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Chilean journal of agricultural research

On-line version ISSN 0718-5839

Chilean J. Agric. Res. vol.75 no.4 Chillán Dec. 2015

http://dx.doi.org/10.4067/S0718-58392015000500009 

RESEARCH

Physiologic specialization of Puccinia triticina Erikss. and effectiveness of Lr-genes in the south of Ukraine during 2013-2014

Olga Babayants1, Lazar Babayants1, Andrii Gorash2*, Alexey Vasilev1, Vita Traskovetskaya1, and Alexey Galaev1


1Plant Breeding and Genetics Institute, National Center of Seed and Cultivar Investigation, 65036, Odessa, Ukraine.
2Institute of Agriculture, Lithuanian Research Centre for Agriculture and Forestry, Department of Cereal Breeding, LT-58344, Akademija, Kedainiai distr., Lithuania. *Corresponding author (andrej@lzi.lt).


Leaf rust is the most widespread and frequently occurring fungal disease of wheat (Triticum aestivum L.) in Ukraine and worldwide. The information about the effectiveness of Lr-genes and also the consequent monitoring of virulence dynamics is necessary for the successful wheat breeding for leaf rust resistance. In 2013-2014 pathotype composition and virulence analysis was studied both on the standard differential set and on the North American System of Nomenclature. According to the standard differential set, 12 phenotypes were identified, of which the most common were 77 (75%) and 144 (6%). A total of 40 phenotypes were identified on the North American Nomenclature. Phenotypes TGTT (24%) and TJTT (8%) were the most frequent, TRTT (1.5%) and TSTT (1.5%) were within the broadest spectrum of virulence among the isolates found in the south of Ukraine. For virulence analysis we used wheat lines of 'Thatcher' that are near-isogenic for 24 leaf rust resistance genes and additionally four cultivars/lines. No virulence to Lr19 was found, whereas increasing virulence to Lr9 was detected (13%). Low frequency of virulence was observed to Lr29 (11%) and Lr47 (21%), high level of virulence was detected to other genes. The effectiveness of 53 known Lr-genes was studied at the seedling and the adult plant stages. Most of them were not effective against leaf rust. Genes Lr9, Lr19, Lr29, and Lr47 were highly effective both at the seedling stage and at adult plant stage. Genes Lr24, Lr42, Lr50, Lr51, and Lr56 were effective only at the adult plant stage.

Key words: Leaf rust, resistance, Triticum aestivum, virulence analysis, wheat.


INTRODUCTION

Leaf rust (caused by Puccinia triticina Erikss.) is the most widespread and frequently occurring fungal disease of wheat (Triticum aestivum L.) in Ukraine and throughout the world (Babayants, 2011; Huerta-Espino et al., 2011). Growing of resistant cultivars is the safest and economically profitable way to control fungal plant diseases. However, wheat breeding for resistance is complicated because of the pathogen's ability to overcome the host resistance. The new pathotypes are continuously emerging due to sexual recombination and mutational processes. The nature of urediniospores enables them to migrate by air for thousands of kilometers, which causes the spread of new virulent pathotypes throughout the world (Kolmer, 2005). The information about the effectiveness of Lr-genes and also the consequent monitoring of virulence dynamics are necessary for successful wheat breeding for resistance to leaf rust and for timely detection of new phenotypes to adjust the breeding programs.

