Evaluation of Watermelon and Related Species for Resistance to Race 1W Powdery Mildew

in Journal of the American Society for Horticultural Science

Powdery mildew [Podosphaera xanthii (Castagne) Braun & Shishkoff (syn. Sphaerotheca fuliginea auct. p.p.)] is now a common disease on watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] in the United States. In this study, the entire available U.S. Plant Introduction collection of Citrullus Schrad. ex Eckl. & Zeyh. species was evaluated for resistance to P. xanthii race 1W. The collection consists of four Citrullus species and one Praecitrullus Pangalo species [C. lanatus var. citroides (L.H. Bailey) Mansf., C. colocynthis (L.) Schrad., C. rehmii De Winter, and P. fistulosus (Stocks) Pangalo]. Wild-type accessions tended to be more resistant more often than the cultivated species, C. lanatus var. lanatus. None were immune, eight of the 1573 accessions exhibited high levels of resistance, and another 86 demonstrated intermediate resistance. Stem and leaf disease severity were weakly correlated (r 2 = 0.64, P = 0.001). The majority of accessions having resistance were collected in Zimbabwe. Resistance was found in four species.

Abstract

Powdery mildew [Podosphaera xanthii (Castagne) Braun & Shishkoff (syn. Sphaerotheca fuliginea auct. p.p.)] is now a common disease on watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] in the United States. In this study, the entire available U.S. Plant Introduction collection of Citrullus Schrad. ex Eckl. & Zeyh. species was evaluated for resistance to P. xanthii race 1W. The collection consists of four Citrullus species and one Praecitrullus Pangalo species [C. lanatus var. citroides (L.H. Bailey) Mansf., C. colocynthis (L.) Schrad., C. rehmii De Winter, and P. fistulosus (Stocks) Pangalo]. Wild-type accessions tended to be more resistant more often than the cultivated species, C. lanatus var. lanatus. None were immune, eight of the 1573 accessions exhibited high levels of resistance, and another 86 demonstrated intermediate resistance. Stem and leaf disease severity were weakly correlated (r 2 = 0.64, P = 0.001). The majority of accessions having resistance were collected in Zimbabwe. Resistance was found in four species.

Powdery mildew affects many cucurbit crops worldwide, limiting yield and increasing the need for fungicide application. Except for a few scattered cases of this disease on watermelon fruit (Ivanoff, 1957; McLean, 1970; Robinson and Provvidenti, 1975), powdery mildew has not been a problem for this crop until recently. Since 1996, a new pathotype of powdery mildew has been damaging watermelon crops in the United States (Davis et al., 2001; Keinath, 2000; McGrath, 2001a). Outbreaks of watermelon powdery mildew pathotypes, races 1W and 2W, on watermelon have been confirmed in South Carolina, Georgia, Florida, Oklahoma, Texas, Maryland, New York, Arizona, and California and were determined using melon (Cucumis melo L.) differentials (Davis et al., 2001; McGrath, 2001a).

Powdery mildew can decrease plant canopy, reduce yields through decreased fruit size and number of fruit per plant, and reduce fruit quality, flavor, and storage life (Keinath and DuBose, 2004; McGrath and Thomas, 1996). The reduced canopy may also result in sunscald of the remaining fruit, making them unmarketable. Detection of powdery mildew on watermelon can be difficult because the presence of the pathogen is less apparent than on melon. There are at least two symptoms on watermelon: chlorotic spots that occur on leaves accompanied by little or no sporulation and only a small amount of mycelial development, or mycelial and conidial development on either leaf surface with or without the associated chlorotic spots (Davis et al., 2001).

Podosphaera xanthii and Golovinomyces cichoracearum (D.C.) V.P. Heluta (formerly Erysiphe cichoracearum D.C.) are the predominant fungi that incite powdery mildew in cucurbits. These organisms differ in virulence against cucurbit species, and in their sensitivity to fungicides (Bertrand, 1991; Epinat et al., 1993; McGrath, 2001a, 2001b). Only one species, P. xanthii, has been reported on watermelon in the United States. Using differential reactions of 10 melon (C. melo) genotypes, seven pathogenically distinct races of P. xanthii can be differentiated (McCreight et al., 1987; Pitrat et al., 1998). More recently, McCreight (2006) reported that there may be as many as 28 races of P. xanthii on melon based on the reported reactions of 30 melon genotypes with as many as eight variants of race 1, and six variants of race 2. The significance of these races defined on melon is not known for watermelon.

