Assessing general combining ability for late blight resistance and tuber characteristics by means of glasshouse seedling tests

Summary Screening tests to detect resistance to late blight in both foliage and tubers were done on glasshouse-grown seedling progenies in parallel with visual assessments by three experienced potato breeders of the yield and quality of glasshouse-grown tubers of the same progenies. There were large differences between the parents of the progenies in their general combining ability (GCA) for both foliage and tuber blight, despite some variation due to specific combining ability for foliage blight. There were also differences between parents in their GCAs for visual preference scores, but these GCAs and those for blight resistance were not correlated. The blight-resistant cv. Stirling had the best combination of high GCAs for all three attributes. The use of these and other seedling progeny tests in a multitrait genotypic recurrent selection scheme is discussed.     

A glasshouse progeny test for resistance to gangrene (Phoma foveata)

Summary Tubers were obtained from 80 glasshouse-grown seedlings of each of twelve progenies, inoculated withPhoma foveata Foister, and the mean percentage of infected tubers recorded. There was a good correlation (r=0.798) between this assessment and that made previously on field-grown tubers of eleven of the progenies. A gangrene test on glasshouse-grown tubers is suggested as a more rapid means of identifying progenies with useful levels of resistance.     

Assessing progenies of potato for resistance to skin spot (Polyscytalum pustulans) in the glasshouse

Summary Crosses were made between six parents differing in susceptibility to skin spot and glasshouse-grown tubers from true seedlings of the nine resulting progenies assessed for resistance to the disease. The assessment was evaluated by a two-year comparison with the behaviour of field-grown tubers of the progenies and their parents. The correlations between progenies in the glasshouse and field were r=0.911 and 0.753 and those between the mid-parental values and the progeny means were high (r>0.864), indicating that parental differences were heritable. It is concluded that correlations were sufficient for glasshouse-grown tubers to be used for assessing large numbers of progenies in genetical studies and for eliminating highly susceptible progenies at an early stage in a breeding programme. R.L. Wastie died 16.01.96.     

A glasshouse test for assessing resistance to early blight (Alternaria solani)

Summary Fourteen genotypes of potato were assessed for resistance toAlternaria solani Sor., both in the glasshouse at the Scottish Crop Research Institute in Scotland and in the field in the Negev, Israel. Glasshouse assessments agreed well with field results, identifying the most resistant and susceptible genotypes. The effects on the glasshouse test of plant age, inoculum concentration, and length of time plants were kept damp after inoculation, were examined. The best agreement with field results was shown by plants inoculated at flowering, and the best discrimination between genotypes was achieved by keeping the foliage damp for 24 h after inoculating with a spore suspension containing 2.5×10³ spores/ml.     

Heterosis and combining ability analysis for resistance to early blight (Alternaria solani) in potato

Summary In an epiphytotic year of early blight, data were recorded on its incidence in 72 progenies (18 females x 4 males) and their parents planted in a randomised complete block design at the Punjab Agricultural University, Ludhiana, India. Combining ability analysis showed that both additive and non-additive gene action were important, with a preponderance of the latter in the inheritance of resistance to early blight. In general, progenies were as good as or better than mid-parent values. Crosses involving both or at least one parent with good general combining ability resulted in heterotic early blight resistant progenies. Promising parents and crosses were identified.     

Assessing progenies of potato for resistance to early blight

Summary Progenies from crosses between cultivars varying widely in resistance to early blight (Alternaria solani Sor.), were assessed for resistance as true seedlings in a glasshouse in Scotland. The resistance of a representative sample of surviving genotypes from each progeny was compared with samples of the same progenies not previously exposed to the fungus, both in the glasshouse in Scotland and in the field in Israel. The exposed population was more resistant. Resistance was identified more effectively in adult plants from tubers in the glasshouse than in true seedlings and agreement between glasshouse and field assessment was better when progenies were compared rather than individual genotypes. The mid parent and progeny mean scores of the unexposed population were correlated at both sites, thus confirming that the resistance is heritable. Selecting resistant individuals at the seedling stage is suggested as a useful tool for resistance breeding, having first chosen the best parents for crossing.     

Assessing the resistance to gangrene of progenies of potato (Solanum tuberosum L.) from parents differing in susceptibility

Summary The resistance to gangrene (caused byPhoma exigua var.foveata) of progenies from two hybridisation schedules involving cultivars resistant, susceptible, or of intermediate reaction to gangrene was assessed. Two methods of sampling tubers from field-grown single-plant plots of 18 clones representing each progeny were used: one tuber was taken from each plant, and a bulk sample of 18 tubers was selected at random from the whole progeny. Gangrene resistance was assessed after incubating tubers which had been rolled in cornmeal-sand inoculum ofP. exigua var.foveata, and was expressed as a percentage of tubers infected. Results from the two methods of sampling correlated closely, and agreed well with tests in a subsequent year on 10-tuber samples of each individual clone. It is concluded that an 18-clone sample adequately represented each progeny, and that the mid-parent gangrene score satisfactorily predicted the resistance of the resulting progeny. There are indications that susceptibility tended to be dominant. The implication of these findings to breeding for resistance to gangrene is discussed.     

