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Susceptibility of potato (Solanum tuberosum) cultivars to powdery scab (caused by Spongospora subterranea f. sp. subterranea), and relationships between tuber and root infection

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Susceptibility of potato (Solanum tuberosum) cultivars to powdery scab (caused by Spongospora subterranea f. sp. subterranea), and relationships between tuber and root infection
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  © Australasian Plant Pathology Society 200310.1071/AP030400815-3191/03/030377www.publish.csiro.au/journals/app  Australasian Plant Pathology , 2003, 32 , 377–385 CSIRO  PUBLISHING Susceptibility of potato (  Solanum tuberosum ) cultivars to powdery scab (caused by  Spongospora subterranea  f. sp.  subterranea ), and relationships between tuber and root infection  Richard E. Falloon A,B  , Russell A. Genet  A  , Andrew R. Wallace A  and Ruth C. Butler  A A  New Zealand Institute for Crop & Food Research Ltd, Private Bag 4704, Christchurch, New Zealand. B Corresponding author; email: falloonr@crop.cri.nz  Abstract.  Ninety-nine potato ( Solanum tuberosum ) cultivars and 13 breeding lines were assessed in field trials for susceptibility to powdery scab (caused by Spongospora subterranea  f. sp.  subterranea ). The trials were carried outover 11 growing seasons (each cultivar/line tested in at least two growing seasons) in soil inoculated with S.subterranea  and regularly irrigated during crop growth. The cultivars and lines were categorised as ‘veryresistant’ (21%), ‘moderately resistant’ (28%), ‘moderately susceptible’ (33%) or ‘very susceptible’ (19%). Scaled average severity scores for the cultivars and lines followed a continuum between very resistant and very susceptible,suggesting that resistance to powdery scab is quantitative. In a glasshouse experiment where small plants wereinoculated with S. subterranea sporosori, 15 cultivars selected to cover the spectrum of field-assessed susceptibilitywere assessed for intensity of infection by S. subterranea  zoosporangium in roots and for numbers of root galls onroot systems. All of the cultivars developed zoosporangia and root galls (i.e. no cultivar was immune frominfection), and root infection was usually related to tuber infection. Field-resistant cultivars (tuber infection)generally had low numbers of root zoosporangia and root galls in the glasshouse experiment, and cultivars that werevery susceptible to tuber infection in the field had high levels of root infection in the glasshouse experiment. Anexception was the early maturing cv. Swift, which had very low levels of tuber infection in the field, but had highnumbers of root zoosporangia and root galls in the glasshouse. These results demonstrate the relative reaction of alarge number of potato cultivars to powdery scab and reaffirm the potential for using plant resistance as a powderyscab management strategy. Development of S. subterranea  in host roots, even in cultivars with resistance to tuber infection, is likely to be important in the epidemiology of powdery scab. AP03040Powdery scab resistance in potato cultivarsR.E.Falloon etal .  Additional keywords: tuber lesions, root galls, zoosporangia. Introduction Powdery scab of potato ( Solanum tuberosum ), caused bythe Protozoan pathogen Spongospora subterranea  f. sp.  subterranea , is an important quality-limiting disease in manytemperate areas of the world where potato crops areintensively grown for human consumption. Powderyscab-infected lines are rejected as seed tubers because theyare likely to infest previously disease-free areas and transmitthe disease to newly established crops. Furthermore, thedisease causes severe lesions on potato tubers and thesedowngrade their quality as fresh product for sale to domesticconsumers. Scabbed tubers also cause difficulties during theskin removal stages of potato processing operations. S.subterranea  may also reduce productivity of potato plants,as we have measured increased total tuber yield from field  plots where severe powdery scab was controlled withchemical treatments (Falloon et al.  1996).The life cycle of S. subterranea  on potato has beenoutlined by Karling (1968) and Hims and Preece (1975).Resting spores of the pathogen are