Resistance of P. xanthii to certain fungicides has been detected, and application of fungicides to undersides of leaves is difficult and often requires the use of systemic materials to achieve adequate control of the disease (McGrath and Thomas, 1996). Recent work by Keinath and DuBose (2004) demonstrated effective control of powdery mildew race 2W on watermelon by alternating preventative applications of two fungicides: mancozeb and azoxystrobin.

Use of resistant watermelon cultivars in addition to fungicide applications should slow development of P. xanthii resistance to fungicides. We initially screened 100 watermelon plant introduction (PI) accessions for resistance to watermelon race 1W of P. xanthii (Davis et al., 2001). This led to the release of watermelon breeding line (PI 525088-PMR), which has intermediate resistance to race 1W P. xanthii (Davis et al., 2006a). The inheritance of resistance in that line to race 1W is multigenic (Davis et al., 2002) and appears to be independent from resistance to P. xanthii race 2W (Davis et al., 2002, 2006b; Thomas et al., 2005). In the current study, we screened the entire available U.S. Department of Agriculture, Agricultural Research Service (USDA-ARS), National Genetic Resources Program (NPGS) watermelon accessions for resistance to race 1W to identify additional resistance genes for control of this race.

Materials and Methods

Plant material.

A total of 1573 Citrullus species and tinda (P. fistulosus) accessions from NPGS at the Southern Regional Plant Introduction Station, USDA-ARS, Griffin, GA, were evaluated. The accessions represent 75 countries of origin and five species, belonging to C. lanatus var. lanatus, C. lanatus var. citroides, C. colocynthis, C. rehmii, and P. fistulosus.

Ten C. melo differentials were included in all experiments to determine the race of P. xanthii present: ‘Delicious 51’, ‘Edisto 47’, Iran H, MR-1, ‘Nantais Oblong’, PI 124112, PI 414723, ‘PMR 45’, ‘PMR 5’, ‘PMR 6’, WMR 29, and ‘Top Mark’. Differentials were supplied by C. Thomas and M. Pitrat and were included in all studies to verify the powdery mildew race present. Race 1W was the only race detected in all experiments.

Greenhouse experiments.

One seed each of the available Citrullus species and P. fistulosus PI accessions was randomly planted in Speedling flats (Sun City, FL) containing Redi-earth growth media (Scotts-Sierra Horticultural Products Co., Marysville, OH) in a greenhouse experiment with six replications. Replications were not performed concurrently. Seedlings were inoculated with the Lane, OK, race 1W P. xanthii isolate maintained on greenhouse-grown watermelon plants. Plants were inoculated two times a week for 3 weeks by brushing an infected leaf onto each seedling, starting at the two-leaf stage of growth. The plants were maintained under normal greenhouse conditions, and night/day temperatures were maintained by an automated system between 25 and 33 °C. Ratings were taken when 50% of the leaf surface area of susceptible differentials was visibly infected. Ratings were performed no later than 2 months after planting. Three ratings were made of each plant (stem, upper side of the leaves, and cotyledons) using the nonlinear 12-point method (Horsfall and Barratt, 1945). Accessions were classified into resistant, intermediate, or susceptible classifications according to the mean disease severity rating (DSR) of six replicates for total plant (leaf, stem, and cotyledon): ≤6%, resistant; >6% and ≤12%, intermediate; and >12%, susceptible. Out of 1573 PI lines tested, 13 did not germinate or experienced early death of seedlings.

A retest was performed on 35 PI accessions chosen for their geographical diversity from the 50 PI lines with the lowest average total plant ratings. Thirteen countries and three species or subspecies (C. lanatus var. lanatus, C. lanatus var. citroides, and C. colocynthis) were represented in the retest. Five replications of one plant of each selected PI accession were planted in a randomized complete block design, inoculated, and rated as above.

Statistical analysis.

Data were transformed to real numbers and the data were analyzed with PROC GLM and PROC CORR using SAS (version 7; SAS Institute, Cary, NC). The data are summarized as averages of the replications for each study in Tables 1 and 2.

Table 1.