A nomenclature to divide the population of leaf rust into separate pathotype "races" was proposed after discovery of physiologic races by Mains and Jackson in 1926 (Mains and Jackson, 1926). Then, after a few modifications, the nomenclature was adopted on the basis of eight cultivars (Johnson and Browder, 1966). However, after emergence of new pathotypes which, according to their characteristics, did not match up with this code, different researchers assigned them different designations. Another disadvantage is that these differentials carry Lr-genes in different genetic background and some of them carry several Lr-genes, for example 'Carina' (Lr2b, LrB), 'Brevit' (Lr2c, LrB), 'Webster' (Lr2a, Lr14a, Lr27), 'Loros' (Lr2c, Lr2d), 'Mediterranean' (Lr2a, Lr3a) share different Lr-genes (Mcintosh et al., 1995; GRIS, 2014). When the near-isogenic lines which possess single Lr-genes were developed they started to be used as differentials among new set of wheat cultivars. Most researchers have replaced the term physiologic race by the term pathotype or phenotype (Kolmer, 2005; Hanzalova and Bartos, 2006). In 1989, a North American System of Nomenclature for Puccinia triticina was proposed. According to this differential set, it was proposed to use 12 near-isogenic lines of 'Thatcher' with single Lr- genes. The differentials were grouped into three sets of four lines. The first set of differentials consisted of 'Thatcher' lines with genes Lr1, Lr2a, Lr2c, Lr3a, the second Lr9, Lr16, Lr24, Lr26, the third Lr3ka, Lr11, Lr17a, Lr30. The pathotypes characteristics are coded in accordance with the type of reaction by one of 16 letters for each of three sets; accordingly, a three-letter code was assigned to the pathotypes (Long and Kolmer, 1989). This nomenclature has become the most common and is being extensively used throughout most of the world (Huerta-Espino et al., 2011). Virulence analysis of leaf rust population requires information about changes of virulence first of all for genes which present interest for specific climatic-territorial areas where the cultivars are planned to be grown. Therefore phytopathologists are attempting to add the differentials with certain Lr-genes to the differential set, according to specificity of pathogen's population of their region. In the last years, a series of additional sets of 'Thatcher' near-isogenic lines has been proposed to be included in the original North American differential set in the USA and other countries (Singh, 1991; Kolmer and Liu, 2000; McVey et al., 2004; Mantovani et al., 2010). Supplemental fourth set of LrB, Lr10, Lr14a, and Lr18 was added by Kolmer et al. (2007) and included in the national virulence surveys in the USA and Canada. The fifth set of differentials with two 'Thatcher' lines with Lr21, Lr28 and two winter wheat lines with genes Lr41 and Lr42 were added to the national virulence surveys in the USA in 2004, but in 2007 the line with Lr42 was dropped after it had been determined that it also possessed Lr24 (Kolmer et al., 2007; Kolmer and Hughes, 2013).

In European countries, a standard differential set which consists of 15 Thatcher near isogenic lines with the single leaf rust genes Lr1, Lr2a, Lr2b, Lr2c, Lr3a, Lr9, Lr11, Lr15, Lr17, Lr19, Lr21, Lr23, Lr24, Lr26 and Lr28 has been adopted. Pathotype analysis is conducted using digital nomenclature with triplet set and octal codes of virulence according to Hanzalova and Bartos (2006) and to Goyeau et al. (2006). Researchers in Nepal, Bangladesh, Pakistan, South Africa, and others countries have reported races using both their own nomenclatures and the North American Nomenclature. However, breeders from India and Australia are still using their own binomial systems of nomenclature (Huerta-Espino et al., 2011; Terefe et al., 2014).

Physiologic races of P. triticina have been studied in Plant Breeding and Genetics Institute in Ukraine since the 1960's on an old standard differential set. In the present study, we used both the old standard differential set and the North American Nomenclature to compare the old findings of the old standard differential set with the North American differential set for P. triticina. The aim of the study was to conduct a pathotype analysis of leaf rust population and to study the effectiveness of 53 known Lr-genes at the seedling and adult plant stages in the south of Ukraine.

MATERIALS AND METHODS

Samples of leaf rust infected wheat leaves of different cultivars and breeding lines were collected from commercial fields and research plots at various locations throughout the south of Ukraine. All collected leaves were air dried at room temperature and stored at 4 °C until spores were collected for inoculation and multiplication of the inoculum. The virulence survey was based on monopustule isolates. Puccinia triticina urediniospores from individual collections were scraped off and mixed with water and Tween 20. The susceptible cultivars 'Michigan Amber' and 'Odesskaya polukarlikovaya' were inoculated with the suspension of urediniospores. Approximately 14 d after inoculation, using a microbiological loop, separate monopustule isolates were transferred on the detached leaves of 'Michigan Amber' and 'Odesskaya polukarlikovaya', kept in the solution of benzimidazol in Petri dishes (Babayants and Babayants, 2014). Microbiological loop was sterilized each time before transferring the new pustules. Each monopustule isolate was transferred into a separate Petri dish. When the inoculum was multiplied, approximately after 14 d, differentials were also inoculated using the same procedure.

