A comparison of stem rust in oats and stripe rust in wheat: A Swedish example
BGRI 2014 Plenary Abstract Jonathan Yuen
Department of Forest Mycology and Plant Pathology, Swedish University of Agricultural Sciences
A number of rusts affect grain crops in Sweden, but stem rust on oats and stripe (yellow) rust on wheat appear to create the greatest problems in production. The epidemiology of these diseases is intimately connected to the overall cropping patterns of these two crops. In Sweden, oats are only sown in the spring, thus forcing any overwintering pathogen to survive a Swedish winter. This is easiest for Puccinia graminis f. sp. avenae, which apparently completes its full, sexual life cycle on the abundant barberry plants. The presence of barberry and clear indications of sexual reproduction by P. graminis suggests that Pgt could be a problem on wheat, but there are only sporadic reports of stem rust on wheat. Wheat cultivars grown in Sweden possess few effective genes for resistance to stem rust, and the lack of rust is probably due to a lack of Pgt in the region. Given the resurgence of barberry in the landscape this implies that stem rust on wheat could be a major problem if (or when) the pathogen returns. P. striiformis, in contrast, can survive the Swedish winters on fall sown cereal crops, and thus it is the fittest clones that survive and dominate in the population. A large number of factors can affect this fitness, most markedly resistance genes in the cultivated wheat, but it is also possible that extended asexual reproduction can reduce the fitness of these persistent clones (Muller's ratchet) so that they can be displaced by fitter clones. Despite the widespread occurrence of barberry plants, we have not found any aecia of P. striiformis, although there does seem to be some genetic variation in the alternate host. Simple models that simulate the appearance and competition between different clonal lineages of the pathogen indicate that fitter individuals will eventually dominate the population, but their initial appearance will be difficult, since they are only detectable after enough generations have passed to increase the population size above a detectable level.
Global Pgt Initiative: An international genetic resource to combat stem rust
BGRI 2014 Plenary Abstract Les Szabo
USDA-ARS, Cereal Disease Laboratory
An important component of the management of wheat stem rust is an understanding of the population diversity of the pathogen, Puccinia graminis f. sp. tritici (Pgt). The discovery of “Ug99” resulted in renewed efforts on pathogen surveys, sample collections and pathotyping of Pgt, with a primary focus on Africa. In the last few years these efforts have been expanded to include other targeted regions, however a global effort is needed. The aims of the “Global Pgt Initiative” is: to capture and maintain living cultures that collectively reflect the entire global diversity of Pgt in the years 2014 - 2016; pathotype and genotype this collection; develop DNAbased diagnostic tools that will be able to rapidly detect shifts in Pgt populations, and provide an early warning system of the vulnerability of wheat to new virulent strains; and provide a genetic baseline for comparison of Pgt populations over time, both forward and backwards. This initiative will provide the wheat rust community with a geographically distributed, well characterized, living culture collection that represents the global diversity of Pgt; a global open access knowledge bank on Pgt pathotypes and genotypes; and advanced molecular diagnostic tools for rapid detection and tracking of Pgt populations. The Global Pgt Initiative represents the most comprehensive effort to capture and characterize the global diversity of Pgt and provide a unique resource to the global wheat rust community.
In 2010, Ethiopia experienced one of the largest stripe rust epidemics in recent history. Over 600,000 ha of wheat were affected, an estimated 60 million Ethiopian Birr ($US3.2 million) were spent on fungicides and large production losses were observed. Factors associated with the 2010 epidemic were conducive climatic conditions (prolonged rain and apparently optimal temperatures), large areas planted to susceptible cultivars, early infection and rapid spread of a virulent pathogen, a low level of awareness, and ineffective control measures. In 2013, highly favourable climatic conditions and early appearance of stripe rust showed remarkable similarity to the conditions observed in 2010, prompting fears of a similar major rust epidemic. However, no stripe rust epidemic developed in 2013. In contrast, only limited and localized outbreaks of stripe rust were observed in 2013; wheat crops remained in good condition and a good harvest was achieved. It seems that a series of positive and timely actions in Ethiopia contributed to the markedly different stripe rust situation in 2013 compared to 2010. The principle factors associated with the positive outcomes in 2013 are (i) effective promotion, plus rapid and widespread adoption of rust resistant wheat cultivars since 2010 - this dramatically reduced the vulnerability of the Ethiopian wheat crop; and (ii) timely and coordinated surveillance efforts, coupled to good information exchange amongst different stakeholders - this resulted in effective control and awareness campaigns that targeted emerging stripe rust outbreaks. A comparative analysis is presented which highlights the similarities and disparities between the 2010 and 2013 stripe rust situations in Ethiopia. The roles and contributions of different organisations are examined and an in-depth analysis of the biophysical conditions in the different years is presented.