Ranking of the mean disease severity rating (DSR) from six replicates of Citrullus species and Praecitrullus species PI lines that demonstrated resistance or intermediate resistance to race 1W Podosphaera xanthii.

Table 1.Table 1.
Table 2.

Ranking of the mean disease severity rating (DSR) for the retested Citrullus species PI lines for race 1W Podosphaera xanthii resistance.

Table 2.

Results and Discussion

Evaluation of watermelon PI accessions for race 1W P. xanthii resistance.

The P. xanthii strain present was race 1 as defined by the susceptibility of the following melon differentials: ‘Delicious 51’, Iran H, ‘Nantais Oblong’, and ‘Top Mark’ and resistance of all the other melon race differentials. Susceptibility of watermelon indicated that it was race 1W pathotype.

In each of the six replications, there was a range of symptoms from no detectable mycelia to 97% coverage on the entire plant. The majority of the PI accessions (92%) had mean detectable mycelia covering 12% to 50% of the plant surface (data not shown). Less than 1% of the PI accessions tested showed high or intermediate resistance. The 93 PI accessions with an average DSR below four (<12% mycelia coverage) for all plant parts tested (leaf, stem, and cotyledon) are listed in Table 1. In our judgment, all 93 of these accessions had commercially useful resistance to the disease. All accessions in Table 1 were determined to be resistant (≤6% mycelia) or have intermediate resistance (between >6% and ≤12% mycelia).

When total plant DSRs were used to rank PI accessions for resistance, only eight accessions (Grif 5601 and PIs 482255, 388770, 482362, 381750, 459074, 386015, and 482248) were considered resistant with a DSR value ≤3.0. Only eight PI accessions were resistant when only the cotyledon DSRs were analyzed (Grif 5601 and PIs 482255, 388770, 482362, 459074, 386015, 482248, and 508443), but this number increased to 13 PI accessions when only the mean of the leaf rating was analyzed (Grif 5601 and PIs 482255, 482362, 381750, 381742, 388770, 459074, 386015, 560008, 169241, 179881, 512343, 500337) and increased again to 21 resistant accessions when we analyzed only stem ratings (Grif 5601 and PIs 482255, 482362, 381750, 388770, 459074, 482248, 525082, 381742, 386015, 560008, 512343, 532738, 266028, 508443, 179885, 512375, 512367, 386025, 219907, 248178). All PI accessions that demonstrated leaf resistance were also rated resistant when using the stem rating. All but one PI accession demonstrating stem resistance showed high or intermediate resistance on the leaf.

In the retest (Table 2), 15 PI accessions were rated as resistant (388770, 505585, 386015, 270545, 482308, 482312, 482313, 482333, 525082, 500323, 482278, 482328, 500331, 508443, 169241). This increase in number of resistant accessions may be due to cotyledon ratings not being considered. Four PI accessions rated resistant in the initial screen had only intermediate resistance in the retest (482248, 482362, 482255, 459074). Only PI 388770 and 386015 demonstrated total plant resistance in both experiments. No PI lines were completely free of mycelia on all plant parts when data from all experiments were combined.

PIs 459074 and 525088, reported resistant in field trials (Davis et al., 2002), were among the 40 accessions most resistant to race 1W (Table 1). PI 482291 and PI 186490, previously resistant in field trials (Davis et al., 2002), were susceptible in these greenhouse tests. The newly released race 1W powdery mildew resistant inbred PI 525088-PMR (Davis et al., 2006a) was not included in these tests, but PI 525088, which came from the same original seed source, was included and was ranked among the 40 most resistant accessions.

Correlation of DSR for leaf, stem, and cotyledon.

A correlation was performed between DSR of the three tissues tested. The transformed means of the original six replicates for each of the 1573 PI accessions were used. There were significant, yet weak, correlations between leaf disease ratings and stem ratings (r = 0.64; P = 0.001), leaf and cotyledon ratings (r = 0.20; P = 0.001), and stem and cotyledon ratings (r = 0.24; P = 0.001). Similarly, the correlation between stem and leaf for the retested PI accessions was significant but weak (r = 0.68; P = 0.001). Cotyledon ratings were not taken on the retest. The data from these two experiments indicate that disease resistance for cotyledons is controlled by different genes than for leaf and stem resistance. This was also true in melon PI 313970 resistance to races 1 and 2 (McCreight, 2003). The DSR for leaf, stem, and cotyledon showed that cotyledon and leaf tissue have, on average, higher DSRs then stem. Because stems had lower DSRs overall than leaves, the limited correlation may be due to amount of mycelia present rather than presence or absence of resistance genes for these two tissues.