A total of 113 monopustule isolates for phenotype and virulence analyses were studied. Phenotype analysis was conducted on the standard differential set of eight cultivars: Malakoff, Carina, Brevit, Webster, Loros, Mediterranean, Hussar, Democrat (Johnson and Browder, 1966) and the North American differential set for P. triticina on 16 near-isogenic lines of 'Thatcher' with single leaf rust resistance genes Lr1, Lr2a, Lr2c, Lr3a, Lr3ka, Lr9, Lr10, Lr11, Lr14a, Lr16, Lr17a, Lr18, Lr24, Lr26, Lr30, and LrB (Long and Kolmer, 1989; Kolmer et al., 2007).

Virulence analysis of leaf rust population was studied on the near-isogenic lines of 'Thatcher' with single genes: LrB, Lr1, Lr2a, Lr2c, Lr3a, Lr3ka, Lr9, Lr10, Lr11, Lr14a, Lr17, Lr18, Lr19, Lr20, Lr21, Lr22a, Lr23, Lr24, Lr25, Lr26, Lr29, Lr30, Lr38, and Lr64. In addition four cultivars/lines 'Pavon 76' (Lr1, Lr10, Lr13, Lr14a, Lr2b, Lr22a, Lr27, Lr46), 'Pavon 753' (Lr47), 'KS86WGRC02' (Lr39) and 'KS92WGRC16' (Lr42, Lr39, Lr21) were used.

The effectiveness of 53 Lr-genes was studied both at the seedling stage and at the stage of adult plant. The resistance at the seedling stage was assessed in the greenhouse under controlled conditions (temperature: +20 ± 2 °C, illuminance: 10 000 lux, 16:8 h photoperiod). Ten days old plants were inoculated by a mix of urediniospores with talcum powder. After inoculation, to create the conditions of a dew chamber, the plants were put in the polyethylene bags and stored in the dark at 21 °C for 16 h. Type of resistance and disease severity were scored on the 12th day after inoculation. Type of resistance at the seedling stage was scored according to the scale: very resistant (VR), resistant (R), moderately resistant (MR), moderately susceptible (MS), susceptible (S), and very susceptible (VS). Disease severity was scored by the scale of Peterson (Babayants and Babayants, 2014).

The field resistance was studied in the artificially infected nursery. The differentials were grown in 1 m long rows (three rows for each differential), the distance between rows was 30 cm. Two rows of the most rust-susceptible spreader ('Odesskaya polukarlikovaya') were planted around the nursery. Also, one row of this susceptible control was planted at every 10th test entry. The leaf rust differentials were inoculated with a mixture of urediniospores and talcum powder according to Babayants procedure (Babayants and Babayants, 2014). Inoculation was done in 22 April 2013 and 28 April 2014. Environmental conditions for pathogen development are presented in Table 1. Adult resistance in the field was scored according to a 9 point scale: 1-2 very high susceptibility, 3 high susceptibility, 4 susceptibility, 5 moderate susceptibility, 6 moderate resistance, 7 resistance, 8 high resistance, and 9 immunity (Babayants and Babayants, 2014). The seeds of differentials and wheat lines/cultivars with Lr-genes were provided via USDA Germplasm Resources Information Network (GRIN) (USDA ARS, 2014).

Table 1. Meteorological data during the inoculation and Puccinia triticina development.


Max: maximum, Min: minimum, Avg: average.



RESULTS

Phenotype composition
Phenotype analysis was conducted using two differential sets - the standard differential set and the North American System of Nomenclature for P. triticina. On the standard differential set, 12 different phenotypes were revealed in 2013. The most common races were 77 (75%) and 144 (6%), noticed also the others 149 (4%), 117 (3%), 192 (2%), 114 (2%), 6 (2%), 21 (2%), 42 (1%), 57 (1%), 20 (1%), 122 (1%) (Table 2). Forty phenotypes for P. triticina were identified on the North American Nomenclature. Among them two phenotypes TGTT (24%) and TJTT (8%) were dominant. Phenotypes PHTT, TQTT, TGPT, RGTT, TGKT, and PBKK had an occurrence frequency of 3% each, while others were represented by single isolates. Phenotype TGTT was avirulent to Lr9, Lr24 and Lr26 and it was virulent against the other 13 Lr-genes of the differential set. TJTT was avirulent only to Lr9 and Lr26. Phenotypes TRTT and TSTT with the frequency of occurrence of 1.5% possess the broadest spectrum of virulence, the former was avirulent only to Lr24 and the latter to Lr26. A total of 40 phenotypes of wheat leaf rust were identified on the North America Nomenclature in the south of Ukraine in 2013 (Table 3).

