Introduction

Potato is one of the strategic crops in Polish agriculture. In 2009 potato crop, despite decreasing tendency observed in the last decade, approximately 600 thousand ha were planted with 12 million tonnes of total yield. The seed production in the country was held on 5000 ha with the total seed tuber yield of 60 thousand tonnes. The low rate of certified seeds planted by farmers (about 8 %) leads to a high Potato virus Y (PVY) infection of widely grown cultivars for fresh market and processing. Losses in potato crop due to infection with PVY might vary between 10 and 90 % and result from decreased size and number of harvested tubers (Jeffries 1998; Novy et al. 2002). Occurrence of the PVYNTN variant, which induces potato tuber necrotic ringspot disease (PTNRD), causes additional quality losses in table and processed cultivars. Breeding against sensitivity to PTNRD has increasing importance in the selection of new cultivars (Valkonen 2007).

Resistance to PVY is one of the most important resistance traits in seed and ware potato production in Poland. This paper presents the elements of breeding research focused on changes in the structure of PVY population, breeding for resistance to PVY and evaluation of reaction to various PVY strains in cultivars registered in Poland.

Structure of PVY Population in Poland

The PVY population at Młochów (central Poland) has been monitored continuously on tobacco (cv. Samsun) bait plants mostly every second  year since 1980. Based on ELISA results with strain specific mAbs and non-specific Ab and on symptoms on tobacco, the isolates were classified into PVYO (PVYO serotype, vein clearing on tobacco), PVYN (PVYN serotype, vein necrosis on tobacco) and PVYN-Wi (PVYO serotype, vein necrosis on tobacco) strain groups. Selected isolates showing positive reaction to PVYN mAb were tested on sensitive potato cv. Vital, Igor and Nicola. The isolates invoking necroses on tubers were classified as PVYNTN strain. All tested PVYN isolates induced PTNRD, indicating a PVYNTN strain type. As shown in Fig. 1, in early 1980s PVYO strain was up to 85–90 % of the PVY population and it had dropped below 10 % since 1986. From 1984 to 2004 PVYN-Wi strain had dominated in the PVY population (Chrzanowska 1991) and then decreased to 32 % in 2008. However, in 2010, PVYN-Wi became the dominant form again (62 %). PVYN and/or PVYNTN strain had gradually increased since 1994 (Chrzanowska and Doroszewska 1997). In 2004, a significant increase of PVYN and/or PVYNTN frequency was noted and it reached 66 % of the population in 2008. Among the isolates tested in 2010, 14 % reacted positively to both PVYN and PVYO specific mAb, what might be indication of mixed infection.

Fig. 1
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Changes of PVY population at Młochów observed on tobacco bait plants (cv. Samsun) in potato fields from 1980 to 2010

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Our study showed that currently the tobacco-necrosis inducing PVYN strain including variant isolates PVYN-Wi and PVYNTN dominates in the PVY population in Młochów (central Poland). Similarly, in some other European countries, a progressive increase of tobacco-necrosis inducing PVYN strain in natural populations infecting potato was also noted during the last two decades (e.g. Dědič et al. 2008; Van der Vlugt et al. 2008; Lindner 2008; Rolot and Steyer 2008). In contrast, although PVYN was the dominant form (91 %) in Tunisia in 2006, it was composed from PVYNTN only, in the absence of PVYN-Wi (Boukhris-Bouhachem et al. 2010). In China, a 1:1 ratio of PVYO and PVYN was detected in 2008 (Bai et al. 2010). In the United States and Canada, PVYO isolates were the most common, accounted for more than 60 % of the population (Gray et al. 2010).