The discovery of Ug99 stem rust with virulence on most widely grown wheat cultivars worldwide triggered substantial new research on host resistance genes and associated virulence dynamics in the pathogen. Ug99 is mutating and migrating, with eight variants presently known, and has spread throughout eastern Africa, across the Red Sea to Yemen and Iran, and to South Africa. It has been speculated that further movement of Ug99 spores from South Africa to South America could happen on prevailing winds that occur about eight days per month on average. While Ug99 is not yet present in South America, this is a critical entry point into the Western Hemisphere as demonstrated by introduction of soybean rust to Paraguay in 2001. Thus, work was initiated to engage countries in South America to participate in monitoring for its occurrence. Stem rust surveys are currently conducted in Argentina, Brazil, and Uruguay on a regular basis. Each country has a national agricultural institute with adequate to good capacity to perform pathotyping work, but have limitations due to inadequate greenhouse cooling. We will present the current virulence dynamics of Pgt in each country. In addition to surveys for rust, we searched for the presence of Berberis spp. in Brazil. Berberis laurina was abundantly distributed in the Rio Grande du Sul state near the city of Caçapava. Leaves sampled in October displayed low to moderate aecial infections. Determination of the pathogen species infecting B. laurina is currently being determined by physiologic and molecular methods.
The shortage of stem rust resistance genes effective against the Ug99 group prompted recent efforts to increase the number of resistance genes available to breeders. We are fortunate that many new and/or cytogenetically improved rust resistance genes are now being shared with the global wheat breeding community by their developers. If we are poor stewards of these resources, the new resistance genes will eventually be defeated, and we will waste the efforts and investments that have been made. However, if we are good stewards, we should have enough resistance to achieve sustainable, durable resistance. Stewardship can be defined as the careful and responsible management of something entrusted to one’s care. What should we do to safeguard the new resistance genes? Diversification of resistance is often suggested as a way to reduce the risk of large scale epidemics. Although diversification is generally a good idea, it cannot be at the expense of leaving new genes exposed and vulnerable. A durable combination (pyramid) must be designed so that the component genes protect each other. They should reduce the probability of simultaneous pathogen mutations to virulence and they should avoid stepwise erosion of the pyramid by preventing significant reproduction of any new race that is virulent on component genes. We need pyramids to be immune or nearly immune not only to current races, but to anticipated mutants. This objective should be achievable with three or more major genes or a combination of major and minor genes. Successful gene stewardship will depend on several things. On the technical side, we will need very good markers for each gene. Each breeding program will require strong genotyping support to assemble and then validate pyramids. Most importantly, successful stewardship will require that we organize our user community to cooperate more closely. We will need to decide which genes require special stewardship and which do not. Every user of the stewardship pool resource will need to participate in earnest. It only takes one cultivar with an unprotected gene to give the pathogen a stepping stone to greater virulence. As they say, a chain is only as strong as the weakest link
Survey of barberry and associated rust pathogens in Nepal
BGRI 2013 Poster Abstract Maria Newcomb
USDA-ARS Arid Land Agricultural Research Center
Wheat contributes directly to food security and the national economy in Nepal. Of the rusts of wheat, stripe rust causes the most frequent and severe yield losses. Race changes can lead to damaging epidemics. To better understand factors that influence regional diversity of the stripe rust and stem rust pathogens, we surveyed rusts on barberry in 2012 and 2013. Nepal has a high diversity of barberry (30 species) and elevational habitats that extend the seasonal distributions of wheat and barberry. The greatest diversity occurs from 2,700 m and above, and distributions range from 1,200 to 4,500 m. We surveyed locations in all regions (central, eastern, western, and far-western) of the hill zone. Barberry was common between 1,300 and 1,800 m where wheat is grown. In the far-western region, barberry was found near all the wheat fields we surveyed. Between 1,300 and 1,800 m, Berberis asiatica is the most common species. B. aristata is present at the upper end of this range. Aecial infections on barberry occurred in patchy distributions in both 2012 and 2013. Collections of aecia on barberry were made at 5 locations and are being identified by inoculation studies using a range of grass hosts. Additionally, the rust samples are being evaluated by real-time PCR assays using species-specific ITS primer/probes for detection of Puccinia graminis or P. striiformis. Preliminary results for 32 single-aecia samples from 2012 were negative for P. graminis; 7 were positive for the P. striiformis complex.