Table 2. The virulence formula and frequency of Puccinia triticina phenotypes identified on the standard differential set in the south of Ukraine in 2013.


R: Resistant, S: susceptible, I: intermediate.


Table 3. Virulence formula and frequency of Puccinia triticina phenotypes in the south of Ukraine in 2013 identified on the North American Nomenclature (16 near-isogenic lines with single Lr-genes).


Virulence analysis of Puccinia triticina population
The virulence analysis was done on 26 near isogenic lines of 'Thatcher' and also on the four wheat cultivars/lines (Tables 4 and 5). None of the 113 studied monopustule isolates had virulence to gene Lr19. In the previous years the frequency of virulence to Lr19 varied from 0% to 4% (Babayants et al., 2004; Babayants, 2011).

Table 4. Frequency of occurrence of virulent isolates of Puccinia triticina in the south of Ukraine in 2013 (at the seedling stage).




Table 5. Frequency of occurrence of virulent monopustule isolates of Puccinia triticina in cultivars/lines with Lr-genes in the south of
Ukraine in 2013.


1The presence of Lr-genes in accordance with Genetic Resources Information System for Wheat and Triticale (GRIS).
2Accession numbers in accordance with Research Service of Germplasm Resource Information Network (GRIN).

The frequency of monopustule isolates virulent to Lr9 was 13%, which is substantially higher than that over the last 20 yr, the maximal frequency of virulence (10%) was noticed in 2003 (Babayants, 2011). The virulence to Lr29 was 11% lower than that in previous years. Gene Lr64 was studied for the first time, the frequency of virulent monopustule isolates to it was 19%. In comparison with previous years the frequency of virulence to Lr26 slightly decreased (17%), in previous years it varied from 40% to 80% (Babayants, 2011).

The frequency of virulent monopustule isolates to gene Lr24 was 26%, in previous years it varied from low (less than 10%) in 1996, 1998, 2002, 2006, 2007 to high in 1999, 2001, and 2003 (> 50%) (Babayants, 2011). The frequency of virulence to Lr25 was 26% (Table 4). Medium frequency of virulence was observed against genes Lr2a, Lr21, Lr22a and Lr23. A high frequency of virulence was noticed to genes LrB, Lr1, Lr2c, Lr3a, Lr3ka, Lr10, Lr11, Lr14a, Lr16, Lr17, Lr18, Lr20 and Lr30 (Table 4). Virulence to lines KS86WGRC02 (Lr39/41), KS92WGRC16 (Lr42, Lr39, Lr21) and 'Pavon 753' (Lr47) was 70%, 54%, and 21% respectively, to 'Pavon 76' (Lr1, Lr10, Lr13, Lr14a, Lr2b, Lr22a, Lr27, Lr46) was 96% (Table 5).

Effectiveness of the Lr-genes in the seedling stage
Seedling resistance of Lr-genes was studied after artificial inoculation by local population of leaf rust. The effectiveness of genes was divided according to the type of reaction into the groups VR, R, MS, S, and VS. The genes within each group were subdivided also according to the disease severity (Table 6).

Table 6. Results of phytopathological evaluation of Lr-genes at the seedling stage.




Highly efficient were Lr9 and Lr19, disease symptoms were not observed. Genes Lr29 and Lr47 also had resistance type of reaction, but the disease severity was 5% and 25%, respectively (Table 6). Moderately susceptible were genes Lr24 and Lr25 with a disease severity of 5%, this group also included genes Lr56 with a disease severity of 10%, 15% for Lr64, Lr39 and Lr42, 25% for Lr45, 40% for Lr15, Lr16, Lr51, and Lr52, 65% for Lr44 and Lr50 (Table 6). The greater part of genes fell within the group exhibiting susceptibility. Lines carrying genes Lr23, Lr22a showed lower level of susceptibility (5%) in this group, a bit higher level of susceptibility (10%) was demonstrated by lines with Lr18, Lr21 and Lr26 genes (Table 6). Lines with genes Lr1, Lr3bg and 'Odesskaya polukarlikovaya' were highly susceptible with a susceptibility level of 65% (Table 6).