capable of long-termsurvival. These are primary sources of infection of host plants, and sporosori (aggregations of resting spores) occur in lesions or as surface contaminants on seed tubers, assoilborne inoculum or in effluent from animals fed infected  potatoes. Zoospores are released from resting spores and infect host root and stolon cells. Zoosporangia develop fromthese infections and release secondary zoospores that initiatenumerous cycles of infection in host root systems. Root gallslater develop on infected potato plants, and numeroussporosori develop within these galls. Infection of potato  378  Australasian Plant Pathology R. E. Falloon  et al. tubers (modified stolons) probably occurs at early stages of tuber development (Hughes 1980; Taylor et al  . 1986) and large numbers of sporosori develop in each tuber lesion.Several control methods have been identified for management of powdery scab. These include preventingtransmission of S. subterranea  to new crops using chemicaltreatments of seed tubers, avoiding or preventing cropinfection from soilborne inoculum by using appropriate croprotations or chemical treatments of soil, and avoidingcontamination of seed tubers or soil by S. subterranea inoculum through the use of appropriate sanitary practices(Karling 1968; Braithwaite et al.  1994; Falloon et al.  1996;Genet et al.  1996).Differences in susceptibility to powdery scab in different potato cultivars, breeding lines and germplasm accessionshave been recognised from as early as 1930 (Karling 1968)through to the present (Hughes 1980; Kirkham 1986; Gans et al.  1987; Jellis et al. 1987 ;  Wastie et al.  1988; Eraslan and Turhan 1989; de Boer 1991; Wastie 1991; Torres et al.  1995;Merz and Martinez, personal communication). Recentrecommendations for integrated management of powderyscab (Burgess and Wale 1994; Falloon et al.  1999; Genet etal.  1996, 2000) have included disease resistance as acomponent of integrated powdery scab management.An extensive field evaluation of potato cultivars availablein New Zealand for their relative susceptibility to powderyscab has been carried out to assist growers with cultivar choice in relation to the disease, and to provide background information for developing potato breeding initiatives toincorporate powdery scab resistance into new cultivars. Inaddition, a glasshouse/laboratory study was carried out todetermine if field assessments of resistance/susceptibility to powdery scab (based on severity of tuber infection) is related to S. subterranea  infection of potato roots. This paper reportsresults of this research. Methods  Field trials to evaluate susceptibility of potato cultivars to powdery  scab Field trials were carried out over 11 consecutive growing seasons,from 1991/92 through to 2001/02, using similar methods and cropmanagement in each season. The trials were laid down each year at the NZ Institute for Crop & Food Research farm, Lincoln, Canterbury, NewZealand (latitude 43°38'S, longitude 172°29'E). The soil at the site wasa Templeton sandy loam (Typic Immature Pallic soil, TypicHaplustepp). The trials were planted on the same site for threeconsecutive growing seasons before moving to an adjacent site withinthe same field. The soil in the area for each trial was prepared to asuitable tilth, and rows 0.75 m apart and 5 cm deep were drawn acrossthe trial area. The rows were divided into plots 1.5 m long, with 1 mgaps between plots in each row. Five healthy seed tubers of each cultivar or line to be tested were hand-planted into each plot, with 0.3 m between tubers. Each trial was of randomised complete block designwith six replicates. S. subterranea  inoculum (100 mL of macerated  potato tubers heavily infected with powdery scab) was spread alongeach plot prior to covering the seed tubers with soil. The infected  potatoes from which inoculum was prepared were from differentcultivars and sources each year, but were always from field-grown linesfrom the Canterbury region of New Zealand. Early planting (mid September) and regular irrigation during crop growth, especially duringand immediately after tuberisation, ensured that moist soil conditionswere maintained in order to encourage powdery scab development. Inearly March each year, when top growth of most lines had died, thetubers in each plot