Stocking the Breeder’s Toolbox: An update on the status of resistance to stem rust in wheat
BGRI 2012 Plenary Abstract Mike Pumphrey
Department of Crop and Soil Sciences, Washington State University, USA
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The number of designated stem rust resistance genes has increased by ~10 over the past four years. Translocations involving several broadly-effective alien resistance genes with limited or no previous agricultural deployment were enginneered to reduce the likelihood of linkage drag, and the foundations of adult plant resistance were established. This progress resulted from international collaboration, increased global coordination, and critical financial support. By buidling on these initial accomplishments and improving genetic and genomic resources over the next four years we expect to achieve: 1. more than 10 additional formally designated stem rust resistance genes conferring resistance to Ug99-complex races, 2. robust/diagnostic DNA marker haplotypes identified for most sources of resistance, 3. multiple linkage blocks of two or more resistance genes to enhance gene pyramiding efforts, and 4. knowledge of numerous additional sources of resistance complelely or partially identified. Never before have so many resources and supporting tools been available to combat the wheat rusts. It is an opportune time for the international community to strategically deploy and responsibly steward our genetic resources for durable control of wheat stem rust.
Stripe rust of wheat (yellow rust) is a recurring production constraint in the majority of wheat growing areas of the world. The transboundary nature of the pathogen coupled with its current virulence capabilities, favorable environmental conditions, sometimes overlapping and/or continuous cultivation of susceptible varieties in stripe rust-prone zones, and genetic uniformity of certain recent ‘mega-cultivars’ were major driving forces in stripe rust epidemics worldwide. Breeding for resistance must continue be the central pillar of stripe rust control, and for this to be effective there must be adequate pathogen monitoring combined with commitment to identify and incorporate diverse sources of resistance, preferably of the durable type. Deployment of resistance will only be successful if it is combined with high yield and appropriate end-use quality to meet the needs of farmers and consumers. Suitable seed systems need to be in place for timely distribution of varieties. This paper deals with the historical impacts and current status of stripe rust epidemics and highlights the need for regional and global collaboration in mitigating the global impact of this disease.