Effectiveness of the Lr-genes in the field
Phytopathological evaluation of wheat lines and cultivars with known genes of resistance to P. triticina was observed in the infected nurseries in 2013 and 2014 (Table 7).

Environmental conditions during pathogen inoculation were more favorable in 2013, indicator of disease development 'Odesskaya polukarlikovaya' scored 2 points. In 2014, there was observed a slightly lower infection pressure of the pathogen, 'Odesskaya polukarlikovaya' scored 4 points. Genes Lr9, Lr19, Lr24, Lr29, Lr42, Lr47, Lr50 and Lr51 were resistant in the field tests during the two experimental years. The genes Lr25, Lr44, Lr52, Lr53, Lr56 and Lr64 were resistant in 2014, but in 2013 they were susceptible (Table 7).

Table 7. Results of phytopathological evaluation of Lr-genes at the adult plant stage.


1Accession numbers in accordance with Research Service of Germplasm Resource Information Network (GRIN).
2Scoring by 1 to 9 point scale, 1: very high susceptibility, 9: immunity.

Gene Status/Name Accession number1 20132 20142 Lr1 NIL GSTR 402 2 4 Lr2a NIL GSTR 403 2 4 Lr2b NIL GSTR 404 2 4 Lr2c NIL GSTR 405 2 3 Lr3 NIL GSTR 406 2 2 Lr3bg NIL GSTR 407 2 4 Lr3ka NIL GSTR 408 2 5 Lr9 NIL GSTR 409 8 7 Lr10 NIL GSTR 410 2 4 Lr11 NIL GSTR 411 2 4 Lr12 NIL GSTR 412 6 5 Lr13 NIL GSTR 413 2 5 Lr14a NIL GSTR 414 2 3 Lr14b NIL GSTR 415 2 4 Lr15 NIL GSTR 416 4 5 Lr16 NIL GSTR 417 2 5 Lr17 NIL GSTR 418 5 5 Lr18 NIL GSTR 419 5 5 Lr19 NIL GSTR 420 9 7 Lr20 NIL GSTR 421 2 6 Lr21 NIL GSTR 422 5 4 Lr22a NIL GSTR 423 6 4 Lr23 NIL GSTR 424 6 5 Lr24 NIL GSTR 425 8 7 Lr25 NIL GSTR 426 5 8 Lr26 NIL GSTR 427 4 6 Lr27 Gatcher PI 377884 3 5 Lr28 NIL GSTR 428 4 5 Lr29 NIL GSTR 429 7 8 Lr30 NIL GSTR 430 2 5 Lr32 NIL GSTR 431 4 5 Lr33 NIL GSTR 432 2 3 Lr34 NIL GSTR 433 5 5 Lr35 NIL GSTR 434 5 5 Lr36 Genetic material GSTR 435 6 6 Lr37 NIL GSTR 436 5 6 Lr38 NIL GSTR 437 2 4 Lr39/41 KS86WGRC02 PI 504517 6 7 Lr42 KS92WGRC16 PI 592728 9 8 Lr44 NIL GSTR 438 4 8 Lr45 NIL GSTR 439 6 5 Lr46 Pavon 76 PI 519847 6 6 Lr47 Pavon 753 GSTR 440 8 8 Lr50 KS96WGRC36 PI 604221 9 8 Lr51 NIL GSTR 441 8 8 Lr52 NIL GSTR 442 6 8 Lr53 98M71 PI 648417 6 8 Lr54 Genetic material PI 648418 3 6 Lr56 Genetic material PI 648419 8 8 Lr60 Genetic material GSTR 443 3 4 Lr63 NIL GSTR 444 2 4 Lr64 NIL GSTR 445 6 8 1Accession numbers in accordance with Research Service of Germplasm Resource Information Network (GRIN). 2Pedigree in accordance with Genetic Resources Information System for Wheat and Triticale (GRIS).