were hand-dug and all tubers greater than 30 g wereharvested. These were washed free of soil and assessed for powderyscab severity using a 0–3 severity scale (0 = nil infection; 1 = less than5% of tuber surface area infected; 2 = 5–25% surface area infected; 3 =greater than 25% surface area infected). An average powdery scabseverity score was calculated for each plot. Glasshouse experiment to measure root infection of potato cultivars by S. subterraneaFifteen potato cultivars were selected to represent the spectrum of susceptibility to powdery scab (from very resistant to very susceptible)as assessed in the field trials described above. White plastic pots (680mL capacity with base drain holes) were filled with 600 mL of dry river sand (1–2 mm grade) and then each pot was placed in a troughcontaining nutrient solution so that the sand was saturated with thesolution. The nutrient solution composition [in reverse osmosis (RO) purified water] was: Ca(NO) 2 , 361 mg/L; KNO 3 , 253 mg/L;MgSO 4 .7H 2 O, 246 mg/L; NH 4  NO 3 , 40 mg/L; Fe-EDTA, 20 mg/L; KCl,4 mg/L; KH 2 PO 4 , 2300 µg/L; H 3 BO 3 , 140 µg/L; MnSO 4 .H 2 O, 90 µg/L;ZnSO 4 .7H 2 O, 115 µg/L; CuSO 4 .5H 2 O, 50 µg/L; and H 2 MoO 4 , 16 µg/L.Ten pots were each planted with a plantlet of each cultivar (approximately three nodes raised in tissue culture). The pots were then placed on a glasshouse bench [16 h light, 8 h dark, 18 (± 2)°C] in arandomised complete block experiment (15 cultivars, 10 blocks). The pots were then irrigated every 2–3 days by immersion in nutrientsolution to saturate the sand in each pot. Fourteen days after planting,each pot was inoculated with 20 mL of S. subterranea  sporosorussuspension (1500 sporosori/mL) in nutrient solution, by pouring thesuspension onto the sand surface at the base of the plant stem. Sporosorihad been obtained from field-grown cv. Agria tubers heavily infected with powdery scab, by dry scraping the tuber surface lesions and collecting sporosori after passage through an 80 µm mesh sieve.At 22 days after inoculation, one block of the experiment (block 10),containing one plant of each of the 15 cultivars, was removed from theglasshouse. The roots of each plant were washed free of sand, and asample of roots was excised from the centre of the root massapproximately 30 mm below the crown of the plant. These roots were placed into 0.1% trypan blue in lactophenol for 5 min and then placed into RO purified water. Roots were examined using a compound microscope, and those containing zoosporangia of S. subterranea  werechosen for assessment of intensity of infection. Numbers of zoosporangia were estimated in ten infected roots from each cultivar bycounting numbers of zoosporangia in 2 mm of each root (tenconsecutive 200 µm sections) from the root tip. The width of each rootwas measured.At 10 weeks after planting (8 weeks after inoculation), fresh weightand length of the shoots of each plant were determined. The root systemof each plant was washed free of sand and photographed using a digitalcamera. The fresh weight of the root system of each plant wasdetermined. Numbers of S. subterranea  root galls on each plant werecounted in photographs of the root systems. Statistical analyses Field trials.  Analysis of variance was used to estimate mean tuber severity scores for each cultivar/line in the trials, and the mean scoresfrom each annual trial were scaled linearly so that those of the twoexperimental standard cultivars, Gladiator and Iwa, were assigned thescores 0.15 and 1.50 respectively (their mean scores over the first four   Powdery scab resistance in potato cultivars  Australasian Plant Pathology 379trials). This procedure adjusted for major seasonal differences ininfection levels in the trials. All cultivars (except the two standards) thatwere tested in two or more growing seasons were then included in ananalysis to estimate a mean severity score for each cultivar. Ageneralised linear model was used for this analysis, with a binomialerror structure to allow for smaller variance of the scores of veryresistant and very susceptible cultivars. Finally, estimated cultivar meanscores were further linearly scaled so that scores for Gladiator and Iwawere 8.8 and 5.0 respectively. This additional scaling was carried out sothat cultivars expressing high levels of resistance to powdery scab had high scores and susceptible cultivars had low scores. A cultivar withalmost complete resistance to tuber infection by S. subterranea willhave a score of about 9 whereas one with all tubers severely infected will have a score of about 1 (based on scaled results for the standards). Glasshouse experiment.  