Because leaves make up the most significant surface area of watermelon plants, breeding for leaf resistance is likely more important than stem or cotyledon resistance. However, in severe powdery mildew cases, cotyledons can be heavily infected, stunting or even killing the seedlings. Whole-plant resistance is, therefore, desirable.

Heterogeneity of resistance within PI lines.

Many of the PI accessions, which are usually open pollinated, showed phenotypic variability (heterogeneity) for disease resistance, which reduced their total plant ranking for these accessions; individuals in one accession ranged from 6% to 94% total plant coverage with mycelia. Average standard deviation of ratings within PI accessions was 20% (range, 0% to 45%). This suggests that there are accessions heterogeneous for resistance. While DSRs varied slightly between individual plants within each of the susceptible control differentials across replications, they were still clearly susceptible plants.

Resistance to P. xanthii race 1W in Citrullus species and P. fistulosus.

There was a high percentage of P. fistulosus (11%) and C. colocynthis (4%) in the 93 accessions most resistant to race 1W (Table 3); these two species comprise <2% of the Citrullus species PI collection. P. fistulosus is distantly related to the genus Citrullus (Levi et al., 2005); it has a chromosome number of n = x = 12, while Citrullus has n = x = 11 and no interspecific crosses have been reported (Robinson and Decker-Walters, 1997).

Table 3.

Number and percentage of each species analyzed demonstrating resistance and intermediate resistance to race 1W Podosphaera xanthii.

Table 3.

When analyzed by geographical origin, 36% and 15% of the 93 most resistant accessions were from Zimbabwe and Zambia, respectively, although they constitute only 9% and 4% of the U.S. Citrullus species PI collection (data not shown). These data indicate the origin of most of the resistant accessions in this study was from this region of Africa. Accessions from 13 countries were selected in the retests and are being inbred for uniform reaction to powdery mildew race 1W. We will perform inheritance studies and allelism tests on these lines to identify multiple resistance genes, to incorporate multiple resistance sources into a single cultivar to offer greater resistance stability.

Powdery mildew on cucurbits is a rapidly evolving disease with serious impact on cucurbit production worldwide. Therefore, resistance screening to emerging races and pathotypes and development of differential lines that can be used to detect these different forms of the fungus are of great importance.

Cucurbit powdery mildew in the United States was not known to infect watermelon before 1996. A new pathotype of cucurbit powdery mildew defined by the ability to infect watermelon has since spread throughout the United States. All strains tested to date that colonize watermelon are able to colonize some of the melon differentials, but not all strains colonizing melon are able to cause disease symptoms on watermelon (Davis et al., 2006a, 2006b).

The melon differentials used in this study defined which melon race of the watermelon pathotype was present; there are no formally released Citrullus species powdery mildew differentials at this time. Watermelon lines are currently being developed that have homogeneous reaction to inoculation with these two watermelon pathotypes of P. xanthii. Once produced, it is likely that the P. xanthii races affecting watermelon will differ from that which infects melon. Because of this, it is suggested that the nomenclature used in this article be adapted to differentiate between the pathotypes. Races could be differentiated by indicating race 1M (or 2M or 3M) according to the reaction of a melon differential set and race 1W or 2W according to the reaction of watermelon differentials. For instance, a strain could belong to race 2M (on melon) and 1W (on watermelon).