Success in seed multiplication and delivery efforts at UAS, Dharwad
BGRI 2012 Plenary Abstract R.R. Hanchinal
University of Agricultural Services, India
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Seed is a basic, vital and central input in agriculture and in all farming systems. Timely availability of quality seeds of varieties/hybrids adapted to to different agro-climatic conditions and in sufficient quantity at affordable prices is a measure of the strength and health of an agricultural economy. Sustained increase in agricultural production requires a continuous development of improved crop varieties/hybrids, an efficient system of production, and a means of distribution to farmers. India is one of the few countries where the seed sector has advanced in parallel with the agricultural production. However, the availability of quality seed of improved varieties and hybrids is grossly inadequate and is a major constraint to enhanced production. Studies made by several workers (Gadwal 2003, Patil et al 2004, Hanchinal et al. 2007) clearly indicate that with high-volume low-value seeds, such as wheat, groundnut, soybean and chickpea, 80% of the cropping area is sown with farm-saved seeds of old and obsolete varieties During last few decades, a number of high yielding disease and pest resistant varieties/hybrids in different crops had 10 to 40% yield superiority over local cultivars. With the exception of high-value low-volume seeds, seed production of low-value high-volume crops is generally left to public sector agencies. The bulky nature of most self pollinated crops, and lack of adequate investment on infrastructure means low remuneration. Although there is enough breeder seed production in most of the high volume crops, further seed multiplication through the foundation and certified seed stages are major constraints to the availability of quality seed. The present rate of seed replacement (SRR) for such crops is 6 to 8%. There is a need to increase SRR to 25 to 30% in varieties and obviously 100% for hybrids. To increase the productivity of low-value high-volume crops farmers need to have access to improved seeds of the right type, at the right time, at the right place and at a reasonable price. For supply of such seeds, both the informal seed sector (farmer managed seed systems) and the formal seed system (seed enterprises) need to be engaged. The informal seed sector is often highly effective in reaching isolated, inaccessible, small holder areas and is a sound opportunity for entrepreneurs to gradually evolve into the formal enterprises Wheat, the most important food crop of world and backbone of global food security, belongs to the highvolume low-value seed group. Of the total area sown to both hexaploid bread wheat and tetraploid durum and emmer wheat worldwide, 44% (95 m ha) is in Asia. Of this,62 m ha are located in just three countries viz. China, India, and Pakistan (Table 1 and Figure 1). Food security and production stability are of paramount importance in most Asian countries, given that the majority of farmers are poor. The wheat rusts have historically been major biotic constraints both in Asia and the rest of the world. Stem rust has been under control since the beginning of the green revolution in South and West Asia in the 1960s. Leaf rust and stripe rust continue to be major threats to production over approximately 60 (63%) and 43 (46%) m ha, respectively, in Asia. Although, the timely application of fungicides can provide adequate control, their use adds to production costs and they are considered environmentally unsafe. Growing resistant cultivars is thus the most effective and efficient control strategy, as it has no cost to farmers and is environmentally safe. Rapid evolution of races with new virulences, or combinations of virulences, dictate a need for discovery and deployment of new resistance genes and/or resistance gene combinations.
How has Lr34/Yr18 conferred effective rust resistance in wheat for so long?
BGRI 2012 Plenary Abstract Beat Keller
Institute of Plant Biology, University of Zurich, Switzerland
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The Lr34/Yr18 gene has been used in agriculture for more than 100 years. In contrast to many other resistance sources against leaf rust and stripe rust, it has remained effective and no virulence has been reported. This makes Lr34 a unique and highly valuable resource for rust resistance breeding. The pleiotropic nature of the gene conferring partial resistance to different pathogen species, the associated leaf tip necrosis and its durability suggest a molecular mechanism that is different from major gene resistance. This is supported by the molecular nature of Lr34 which was recently found to encode an ABC transporter. Interestingly, all tested wheat lines contain an allele of the Lr34 gene on chromosome 7DS. In its susceptible form, the gene does not confer resistance. The difference between the encoded resistant and susceptible LR34 isoforms consists of only two amino acid changes, whereas the rest of the proteins are identical. These two changes must change the biochemical properties of the resistant LR34 transporter in such a way that the plant becomes resistant. We speculate that there is a slight conformational change in the resistant form of the protein, resulting either in modified specificity or kinetics of the transported molecule, or that the binding properties to an unknown second protein interacting with LR34 are changed, resulting in altered function. While the molecular nature of the molecule(s) transported by the LR34 protein remains unclear, it is likely that a physiological change related to Lr34 activity is at the basis of resistance. We are currently establishing transgenic approaches in heterologous grass species to further investigate the molecular activity of Lr34 and to better understand a physiological mechanisms resulting in disease resistance.