The patterns of zoosporangium countsalong infected roots of glasshouse-grown plants were examined graphically for each cultivar and total numbers of zoosporangia in eachinfected root were calculated. Before analysis, these data wereconverted to numbers per mm 2  of root surface area, using the measured root widths, because zoosporangia infect the cortical cells of roots. Numbers of galls were converted to numbers per g root fresh weight before being analysed. All data were analysed using generalised linear models (McCullagh and Nelder 1989) with a logarithmic link (transformation) using error structures appropriate to the type of data,or chosen to best represent the distribution of the data. A Poisson error structure was used for zoosporangium counts, a normal error structurefor plant fresh weight and root width data, and a negative-binomialerror structure for numbers of galls on root systems. Means and confidence limits were calculated on the logarithmic scale as part of theanalyses and then back-transformed to the srcinal scale of measurement. Comparisons of field severity scores and laboratory severityassessments.  Scatter plots, correlation coefficients ( r  ) and a range of multivariate techniques were used to compare field-determined scaled average powdery scab severity scores for tubers with cultivar means for laboratory assessed parameters of S. subterranea  root infection(numbers of zoosporangia and numbers of root galls on glasshousegrown plants), and to compare the different laboratory assessed rootinfection parameters.All statistical analyses were carried out using the GenStat statistical package (GenStat Committee 2000) and a level of 5% was used throughout to determine significance. Results  Field evaluations of susceptibility of potato cultivars to  powdery scab The mean powdery scab severity scores for the twostandard cultivars (Gladiator and Iwa) varied across the 11growing seasons in which they were assessed. The meanscore for Gladiator ranged from 0.01 to 0.16 (standard deviation = 0.06), whereas that for Iwa ranged from 0.38 to2.56 (standard deviation = 0.65). Most of the cultivars wereincluded in the trials for two growing seasons (Table 1), solinear scaling (see Methods above) to adjust for variations of infection between growing seasons, as detected for the twostandard cultivars, was clearly necessary.Table 1 lists the scaled average powdery scab severityscores for tubers of the 99 potato cultivars evaluated in field trials. The cultivars formed a continuum of susceptibility to powdery scab (scaled average severity score) from veryresistant to very susceptible. The cultivars were arbitrarilycategorised (Table 1) as follows; ‘very resistant’ to powderyscab (scaled average severity score  8.0), ‘moderatelyresistant’ (score 7.0–7.9), ‘moderately susceptible’ (score6.0–6.9) and ‘very susceptible (score  5.9). Thiscategorisation classified 22% of the cultivars as veryresistant, 30% as moderately resistant, 30% as moderatelysusceptible and 17% as very susceptible.Table 1 also lists the scaled average powdery scab severityscores for 13 potato breeding and germplasm lines from theCrop & Food Research potato breeding programme. Theselines expressed the full spectrum of powdery scabsusceptibility from very resistant to very susceptible. Similar results have also been obtained over three growing seasons inevaluations of early stage breeding material grown inreplicated, single-seed tuber plots. Glasshouse experiment to measure root infection of potato cultivars by S. subterranea  Plant parameters.  Root widths differed for the differentcultivars when measured at the time of microscopicexamination of roots for intensity of zoosporangiuminfection (36 days after planting). The cultivars with thenarrowest roots were Nadine, Gladiator and Swift (meanwidths = 155, 160 and 171 µm, respectively), whereas cvv.Russet Burbank, Agria, Katahdin, Asterix, Arran Banner,Moonlight, Iwa, Red Rascal and Pacific had roots of similar and intermediate width (mean width = 186–206 µm). Cvv.Rocket, Desiree and Kennebec had the thickest roots (meanwidths = 222, 228 and 233 µm, respectively).The cultivars had different root fresh weights at harvest(10 weeks after planting). Root fresh weight was least for cv.Swift (mean root fresh weight = 1.9 g) and greatest for cv.Moonlight (8.5 g).  