The soybean crop is one of the most important crops worldwide. Soybean seeds are important for both protein meal and vegetable oil. The crop is grown on an estimated 6% of the world’s arable land, and since the 1970s, the area in soybean production has the highest percentage increase compared to any other major crop. Recent increases in production coincide with increases in demand for meal and oil. Soybean production was 17 million metric tons (MMT) in 1960 and increased to 230 MMT in 2008. Future soybean production is expected to increase more than other crops, due to expanded production area and higher yields. There are a number of important abiotic and biotic constraints that threaten soybean production by directly reducing seed yields and/or seed quality. Abiotic constraints include extremes in nutrients, temperatures and moisture. These may reduce production directly, but also indirectly through increases in pathogens and pests. Biotic constraints tend to be geographically and environmentally restricted. Some diseases like soybean rust may be explosive by producing copious amounts of air-borne spores. This disease, more so than most, caused great concern when first found invading soybean production areas in Brazil and the United States of America. In contrast, red leaf blotch is a disease restricted to a few countries in Africa, but deserving attention since it has not been intensely studied and adequate management strategies, such as the use of resistant varieties, are not available. Significant losses in soybean yield beyond current levels may have implications for food security because of our dependence on the soybean crop, directly and indirectly for food products. In addition, because the crop is highly nutritious and versatile it offers resources to address world food issues through current and future utilization practices. Future soybean production is expected to increase in proportion to increased demand, and with application of newer genomic technologies, the crop has enormous potential to improve dietary quality for people throughout the world whether consumed as a vegetable crop or processed into various soybean food products.

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Crops that feed the World 2. Soybean—worldwide production, use, and constraints caused by pathogens and pests

Virus Diseases of Peanut

Biotic and abiotic stresses cause significant yield losses in legumes and can significantly affect their productivity. Biotechnology tools such as marker-assisted breeding, tissue culture, in vitro mutagenesis and genetic transformation can contribute to solve or reduce some of these constraints. However, only limited success has been achieved so far. The emergence of “omic” technologies and the establishment of model legume plants such as Medicago truncatula and Lotus japonicus are promising strategies for understanding the molecular genetic basis of stress resistance, which is an important bottleneck for molecular breeding. Understanding the mechanisms that regulate the expression of stress-related genes is a fundamental issue in plant biology and will be necessary for the genetic improvement of legumes. In this review, we describe the current status of biotechnology approaches in relation to biotic and abiotic stresses in legumes and how these useful tools could be used to improve resistance to important constraints affecting legume crops.

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Biotechnology approaches to overcome biotic and abiotic stress constraints in legumes

Soybean production in South and North America has recently been threatened by the widespread dissemination of soybean rust (SBR) caused by the fungus Phakopsora pachyrhizi. Currently, chemical spray containing fungicides is the only effective method to control the disease. This strategy increases production costs and exposes the environment to higher levels of fungicides. As a first step towards the development of SBR resistant cultivars, we studied the genetic basis of SBR resistance in five F2 populations derived from crossing the Brazilian-adapted susceptible cultivar CD 208 to each of five different plant introductions (PI 200487, PI 200526, PI 230970, PI 459025, PI 471904) carrying SBR-resistant genes (Rpp). Molecular mapping of SBR-resistance genes was performed in three of these PIs (PI 459025, PI 200526, PI 471904), and also in two other PIs (PI 200456 and 224270). The strategy mapped two genes present in PI 230970 and PI 459025, the original sources of Rpp2 and Rpp4, to linkage groups (LG) J and G, respectively. A new SBR resistance locus, rpp5 was mapped in the LG-N. Together, the genetic and molecular analysis suggested multiple alleles or closely linked genes that govern SBR resistance in soybean.

Molecular mapping of soybean rust (Phakopsora pachyrhizi) resistance genes: discovery of a novel locus and alleles

Aerial transport alone is seldom responsible for the introduction of nonindigenous species into distant regions; however, the capacity to use the atmospheric pathway for rapid spread in large part determines the invasive potential of organisms once they are introduced. Because physical and biological features of Earth's surface influence the routes and timing of organisms that use the atmospheric pathway, long-distance movement of aerobiota is largely regular and thus predictable. Soybean rust (Phakopsora pachyrhizi), potentially the most destructive foliar disease of soybean, recently invaded North America. The concepts presented in this article form the basis of the soybean rust aerobiology prediction system (SRAPS) that was developed to assess potential pathogen movement from South America to the United States. Output from SRAPS guided the scouting operations after the initial discovery of soybean rust in Louisiana. Subsequent observations of P. pachyrhizi in the southeastern United States pro...

https://www.jstor.org/stable/10.1641/0006-3568%282005%29055%5B0851%3APOTAPF%5D2.0.CO%3B2

Principles of the Atmospheric Pathway for Invasive Species Applied 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.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PD-89-0664

Breeding for Resistance to Soybean Rust.

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.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PD-90-0708

Evaluation of Virulence of Phakopsora pachyrhizi and P. meibomiae Isolates

Phakopsora pachyrhizi, the causal fungus of soybean rust, was discovered in the continental U.S. in November 2004. The presence of this disease in the U.S. may have an impact on soybean (G...

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Evaluation of Soybean Germplasm for Resistance to Phakopsora pachyrhizi

Soybean Rust: Is the U.S. Soybean Crop At Risk?

Mueller, T. A., Miles, M. R., Morel, W., Marois, J. J., Wright, D. L., Kemerait, R. C., Levy, C., and Hartman, G. L. 2009. Effect of fungicide and timing of application on soybean rust severity and yield. Plant Dis. 93:243-248. Soybean rust, caused by Phakopsora pachyrhizi, is a devastating foliar disease of soybean that may cause significant yield losses if not managed by well-timed fungicide applications. To determine the effect of fungicide timing on soybean rust severity and soybean yield, field trials were completed in Paraguay (four locations), the United States (two locations), and Zimbabwe (one location) from 2005 to 2006. Treatments at each location included applications of tebuconazole, pyraclostrobin, or a combination of azoxystrobin + propiconazole, and in some locations pyraclostrobin + tebuconazole at the following soybean growth stages (GS): (i) GS R1 (beginning flowering), (ii) GS R3 (beginning pod), (iii) GS R5 (beginning seed), (iv) GS R1 + R3, (v) GS R3 + R5, and (vi) GS R1 + R3 + R5. Soybean yields from plots treated with fungicides were 16 to 114% greater than yields from no fungicide control plots in four locations in Paraguay, 12 to 55% greater in two locations in the United States, and 31% greater in Zimbabwe. In all locations, rust severity measured over time as area under the disease progress curve (AUDPC) was negatively correlated (r = –0.3, P < 0.0001) to yield. The effectiveness of any given treatment (timing of application and product applied) was often dependent on when rust was first detected and the intensity of its development. For example, when soybean rust was first observed before GS R3 (two locations in Paraguay), the plants in plots treated with a fungicide at GS R1 had the lowest AUPDC values and highest yields. When soybean rust was first observed after GS R3, plants treated with a fungicide at GS R3 and/or GS R5 had the lowest AUDPC values and highest yields with a few exceptions.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-93-3-0243

M. R. Miles

Papers

Breeding for Resistance to Soybean Rust.

Evaluation of Virulence of Phakopsora pachyrhizi and P. meibomiae Isolates

Evaluation of Soybean Germplasm for Resistance to Phakopsora pachyrhizi

Soybean Rust: Is the U.S. Soybean Crop At Risk?

Effect of fungicide and timing of application on soybean rust severity and yield.