The susceptibility of potato cultivars to gangrene in laboratory tests in relation to origin of tubers,damage, method of inoculation and test centre

Summary Undamaged and damaged tubers of 10 cultivars were inoculated withPhoma foveata by two methods at five test centres over three years. Damaging tubers before inoculation increased the discrimination between cultivars. While this increase was small with tubers rolled in cornmealsand cultures, it was often substantial with those dipped in a suspension of comminuted agar cultures. Correlations between centres for the four tests were significant (P<0.05) and were highest for tubers inoculated with cornmeal-sand with or without damage. The gangrene scores of the cultivars in the four tests were correlated (P<0.05) with their field susceptibility ratings but there was a strong interaction between cultivar reaction and year of test. When tubers grown in Cumbria and Midlothian were tested at one centre over two years, the gangrene scores were also correlated (P<0.05).     

Assessing potato cultivars in Tunisia for susceptibility to leak caused byPythium aphanidermatum

Summary The susceptibility of 21 potato cultivars to leak (watery wound rot) caused byPythium aphanidermatum was compared. Whole tubers were inoculated after wounding by dipping into a water suspension of 10³ oospores/ml and incubated for 3 days at 25°C. The mean penetration of tissue calculated from the recording of lesion width and depth and the qualitative assessment of rot extension were highly correlated. For 10 of 12 cultivars tested at least twice, consistent reactions were obtained: cvs Korrigane. Superstar, Safrane and Yesmina were moderately susceptible, whereas Atlas, Diamant, Mondial, Obélix, Spunta and Timate were susceptible to highly susceptible. Tubers originated from two locations and/or two cropping seasons: thus, the performance of these cultivars under different production conditions reveals a satisfactory level of agreement with the test. Moreover, ratings assigned are supported by field or store observations.     

Screening tuber-bearingSolanum spp. for resistance to soft rot caused byErwinia carotovora ssp.atroseptica (van Hall) Dye

Summary Soft rot caused by Erwinia carotovora ssp. atroseptica is a major disease of stored potatoes. Since varietal resistance can contribute to control, the work reported was designed to find new sources of resistance among related tuber-bearing Solanum spp. True seeds were imported from two international collections and families were screened for resistance to tuber soft rot. Forty-eight resistant clones were found in 21 out of 100 accessions. These clones will be used in breeding programmes at the diploid or tetraploid level.     

A prototype inoculator for use with cysts ofGlobodera spp.

Summary An apparatus was developed for inoculating large numbers of experimental units with cysts ofGlobodera spp. The inoculator is adjustable and releases different and equal volumes of cysts from an internal reservoir. It was found to dispense accurate amounts of inoculum and to be much faster than manually counting cysts.     

Assessing the tolerance toGlobodera pallida of resistant potato genotypes by means of field and pot tests

Summary The tolerance toGlobodera pallida of 15 potato genotypes with a high level of resistance toG. pallida was assessed by measuring their tuber yield in heavily infested fields and in nematicide treated strips in the same fields. Tolerance was also assessed in pots in the glasshouse by measuring the total biomass after 35 and 70 days growth in heavilyG. pallida infested soil and in uninfested soil. Large differences in tolerance between the genotypes were observed. High correlations were observed between tolerance assessments made in the field experiments and between field and pot experiments. No relationship was found between tolerance and maturity. Shoot and root dry weight were about equally affected by the nematode.     

Inheritance of extreme resistance to potato virus Y in potato

Summary Segregation for extreme resistance to PVY was evaluated in progenies derived from crossing two extremely resistant potato clones with parents differing in resistance. Resistance was evaluated after mechanical inoculation with PVY^O and PVY^N, and after graft inoculation with PVY^O. Biological and serological tests (ELISA) were used for virus detection. The extreme resistance is governed by a single dominant gene, but observed segregations deviated from the expected ratios. Considerable modifying effects were detectable, depending on the potato genotype and virus isolate, for a significant excess of susceptible genotypes was observed in some progenies. Moreover, genotypes with non-parental types of resistance to PVY were observed.     