Intensity of S. subterranea  infection in roots of potatocultivars.  The distributions of zoosporangia along the rootsfrom root tips were similar for all of the roots from eachcultivar. Few zoosporangia were observed within the first200–400 µm from the root tips of any of the cultivars. Thecultivars differed in the total numbers of zoosporangia intheir roots (Fig. 1 a ). Fewest zoosporangia were observed inroots of cvv. Gladiator (mean = 6 zoosporangia/mm 2  rootsurface area), Red Rascal (10) and Nadine (10), whereasgreatest numbers were observed in roots of cvv. Rocket(118), Iwa (128), Asterix (141), Swift (141) and Agria (143).The other cultivars had intermediate numbers of zoosporangia in their roots (Fig. 1 a ). Fig. 2 shows a root of cv. Iwa with severe zoosporangium infection.The cultivars also differed in the numbers of galls on their root systems (Fig. 1 b ). Fewest galls were on roots of cvv. Nadine (mean = 0.3 galls/g root fresh weight), Gladiator (0.4), Moonlight (1.5) and Red Rascal (1.6). The greatestnumbers of galls were observed on roots of cvv. Agria (mean= 13.9 galls/g root fresh weight), Pacific (15.6), Rocket  380  Australasian Plant Pathology R. E. Falloon  et al. (17.8), Katahdin (19.2) and Swift (23.8). The other cultivarshad intermediate numbers of galls on their root systems.There was a strong relationship (Fig. 3 a ) betweennumbers of zoosporangia in roots and numbers of root gallson root systems ( r   = 0.83). Cvv. Agria, Asterix and Iwa did not strictly follow this relationship, having high numbers of zoosporangia in roots but relatively few root galls (Fig. 1).Examination of relationships between physical root parameters and root infection indicated that correlationswere poor for the relationships between mean cultivar rootwidths and mean numbers of zoosporangia per root surfacearea ( r   = 0.29), and mean root widths and mean numbers of root galls ( r   = 0.43). Correlations were also poor for therelationships between mean root fresh weights and meannumbers of zoosporangia per root surface area ( r   = –0.29),and mean root fresh weights and mean numbers of root galls per g root fresh weight ( r   = –0.03).  Relationships between field-assessed tuber infection and root infection.  The relationships between laboratory-measured parameters of root infection of 15 cultivars by S.subterranea  and their field-assessed scaled average powdery scab severity scores were examined graphically.Fig. 3 b  presents the relationship between numbers of zoosporangia/mm 2  root surface area and field score, for which the correlation coefficient was –0.59. If the data for the cv. Swift were excluded, the coefficient for this Table 1.Scaled average powdery scab severity scores for potato cultivars, breeding lines and germplasm accessions assessed in field trials for susceptibility to the disease . All cultivars were assessed for at least two growing seasons (numbers in parentheses indicate growing seasons more than two) Very resistantModerately resistantModerately susceptibleVery susceptibleCultivarScoreCultivarScoreCultivarScoreCultivarScoreSwift9.0Russet Burbank7.9 (3)2581.36.9Kennebec5.9Vtn62-33-38.8Ranger Russet7.9Frisia6.9 (4)Concorde5.9Gladiator  A 8.8 (11)Nooksac7.8 (3)Bildstar6.9Crebella5.9Donald8.7Innovator7.7Tekau6.9Rosa5.9Winston8.6Heather7.7810/76.9Whitu5.5 (3)Saxon8.5Cliffs Kidney7.7White Delight6.8710/115.5Red Rascal8.4Velox7.6Wha6.8 (3)Agria5.4 Nortakah8.4Pacific7.6Red Ruby6.8Draga5.3Van Gogh8.3Valor7.6Crop 96.8 (3)Coliban5.3Toru8.3Fraser7.6Jewel6.7Ukama5.1Moonlight8.3Summit 7.5Rua6.7 (3)Bright5.1Harmony8.3Dawn7.5Arran Banner6.7Iwa B 5.0 (11)Foxton8.2Rima7.5Fronika6.6Cardinal4.9 (3)Snowden8.2Croft7.5Spunta6.6B5281-14.8Ilam Hardy8.2Jersey Benne7.4Hulda6.6Florissant4.8Stroma8.2Waru7.4Obelix6.6 (3)Liseta4.6 Nadine8.1Tarago7.4Tahi6.5Marco Polo4.6Cantate8.1Red King7.4Matilda6.5Markies4.5Fianna8.1Desiree7.4 (5)Atlantic6.5676/23.9Driver8.0Pentland Dell7.4759/36.5743/63.5Lady Rosetta8.0Maris Anchor7.3Epicure6.4Asterix2.6Horizon8.0Majestic7.3Mondial6.4Record8.0 (3)Kaimai7.2 (3)Shepody6.4 (3)Sebago7.2Rocket6.4 (3)Kiwitea7.2 (3)Frontier Russet6.3Delcora7.1Victoria6.3Lutetia7.1672/66.3Dakota Red7.1Fanfare6.3718/67.0Bolesta6.2Karaka7.0Diamant6.1Katahdin7.0Red Star6.12000A6.1Morene6.1Mona Lisa6.0Premiere6.0Crop 176.0Yvonne6.0 (3) A Resistant standard. B Susceptible standard.  