DISCUSSION

Twelve phenotypes were identified on the old differential set in the population of leaf rust in 2013. The most common phenotypes were 77 (75%) and 144 (6%). Over the last 20 yr a total of 47 known and 15 new races in the population of P. triticina were found in the south of Ukraine (Babayants, 2011). According to the North American nomenclature, the dominant phenotypes in the population of leaf rust were TGTT (24%) and TJTT (8%), both phenotypes possess a broad spectrum of virulence and according to the old nomenclature both may belong to the "race 77". However, the first phenotype was avirulent to Lr24, whereas the second phenotype was virulent to it. The obtained results showed that the North American differential set provides more important information and much better differential ability than the old standard differential set used before in Ukraine. The presence of the effective gene Lr9 and genes with partial resistance Lr24 and Lr26 in the nomenclature makes more significant phenotype differentiation for south of Ukraine. The advantage of the North American nomenclature over the standard differential set also was established by other researchers (Todorova and Kiryakova, 2001). Moreover, the researchers who work independently using the letter code nomenclature will give the same code-name to any new physiologic pathotypes that code the same on this system. This allows researchers to speak the same language and have a common understanding of the virulence phenotypes. However, some Lr-genes which are effective in the south of Ukraine are not included in the differential set. Therefore for virulence analysis we used additional 'Thatcher' near-isogenic lines with Lr19, Lr20, Lr21 , Lr22a, Lr23, Lr24, Lr25, Lr29 and Lr64 Lr-genes, 'Pavon 76' (Lr1, Lr10, Lr13, Lr14a, Lr2b, Lr22a, Lr27, Lr46) and wheat lines 'Pavon 753' (Lr47), KS86WGRC02 (Lr39) and KS92WGRC16 (Lr42, Lr39, Lr21) which are new or possess resistance and present local interest for our environments. Virulence analysis exhibited that the frequency of virulence to Lr9 was increased by 13% in 2013. It is generally accepted, that when the frequency of virulence reaches higher than 10% efficiency of gene may be overcome by pathogen. When the frequency of virulent pathotypes and/or their aggressiveness increase, this gene may be shortly overcome by the pathogen in the south of Ukraine. Breaking down of Lr9 was noticed in the USA and Canada (McCallum et al., 2011; Kolmer et al., 2012).

Lr19 remains highly efficient, no virulent isolates were found in the population of leaf rust. Virulence to that resistant gene has not been found in other European countries (Mesterhazy et al., 2000; Huerta-Espino et al., 2011). But according to the published data a large quantity of virulent pathotypes (40%) were detected in the population of leaf rust in Volga region of Russia (Kurbanova, 2011; Ivanova, 2013).

The gene Lr24 derived from Agropyron elongatum (Host.) Neviski (McIntosh et al., 2013). According to the last monitoring, virulence to Lr24 was very rare in Germany, Spain, Hungary, Slovak Republic (Huerta-Espino et al., 2011), and Czech Republic (Hanzalova et al., 2013), Lithuania (Liatukas, 2003) and China (Liu and Chen, 2012). In Ukraine, Lr24 exhibited moderate susceptibility at the seedling stage and resistance at adult plant stages.

Gene Lr29 derived from A. elongatum (McIntosh et al., 2013) under artificial inoculation by population of leaf rust at the seedling stage provided resistance; the frequency of virulence to it was 11%. However, in the previous years virulence to it was relatively high. This indicates that this gene is unreliable against leaf rust (Babayants et al., 2004; Babayants, 2011).

Slow rusting gene Lr34 has been providing resistance for more than 50 yr, and Lr46 has remained effective since 1976 in the USA. Also, it was established that Lr34 has the ability to enhance the resistance to P. triticina in combination with other Lr-genes and to provide positive pleiotropic effect to other wheat diseases (German and Kolmer, 1992; Kolmer et al., 2008). The synergy between Lr34 and other Lr-genes also was detected in our environments (unpublished data). However, Lr34 in single use did not provide sufficient level of resistance in the population of southern Ukraine. It was highly susceptible at the seedling stage and susceptible (scored 6) at the adult plant stages. Gene Lr46 in combination with other genes Lr1, Lr10, Lr13, Lr14a, Lr2b, Lr22a, and Lr27 in background of 'Pavon 76' also did not provide sufficient level of resistance.