Screening tuber-bearingSolanum spp. for resistance toGlobodera rostochiensis Ro1 Woll. andG. pallida Pa2/3 stone

Summary Accessions of tuber-bearingSolanum spp. related toS. tuberosum subsp.tuberosum were obtained from the German-Dutch collection (Braunschweig, Germany) and the Inter-regional potato collection (Sturgeon Bay, USA). They were screened for resistance toG. rostochiensis Rol andG. pallida Pa2/3. Among 1567 clones from 52 accessions, 135 clones (23 accessions) were resistant toG. rostochiensis. They mainly representedS. andigena, gourlayi, spegazzinii andvernei. Among 1689 clones (74 accessions), 105 clones (32 accessions) were resistant toG. pallida. They representedS. gourlayi, spegazzinii, sparsipilum andvernei. About 25 clones were resistant to both species.     

An early generation screen for combined cyst nematode (Globodera spp.) and blight (Phytophthora infestans) resistance in potato

Summary A technique is described which enables potato clones to be screened for resistance to both species of potato cyst nematode (Globodera rostochiensis andG. pallida) and to blight (Phytophthora infestans) in the foliage during the winter and first spring following harvest of seedlings grown from true seed. A container test, used to assess resistance to both species ofGlobodera in a mixed inoculum, is followed by an assessment of resistance to foliage blight, using the same plants in the glasshouse. This screen can be completed in time for duplicate tubers of the resistant clones to be planted for multiplication in the second growing season. The close correlation between results obtained using this system and those predicted from the known inheritance of the characters selected and the gain in efficiency achieved in breeding, are illustrated by examples. In Kartoffelzuchtprogrammen müssen viele Eigenschaften berücksichtigt werden. Die Reihenfolge ihrer Selektion kann grossen Einfluss auf die gesamte Arbeitsleistung haben. Dort wo die Aufspaltung der Nachkommenschaft bedeutsam ist, kann eine frühe Selektion leicht erkennbarer Eigenschaften, wie Resistenz gegenüber Sch?dlingen und Krankheiten, die Populationsgr?sse wirkungsvoll verringern. Dies k?nnte den starken Selektionsdruck auf Knolleneigenschaften als erste Hauptprüfung in Kartoffelzuchtprogrammen ersetzen. In dieser Arbeit wird eine Technik zur Prüfung auf kombinierte Resistenz gegenüber Kartoffelnematoden und Krautf?ule beschrieben, und es werden die Ergebnisse für Nachkommen aus typischen Kreuzungen mitgeteilt (Tabelle 1). Die Nematodenresistenz wurde mit Hilfe eines Gef?sstestes ermittelt, bei dem ein Inokulumgemisch aus Larven vonGlobodera rostochiensis undG. pallida verwendet wurde. Dies ergab die in Tabelle 2 dargestellten Befunde und eine Verteilung der Zystenproduktion, wie sie in den Abbildungen 1A und 1B gezeigt wird. Diese Ergebnisse best?tigten, dass die nicht wiederholten Tests Reaktionen ergaben, wie sie von der bekannten Vererbung der Eigenschaften erwartet wurden, und ein Chi-Quadrat-Test (Tabelle 2) ergab bei drei von vier Familien, dass keine Abweichung von den erwarteten Verh?ltnissen für H1, von dem sich dieG. rostochiensis-Resistenz herleitet, vorlag. Eine vorkommende Abweichung (Tabelle 2, Familie 4) liess sich mit der ungenügenden Unterscheidung der unreifen Zysten von beidenGlobodera-Arten erkl?ren. Bei der Selektion auf Resistenz gegenüber den beiden Arten ist diese Fehlerquelle unbedeutend. Die als nematodenresistent ermittelten Pflanzen wurden in T?pfen bei hoher Luftfeuchtigkeit in einem Polythen-Tunnel in Gew?chshaus angezogen und mit einer Zoosporensuspension aus einem Isolatkomplex vonPhythophthora infestans besprüht. Die Anf?lligkeit wurde ermittelt, indem die ungef?hre Blattfl?che der entstehenden nekrotischen L?sionen bewertet wurde. Die Ergebnisse in Tabelle 3 und die Verteilung der resistenten Klone (Abb. 1C) weisen—wie erwartet—auf die polygene Vererbung hin. Bei den angewandten Selektionsstufen erhielt man ungef?hr 15% der Nachkommen mit hinreichender Resistenz gegenüber den drei vorgegebenen Eigenschaften. Die Teste wurden in der Lagerzeit der Knollen, die ursprünglich von S?mplingspflanzen stammten, abgeschlossen, so dass sie in der der Aussaat nachfolgenden Saison ausgepflanzt werden konnten. Dans les programmes de sélection de la pomme de terre, de nombreux caractères sont pris en compte et le choix de la succession de leurs mesures a une forte importance sur l'efficacité globale du travail. Quand il y a ségrégation pour des caractères importants et facilement mesurables comme la résistance aux ravageurs et aux maladies, on peut réduire le travail de maintenance du matériel en réalisant des cribles précoces pour ces caractères. Ces tests peuvent être d'abord réalisés à la place des tests de qualité, classiquement effectués en début de programme. Les auteurs décrivent ici une technique de tests de résistance à la fois vis-à-vis des nématodes à kyste et du mildiou. Les résultats de tests de descendance de quatre croisements sont détaillés. La résistance aux nématodes est mesurée par des tests en conteneours dans lesquels le substrat est infesté par un mélange de larves infestantes deGlobodera rostochiensis et deG. pallida. On trouve les résultats globaux en Tableau 2 et les distributions exprimées en valeur relative du nombre de kystes neoformés sont représentées en figures 1A et 1B. Malgré l'absence de répétitions, ces résultats sont en accord avec ceux espérés: le {ie67-1} (Tableau 2) ne montre pas de différence significative pour 3 familles sur 4 avec les résultats théoriques quand c'est la résistance due au gène H1 qui est mesurée. Le cas de non concordance (Tableau 2, famille 4) est explicable par une possible confusion des espèces à la lecture. Cependant, cette source d'erreurs est peu importante quand la sélection est dirigée en même temps contre ces deux espèces. Les clones jugés résistants sont alors testés vis-à-vis dePhytophthora infestans. Ils sont placés en pots et cultivés en serre sous tunnel de polyethylène pour entretenir une forte humidité relative. Une suspension de zoospores d'un isolat complexe est pulvérisée et la sensibilité est appréciée par la surface approximative des lésions nécrotiques du feuillage. Les résultats du Tableau 3 et la distribution relative de la fréquence des clones résistants confirment l'hérédité polygénique de la résistance au mildiou. Environ 15% des familles sont ainsi retenues comme ayant un bon niveau de résistance aux deux espèces deGlobodera et àP. infestans. Les tests, réalisés en hiver, sont achevés à temps pour que les duplicatats, conservés à basse température, identifiés comme résistants puissent être plantés au champ en temps utile, l'année qui suit les semis de graines.     