The worldwide spread of PVYNTN and PVYN-Wi has been explained by their selective advantage over the parent strains (Kerlan 2003/4). In Poland, transmissibility by Myzus persicae of PVYNTN and PVYN-Wi isolates has been found to be higher than that of PVYO and PVYN isolates (Kaliciak and Syller 2009) and PVYNTN was more effectively transmitted than PVYN-Wi (Kostiw and Trojanowska 2011). Moreover, PVYNTN and PVYN-Wi can frequently escape detection by visual inspection in seed potato certification schemes because of mild symptoms on potato plants (Kerlan 2003/4). This is especially true for Polish isolates of PVYN-Wi which seem to be more infective to most potato cultivars and causes mild mosaic symptoms, making negative selection in growing seed crops difficult (Chrzanowska 1994).

Breeding for PVY Resistance

The field resistance to PVY expressed under natural conditions is of polygenic nature and is present in majority of grown cultivars. Resistance related to localized infection, expressed as hypersensitive resistance (HR) or extreme resistance (ER) is governed by the major dominant genes (Solomon-Blackburn and Barker 2001). Our breeding for PVY resistance was initiated in early 1950s within a program for development of the parental lines with multiple resistances to viruses, late blight and soft rot (Świeżyński 1991). At first, resistance to PVY infection was introduced into our program from S. tuberosum. In the course of time, the resistance to infection was replaced by extreme resistance governed by the single, dominant genes (Dziewońska 1967). Various independent S. stoloniferum sources of extreme resistance were utilized in the parental line breeding program, namely: the clone XIIB from IHAR collection (Dziewońska and Pochitonow 1971), two clones C.854 and C.858 from UK (supplied by Cockerham), clones MPI 55.957/24 and MPI 55.957/54 from the Max-Planck Institute for Plant Breeding Research, Germany (Ross 1958). All these clones were male sterile, however in 1982, the exploitation of male fertile source of ER began. Such a source was identified in the accessions of S. stoloniferum from collection of Vavilov Research Institute of Plant Industry (VIR), Russia, which were kindly supplied by Bukasov (Butkiewicz 1988).

The resistance to PVY infection was also introduced into diploid pre-breeding program, conducted parallel to 4x parental lines program. The donor of this resistance was S. chacoense clone PK133 from the Wyszoborz collection, Poland.

The gene Ry-f sto from VIR source of resistance provides complete resistance to all known strains of PVY and Potato virus A (PVA). The gene was mapped to chromosome XII (Flis et al. 2005) and was found to be located at the same position as Ry sto mapped by Song et al. (2005). The developed CAPS marker GP122718 tightly linked to resistance gene was successfully used for identification of cultivars expressing ER to PVY. The PCR product of expected size (718 bp) was found after EcoRV digestion during testing 19 Polish and 8 German (Flis et al. 2005) and, additionally, in next 5 Polish extremely resistant cultivars (Witek et al. 2006). The PCR product was not found in susceptible cultivars.

Nowadays, the GP122 marker is used for Marker Assisted Selection (MAS) in parental line breeding program. The marker is also recommended for selecting PVY resistant cultivars in commercial breeding.

Recently, the investigation of reaction to PVY in Polish cv. Rywal led to the identification of novel gene Ny-1 mapped to the chromosome IX. Since the gene confers HR to all PVY variants, it may be considered an alternative to Ry genes. As expected for HR, the expression of Ny-1 is temperature dependent: at 20 °C common and necrotic strains of PVY were localized in inoculated plants, while at 28 °C symptomless spread of the virus was observed (Szajko et al. 2008). Further testing of cultivars expressing HR to PVYN infection, but lacking Ry gene, resulted in mapping of the Ny-1 gene in two other cultivars Albatros and Sekwana to the same region of chromosome IX as Ny-1 in cv Rywal. Another novel Ny-2 gene has mapped to chromosome XI in cv. Romula, and this gene governs temperature independent HR to PVYN infection (Szajko et al. 2012). The group of cultivars with Ny genes is an additional source of PVY resistance that can be used by breeders.