Gene Lr42 derived from Aegilops tauschii Coss. (McIntosh et al., 2013) exhibited moderate susceptibility in the seedling test and resistance in the field test. This indicates that it provides sufficient level of resistance only at the adult plant stages. Gene Lr47 derived from Triticum speltoides (Tausch) Gren. ex K. Richt. (McIntosh et al., 2013) provided resistance in seedling test and in the field in 2013 and 2014, but virulence to it at the seedling plant stage was 21%. This indicates that resistance may be soon overcome by pathogen. Genes Lr50, Lr51 and Lr56 were studied by us for the first time and exhibited resistance in the field test during 2 yr, but Lr50 was susceptible, and Lr51 , Lr56 were moderately susceptible in the seedling test. That may indicate that they act as the adult plant stages genes. Gene Lr64 derived from Triticum dicoccoides (McIntosh et al., 2013) was also studied by us for the first time. At the seedling stage plants showed moderate susceptibility, the frequency of virulence was 19%, at the adult plant stages this gene provided susceptibility in 2013 and resistance in 2014. The resistance in 2014 and susceptibility in 2013 of genes Lr25, Lr44, Lr52, Lr53, Lr56 and Lr64 in the field test can be caused by different pathogen development. Artificial leaf rust inoculation was more successful in 2013, which can be explained by a very small amount of precipitation in April 2014 during the crucial period for inoculation (Table 1). Urediniospores begin to develop a germ tube and penetrate the cell only when the moisture is present in the form of dew or light rain on the leaf surface. Germination occurs after 8 h at 18 °C, spores possess the ability to retain viability only 1-3 d after inoculation under field conditions in the absence of the dew period (Babayants and Babayants, 2014). Due the better environment conditions for pathogen development, the infection of leaf rust developed better in 2013 than in 2014. In 2013, when infection pressure was higher genes Lr25, Lr44, Lr52, Lr53, Lr56, and Lr64 were susceptible, which may indicate their insufficiency; under optimal conditions for pathogen development they cannot provide sufficient level of resistance.

The Ukrainian population of leaf rust consists of broad range of pathotypes with different spectrum and frequency of virulence. This point to high pathogen evolutionary ability, and consequently a large part of known Lr-genes have lost their efficiency and cannot be used as donors of resistance. Thus, single use of these Lr-genes cannot provide durable defense against wheat leaf rust. Theoretically, if resistance is controlled at one single locus, only one mutation in the corresponding avirulent gene may lead to emergence of a new virulent pathotype. Therefore several breeding strategies, pyramiding and slow rusting to enhance the durability of resistance to leaf rust have been proposed. Gene pyramiding is incorporation of several Lr-genes into a single genotype. Slow rusting is conferred by genes which provide a longer latent period of disease, lower spore production and as a result smaller areas under the disease progress curve than a susceptible control (Singh et al., 2011). For gene pyramiding it is needed to choose the major genes to which corresponds a very low ratio of virulence in pathogen population and then they could be used in combination with seedling partial resistance genes or in combination with adult non-specific genes (Krattinger et al., 2009; Dakouri et al., 2013).

CONCLUSIONS

The population of leaf rust consisted of different phenotypes in the south of Ukraine. According to the standard differential set, there were identified 12 phenotypes of which the most common were 77 (75%) and 144 (6%), 149 (4%), 117 (3%), 192 (2%), 114 (2%), 6 (2%), 21 (2%), 42 (1%), 57 (1%), 20 (1%), 122 (1%). The North American nomenclature provides much more differential ability, and using this nomenclature we identified 40 phenotypes, of which the most frequent were TGTT (24%) and TJTT (8%), phenotypes TRTT (1.5%) and TSTT (1.5%) possess the broadest spectrum of virulence among the isolates found in the south of Ukraine. Among all studied Lr-genes no isolates were virulent to Lr19, against other Lr-genes virulence frequency varied from 11 to 97%. Low frequency of virulence was observed to Lr29 (11%), Lr9 (13%) and Lr47 (21%), high level of virulence was detected to other genes. Most of the known Lr-genes do not provide resistance either at the seedling or at the adult plant stage. Genes Lr9, Lr19, Lr29 and Lr47 were highly effective both at the seedling stage and in the field test. Genes Lr24, Lr42, Lr50, Lr51 and Lr56 were effective only at the adult plant stage during the two experimental years.

LITERATURE CITED

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Received: 19 December 2014.
Accepted: 21 May 2015.


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