Powdery scab resistance in potato cultivars  Australasian Plant Pathology 381 relationship was –0.80. The correlation coefficient for therelationship of score with numbers of galls/g root freshweight was –0.23, or –0.50 with the data for cv. Swiftexcluded.Various multivariate techniques using all measured  parameters were applied to identify possible reasons for thehigh numbers of zoosporangia and root galls that occurred on cv. Swift even though it was assessed as very resistant to powdery scab on field-grown tubers. No conclusive evidencewas found, other than that glasshouse-grown cv. Swift plantshad the smallest root fresh weights of all the fifteen cultivarsevaluated, with the mean root fresh weight for this cultivar (1.9 g) being approximately half that of the next largestcultivar, Kennebec (3.9 g). Discussion This research provides a comprehensive assessment of therelative susceptibility to powdery scab of potato cultivarsavailable in New Zealand. Growers can use this information,along with chemical and cultural methods for control of thedisease, as they develop powdery scab management for their  potato-growing operations. They can either choose cultivarsthat are resistant to the disease, or where susceptible cultivarsmust be grown, apply other appropriate powdery scabcontrol methods. These results, together with other research,also provide background information for incorporating powdery scab resistance into new potato cultivars as a keycharacter in potato breeding programmes. We continue toutilise this information as part of Crop & Food Research’s potato improvement initiatives, and are also exploring theuse of genetic markers to assist with identification of  powdery scab resistance.The field assessment of susceptibility of 99 potatocultivars and 13 breeding lines has demonstrated thatsusceptibility to powdery scab follows a continuum fromvery resistant to very susceptible. This characteristic has also been demonstrated by other studies where large numbers of germplasm lines have been assessed for reaction to thedisease (e.g. Torres et al  . 1995). Furthermore, our glasshouseexperiment demonstrated that severity of S. subterranea infection in roots (numbers of zoosporangia and root galls)of different cultivars also followed a continuum. Thisindicates that resistance to the disease is of the ‘quantitative’type, which is characterised by a continuous variation between cultivars in resistance characters, and is probably based on the additive effects of several resistance genes, eachwith relatively small effects (Parlevliet 1989; Bradshaw1994). The fact that none of the cultivars showed totalimmunity to S. subterranea supports the conclusion thatresistance is multi-mechanistic and likely to be multigenic.Resistance to this pathogen could result from effects atseveral stages in the infection cycle, possibly including Mean number of zoosporangia / mm2 0 20 40 60 80 100 120 140 160 AgriaSwiftAsterixIwaRocketKatahdinPacificDesireeKennebecRusset Burbank Arran BannerMoonlightNadineRed RascalGladiatorMean number of galls / root fresh weight (g)0 5 10 15 20 25 30 35 40AgriaSwiftAsterixIwaRocketKatahdinPacificDesireeKennebecRusset Burbank Arran BannerMoonlightNadineRed RascalGladiator( a  ) ( b  ) Fig. 1 .Infection of roots of different potato cultivars by Spongospora subterranea in a glasshouse experiment. Mean numbers of zoosporangiain roots ( a ) were assessed for each cultivar 22 days after inoculation of plants with S. subterranea sporosori, whereas numbers of root galls ( b )were assessed 8 weeks after inoculation. Bars are 95% confidence limits for the means ( a , for variation between 10 roots of a single plant; b , for root systems of nine plants). Fig. 2 .Light micrograph of a root of the potato cv. Iwa heavilyinfected with zoosporangia of Spongospora subterranea.  Bar =50µm.
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