Literature Cited

  • BertrandF.1991Les oïdiums des cucurbitacées: maintien en culture pure, étude de leur variabilité et de la sensibilité chez le melonPh.D. DissUniversité of Paris XIOrsay, France

    • Export Citation
  • DavisA.R.LeviA.WehnerT.PitratM.2006aPI 525088-PMR, a melon race 1 powdery mildew resistant watermelon lineHortScience4115271528

  • DavisA.R.TettehA.WehnerT.LeviA.PitratM.2006bWatermelon resistance to powdery mildew race 1 and race 2412420HolmesG.J.Proc. Cucurbitaceae 2006. Universal PressRaleigh, NC

    • Export Citation
  • DavisA.R.BrutonB.D.PairS.D.ThomasC.E.2001Powdery mildew: an emerging disease of watermelon in the United StatesCucurbit Genet. Coop. Rpt.244248

    • Search Google Scholar
    • Export Citation
  • DavisA.R.ThomasC.E.LeviA.BrutonB.D.PairS.D.2002Watermelon resistance to powdery mildew race 1192198MaynardD.N.Cucurbitaceae ’02. ASHS PressAlexandria, VA

    • Export Citation
  • EpinatC.PitratM.BertrandF.1993Genetic analysis of resistance of five melon lines to powdery mildewsEuphytica65135144

  • HorsfallJ.G.BarrattR.W.1945An improved grading system for measuring plant diseasePhytopathology35655(abstr.)

  • IvanoffS.S.1957Powdery mildew pimples of watermelon fruitsPhytopathology47599602

  • KeinathA.P.2000Effect of protectant fungicide application schedules on gummy stem blight epidemics and marketable yield of watermelonPlant Dis.84254260

    • Search Google Scholar
    • Export Citation
  • KeinathA.P.DuBoseB.2004Evaluation of fungicides for prevention and management of powdery mildew on watermelonCrop Prot.233542

  • LeviA.ThomasC.E.SimmonsA.M.ThiesJ.A.2005Analysis based on RAPD and ISSR markers reveals closer similarities among Citrullus and Cucumis species than with Praecitrullus fistulosus (Stocks) PangaloGenet. Resources Crop Evol.52465472

    • Search Google Scholar
    • Export Citation
  • McCreightJ.D.2003Genes for resistance to powdery mildew races 1 and 2 US in melon PI 313970HortScience38591594

  • McCreightJ.D.2006Melon–powdery mildew interactions reveal variation in melon cultigens and Podosphaera xanthii races 1 and 2J. Amer. Soc. Hort. Sci.1315965

    • Search Google Scholar
    • Export Citation
  • McCreightJ.D.PitratM.ThomasC.E.KishabaA.N.BohnG.W.1987Powdery mildew resistance genes in muskmelonJ. Amer. Soc. Hort. Sci.112156160

    • Search Google Scholar
    • Export Citation
  • McGrathM.T.2001aDistribution of cucurbit powdery mildew races 1 and 2 on watermelon and muskmelonPhytopathology91197(abstr.)

  • McGrathM.2001bFungicide resistance in cucurbit powdery mildew: experiences and challengesPlant Dis.85236245

  • McGrathM.T.ThomasC.E.1996Powdery mildew2830ZitterT.A.HopkinsD.L.ThomasC.E.Compendium of cucurbit diseases. The American Phytopathological SocietySt. Paul, MN

    • Export Citation
  • McLeanD.M.1970Powdery mildew on watermelon fruitsPlant Dis. Rpt.54372373

  • PitratM.DogimontC.BardinM.1998Resistance to fungal diseases of foliage in melon167173McCreightJ.D.Cucurbitaceae ’98. ASHS PressAlexandria, VA

    • Export Citation
  • RobinsonR.W.ProvvidentiR.1975Susceptibility to powdery mildew in Citrullus lanatus (Thunb.). Matsum. & NakaiJ. Amer. Soc. Hort. Sci.100328330

    • Search Google Scholar
    • Export Citation
  • RobinsonR.W.Decker-WaltersD.S.1997Cucurbits40CAB InternationalNew York

    • Export Citation
  • ThomasC.E.LeviA.CanigliaE.2005Evaluation of U.S. plant introductions of watermelon for resistance to powdery mildewHortScience40154156

    • Search Google Scholar
    • Export Citation

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Contributor Notes

This work was partially supported by the Cucurbit Crop Germplasm Committee, the National Germplasm System, USDA, ARS, and a fellowship under the OECD Co-operative Research Programme: Biological Resource Management for Sustainable Agriculture Systems.We thank Amy Helms and Anthony Dillard for technical assistance and Robert Jarret for supplying seed from the Citrullus species collection. Some melon differentials were kindly supplied by Dr. Claude Thomas.

Corresponding author. E-mail: adavis-usda@lane-ag.org.