Development of parental lines in Młochów was initiated with an idea to increase breeding efficiency of new cultivars. These cultivars should have improved quality traits along with resistance to main potato pathogens and pests, better storability and adaptation to water deficient soils (Swieżyński 1987). Parental lines, bred in a 5 year cycle, are donors of traits of interest, having the genetic background enriched with genes originating from various Solanum species. The fact that the parental lines combine resistances to main potato viruses with an acceptable level of agronomic traits is an important element of the breeding program. This should enhance the chance of selecting new cultivars, which are resistant to viruses. Complexity of virus resistance in parental lines has been increasing in the last four decades (Table 1).

Table 1 Complexity of virus resistance in tetraploid parental lines offered to Polish breeders (Zimnoch-Guzowska 2012)
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In the 5-year cycles of pre-breeding, breeding lines are selected for resistance to PVY, PVA, Potato virus X (PVX), Potato leafroll virus (PLRV), Potato virus M (PVM) and Potato virus S (PVS). Selection of resistant forms based on mechanical and graft inoculation and application of ELISA tests both in inoculated plants and in their tuber progeny. Additionally, stocks are exposed to a natural high virus infection pressure in Młochów experimental field and assessed by tuber indexing.

In recent years, PCR markers linked to respective resistance genes allowed combining phenotypic and genotypic selection. The MAS is introduced at the stage of the 5-year selection cycle, which is chosen on the basis of breeding priorities and number of assessed genotypes at the respective year. Nowadays markers are applied for selection of resistant forms to PVY (Flis et al. 2005; Witek et al. 2006), PVS (Marczewski et al. 2002; Witek et al. 2006) and PVM (Marczewski et al. 2006) along with resistance to Globodera rostochiensis (Milczarek et al. 2011) and sometimes to late blight (Śliwka et al. 2006). The application of mixed phenotypic selection and MAS results in an estimated 50 % reduction of total costs of selection for resistance traits. In commercial cultivar breeding, where a limited number of resistance traits are tested, the use of MAS offers probably a lower reduction of the total cost of the whole breeding cycle. However, the number of lines tested for obligatory resistance traits could be reduced by the application of MAS.

Since 1967 some parental lines have been provided to Polish breeders every year. Each parental line was characterized for ca 50 agronomic, resistance and quality characters. In total 277 parental lines were offered to breeders since the beginning of the program. From direct crosses with parental lines 63 cultivars were selected and registered in Poland. The analysis of the input of the parental lines breeding program on breeding progress in Polish cultivars indicated significant influence of parental lines on starch yield in the group of starch cultivars and resistance to PVY and late blight (Fig. 2). Well represented group of cultivars expressing ER and HR to PVY registered in 2009 in Poland mainly originated from the parental lines breeding program.

Fig. 2
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Resistance to PVY and late blight (LB) in 95 potato cultivars registered in Poland since 2002 according to their origin: foreign, Polish and Polish derived form parental lines (PL)

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Evaluation of Potato Cultivars Registered in Poland for Resistance and Reaction to PVY Strains

Out of 133 potato cultivars registered in Poland in 2009, 113 cultivars were evaluated for resistance and reaction to four PVY isolates from the IHAR-PIB/Młochów virus collection (Table 2). In this work were used: the isolate LW (collected in 1970, accession number AJ890349), Ny (collected in 1974, FJ666337), Wi (collected in 1984, EF558545) and 12/94 (collected in 1994, AJ889866). The isolate LW induces VCl on tobacco and is a PVYO serotype. Based on whole genome sequence it belongs to phylogenetic group PVYN-Wi (Hu et al. 2009; Karasev et al. 2011). The isolate Ny induces VN on tobacco and is a PVYN serotype. Based on whole genome sequence it belongs to phylogenetic group recombinant PVYNTN as suggested by Hu et al. (2009). The isolate Wi induces VN on tobacco and is a PVYO serotype. Based on whole genome sequence it belongs to phylogenetic group PVYO (Hu et al. 2009; Ogawa et al. 2012). The isolate 12/94 induces VN on tobacco and is a PVYN serotype. It belongs to phylogenetic group recombinant PVYNTN based on whole genome sequence (Hu et al. 2009). The isolate 12/94 causes PTNRD in the majority of tested sensitive cultivars, while isolate Ny induces PTNRD only in few cultivars.

