi and recently discovered for the first time in continental United States, has been of concern to the U.S. agricultural industry for more than 30 years. Since little soybean rust resistance is known, and resistance is often difficult to detect or quantitate, we initiated a project to develop a better, more quantitative, method. The methodology determined the average numbers and diameters of uredinia in lesions that developed on leaves of inoculated plants 14 days after inoculation. It was used to compare virulence of P. pachyrhizi isolates from Asia and Australia and P. meibomiae from Puerto Rico and Brazil, collected as many as 30 years earlier, with isolates of P. pachyrhizi recently collected from Africa or South America. Susceptible reactions to P. pachyrhizi resulted in tan-colored lesions containing 1 to 14 uredinia varying greatly in size within individual lesions. In contrast, on these same genotypes at the same time of year, resistance to other P. pachyrhizi isolates was typified by 0 to 6 small uredinia in reddish-brown to dark-brown lesions. Using appropriate rust resistant and rust susceptible genotypes as standards, examination of uredinia 14 days after inoculation allowed quantitative comparisons of sporulation capacities, one measure of susceptibility or resistance to soybean rust. The study verified the presence and ability to detect all known major genes for resistance to soybean rust in the original sources of resistance. It demonstrated that soybean lines derived from the original PI sources, and presumed to possess the resistance genes, in actuality may lack the gene or express an intermediate reaction to the rust pathogen. We suggest that a determination of numbers and sizes of uredinia will detect both major gene and partial resistance to soybean rust.

The Effect of Solar Irradiance on the Mortality of Phakopsora pachyrhizi Urediniospores

Soybean rust, caused by Phakopsora pachyrhizi, may be the most important foliar disease of soybean. Within the last 10 years, the fungus has moved to many new geographical locations via spread of airborne urediniospores. The objective of this study was to determine the relationship between urediniospore viability and exposure to solar radiation. Urediniospores of P. pachyrhizi were exposed in Capitán Miranda, Paraguay, to determine the deleterious effects of sunlight. Concomitant total solar (0.285 to 2.8 μm) and ultraviolet (0.295 to 0.385 μm) irradiance measurements were used to predict urediniospore germination. Urediniospores exposed to doses of solar and ultraviolet (UV) radiation ≥27.3 MJ/m2 and ≥1.2 MJ/m2, respectively, did not germinate. The proportions of urediniospores that germinated, normalized with respect to the germination proportion for unexposed urediniospores from the same collections, were a linear function of solar irradiance (R2 = 0.83). UV measurements predicted normalized germination proportions equally well. Results of inoculation experiments with exposed P. pachyrhizi urediniospores supported the results of the germination trials, although the effects of moderate levels of irradiance varied. The relationship between urediniospore viability and exposure to solar radiation has been incorporated into the U.S. Department of Agricultures soybean rust aerobiological model that provides North American soybean growers with decision support for managing soybean rust.

Adult Plant Evaluation of Soybean Accessions for Resistance to Phakopsora pachyrhizi in the Field and Greenhouse in Paraguay

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.

First Report of Soybean Rust Caused by Phakopsora pachyrhizi on Phaseolus spp. in the United States

Phakopsora pachyrhizi Syd. & P. Syd., the cause of soybean rust, was first observed in the continental United States in November 2004 (2). During the growing season of 2005, P. pachyrhizi was confirmed on soybean (Glycine max) and/or kudzu (Pueraria montana) in nine states in the southern United States. It is known that P. pachyrhizi has a much larger host range within the Fabaceae family. On 29 September 2005 in Quincy, FL, 45 entries of mostly large-seeded legumes were planted next to soybeans that were infected with P. pachyrhizi. Several seeds of each entry were planted on one hill. Soybean plants growing adjacent to these potential hosts had 15 to 25% of the leaf area affected, 95% incidence, and 73% defoliation on 16 November. On 7 December 2005, all the plants of Phaseolus coccineus L. (scarlet runner bean, PI311827), Phaseolus lunatus L. (lima bean, PI583558), and two Phaseolus vulgaris L. (kidney bean) cvs. Red Hawk and California Early Light Red Kidney (CELRK) were found to have leaves with suspected rust lesions. These plants were at physiological maturity but had not senesced. None of the hosts had been inoculated other than from spores produced by the adjacent rust-infected soybean plants or from unknown locations. On the basis of microscopic examination, suspected infected leaves from plants of the Phaseolus spp. had rust pustules characteristic of P. pachyrhizi uredinia. Uredinia were counted within a randomly selected 2-cm2 area of one leaf of each sample. The mean and range of uredinia per lesion for Phaseolus coccineus was 29 uredinia with a range of 0 to 3 uredinia per lesion, Phaseolus lunatus had 2 uredinia with 0 to 1 uredinium per lesion, Phaseolus vulgaris cv. Red Hawk had 22 uredinia with 0 to 5 uredinia per lesion, and Phaseolus vulgaris cv. CELRK had 43 uredinia with 0 to 4 uredinia per lesion. Polymerase chain reactions using two sets of primers (Ppa1/Ppa2 and Pme1/Pme2) were performed on DNA extracted from leaves of the three species with sporulating rust pustules (1). The results of these tests and further tests conducted by the USDA/APHIS confirmed that P. pachyrhizi was the causal organism for the observed rust. References: (1) P. F. Harmon et al. On-line publication. doi:10.1094/PHP-2005-0613-01-RS. Plant Health Progress, 2005. (2) R. W. Schneider et al. Plant Dis. 89:774, 2005.

First report of Phakopsora pachyrhizi, cause of soybean rust, on Neonotonia wightii in Paraguay.

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.