  • BertrandF.1991Les oïdiums des cucurbitacées: maintien en culture pure, étude de leur variabilité et de la sensibilité chez le melonPh.D. DissUniversité of Paris XIOrsay, France

    • Export Citation
  • DavisA.R.LeviA.WehnerT.PitratM.2006aPI 525088-PMR, a melon race 1 powdery mildew resistant watermelon lineHortScience4115271528

  • DavisA.R.TettehA.WehnerT.LeviA.PitratM.2006bWatermelon resistance to powdery mildew race 1 and race 2412420HolmesG.J.Proc. Cucurbitaceae 2006. Universal PressRaleigh, NC

    • Export Citation
  • DavisA.R.BrutonB.D.PairS.D.ThomasC.E.2001Powdery mildew: an emerging disease of watermelon in the United StatesCucurbit Genet. Coop. Rpt.244248

    • Search Google Scholar
    • Export Citation
  • DavisA.R.ThomasC.E.LeviA.BrutonB.D.PairS.D.2002Watermelon resistance to powdery mildew race 1192198MaynardD.N.Cucurbitaceae ’02. ASHS PressAlexandria, VA

    • Export Citation
  • EpinatC.PitratM.BertrandF.1993Genetic analysis of resistance of five melon lines to powdery mildewsEuphytica65135144

  • HorsfallJ.G.BarrattR.W.1945An improved grading system for measuring plant diseasePhytopathology35655(abstr.)

  • IvanoffS.S.1957Powdery mildew pimples of watermelon fruitsPhytopathology47599602

  • KeinathA.P.2000Effect of protectant fungicide application schedules on gummy stem blight epidemics and marketable yield of watermelonPlant Dis.84254260

    • Search Google Scholar
    • Export Citation
  • KeinathA.P.DuBoseB.2004Evaluation of fungicides for prevention and management of powdery mildew on watermelonCrop Prot.233542

  • LeviA.ThomasC.E.SimmonsA.M.ThiesJ.A.2005Analysis based on RAPD and ISSR markers reveals closer similarities among Citrullus and Cucumis species than with Praecitrullus fistulosus (Stocks) PangaloGenet. Resources Crop Evol.52465472

    • Search Google Scholar
    • Export Citation
  • McCreightJ.D.2003Genes for resistance to powdery mildew races 1 and 2 US in melon PI 313970HortScience38591594

  • McCreightJ.D.2006Melon–powdery mildew interactions reveal variation in melon cultigens and Podosphaera xanthii races 1 and 2J. Amer. Soc. Hort. Sci.1315965

    • Search Google Scholar
    • Export Citation
  • McCreightJ.D.PitratM.ThomasC.E.KishabaA.N.BohnG.W.1987Powdery mildew resistance genes in muskmelonJ. Amer. Soc. Hort. Sci.112156160

    • Search Google Scholar
    • Export Citation
  • McGrathM.T.2001aDistribution of cucurbit powdery mildew races 1 and 2 on watermelon and muskmelonPhytopathology91197(abstr.)

  • McGrathM.2001bFungicide resistance in cucurbit powdery mildew: experiences and challengesPlant Dis.85236245

  • McGrathM.T.ThomasC.E.1996Powdery mildew2830ZitterT.A.HopkinsD.L.ThomasC.E.Compendium of cucurbit diseases. The American Phytopathological SocietySt. Paul, MN

    • Export Citation
  • McLeanD.M.1970Powdery mildew on watermelon fruitsPlant Dis. Rpt.54372373

  • PitratM.DogimontC.BardinM.1998Resistance to fungal diseases of foliage in melon167173McCreightJ.D.Cucurbitaceae ’98. ASHS PressAlexandria, VA

    • Export Citation
  • RobinsonR.W.ProvvidentiR.1975Susceptibility to powdery mildew in Citrullus lanatus (Thunb.). Matsum. & NakaiJ. Amer. Soc. Hort. Sci.100328330

    • Search Google Scholar
    • Export Citation
  • RobinsonR.W.Decker-WaltersD.S.1997Cucurbits40CAB InternationalNew York

    • Export Citation
  • ThomasC.E.LeviA.CanigliaE.2005Evaluation of U.S. plant introductions of watermelon for resistance to powdery mildewHortScience40154156

    • Search Google Scholar
    • Export Citation
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