Table 2 Field resistance and reaction to four PVY isolates of potato cultivars registered in Poland in 2009
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According to percentage of infected plants, infection symptoms and ELISA readings, the examined potato cultivars were classified as: (1) ER, if ELISA readings were as low as for the standard cv. Kuras and the gene Ry-f sto was identified by MAS; (2) HR, if small necrotic lesions were observed, but the virus was not detected (ELISA values as low as for non-inoculated control); (3) relatively resistant (R), if less than 50 % of inoculated plants were systemically infected and ELISA readings were higher than in healthy control; observed symptoms were different and cultivar dependent; (4) moderately resistant (mR), if 50–80 % of plants were systemically infected and ELISA values were higher than those observed in infected plants of the group R with varied symptoms of infection depending on cultivar; (5) susceptible (S), if all plants were infected, ELISA readings as high as in plants of susceptible cv. Zebra with symptoms dependent on cultivar. Tested cultivars were previously assessed for field resistance by the Research Centre for Cultivar Testing (COBORU) in a 1 to 9 grade scale, where 1 means susceptible and 9 extreme resistant.

As shown in Table 2, out of 113 cvs tested, 36 were resistant to all four PVY isolates. Among them there were 23 ER cultivars (with Ry-f sto gene present in 20 ones) and 7 HR cultivars, which were scored at 9 and 8 in the field exposure tests, respectively. Beside, the resistance to all four isolates was observed in 6 cultivars from group R scored in the field at 7.5 and 8. An additional 9 cultivars scored for the field resistance as 7 and 7.5 were classified as R or mR depending on PVY isolate.

The 44 cultivars, with field assessments from 3.5 to 7.0, were classified as R or mR after inoculation with the isolate LW, but their reactions to the other three isolates were diversified. The remaining 24 tested cultivars scored for field resistance from 3.5 to 5.5 showed susceptibility to all the four isolates. Among 113 cultivars tested, 26 cultivars reacted with tuber necrotic symptoms indicative of PTNRD by inoculation with 12/94.

Most of the tested cultivars were resistant to the isolate LW. In turn, isolates 12/94 and Wi were the most infective to tested cultivars, with the exception of ER and HR cultivars. This was an additional confirmation of effective protection by the Ry-f sto gene against old and new PVY isolates. The application of Ny isolate for inoculation of cultivars with field assessments below 7.5 led to changes in their classification based on reaction to the isolate LW. Namely, 23 cultivars from R and mR were downgraded to mR and S classes, respectively. In the case of cv. Augusta, the drop from R to S class was observed. The use of two new isolates Wi and 12/94 in the assessment evidenced significant breaking down of PVY resistance expressed to old isolates LW and Ny.

Similarly to our results, Valkonen et al. (1994) reported that potato cv. Allegany reacted with hypersensitivity to an ordinary strain isolate of PVYO, but was susceptible to an isolate of PVY causing necrosis in tobacco (PVYN). In Slovenia the occurrence of PVYNTN variant in the beginning of 1990s created significant losses in seed production. In 1990 and 1992, up to 80 to 100 % of seed potato fields of sensitive to PVYNTN cultivars like Igor and Vesna, Jearla, Resy or Desiree were rejected due to PVY infection (Kus 1995). In Austria Schiessendoppler (1996) noticed an increased susceptibility to PVY strain of older cultivars due to higher virulence of the PVYNTN along with stable resistance to PVYNTN of some resistant cultivars. Also McDonald and Singh (1996) indicated that several North American cultivars sensitive to PVYNTN strain expressed high or relative resistance to PVYO strain.