Resistance to gangrene (Phoma exigua var.foveata) and dry rot (Fusarium solani var.coeruleum) in potato tubers. 2. Evaluation of genotypic differences in pectin-bound magnesium and calcium for screening purposes

Summary The levels of incorporation of Mg and Ca into tuber cell walls and middle lamellae were compared with the intensity of attack byPhoma exigua var.foveata (P.e.f.) (gangrene) andFusarium solani var.coeruleum (F.s.c.) (dry rot) in six genotypes varying in their levels of resistance. The Mg:Ca ratio was higher in those genotypes more resistant toP.e.f., while the cation effect on attack byF.s.c was variable. Although genotype resistance may be improved by extra Mg bridges in the pectin structure the genotypic differences in susceptibilty to gangrene could not be explained by differences in Mg content assessed from alcohol-insoluble solids. Analyses of breeders' selections showed no general relationship between Mg or Ca contents and disease incidence. Therefore, genotypic differences in incorporation of these cations could not be used for screening for improved resistance.     

The expression of resistance to the white potato cyst nematode (Globodera pallida) by individual potato seedlings under different inoculation treatments

Summary The resistance toGlobodera pallida of individual potato seedlings from three progenies derived fromSolanum tuberosum ssp.andigena CPC 2802 was assessed using three inoculation treatments. Plants grown from tubers from the seedlings were used to reassess resistance. All the seedling test inoculation treatments over-estimated the proportion of clones classified as resistant compared to the results from tuber-grown plants. However, the mean relative resistance of progenies estimated by both tests showed good agreement, and it is suggested that seedlings are best used for assessing the resistance of progenies of crosses to identify promising combinations rather than individual clones.     

Laboratory screening for resistance to gangrene and toFusarium dry rot in potato

Summary For laboratory screening for resistance to gangrene andFusarium dry rot, the ‘scrape and paint’ method was compared to a ‘prick and paint’ method. In the latter method the potatoes were wounded before inoculation by rolling them over a group of nail points. The disease scores increased when the length of the nail points was increased from 2 to 6 mm, but the scores were lower than for scraped potatoes. For the 10 cultivars used, the results for the pricked and the scraped potatoes correlated equally well with the results from a ‘field inoculation’ method. The two laboratory methods discriminated equally well between resistant and susceptible cultivars. However, pricking is much less laborious than scraping. In routine tests with the ‘prick and paint’ method, differences between cultivars were highly significant over three years for resistance to gangrene but less significant forFusarium dry rot.

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Zusammenfassung Für die Laborprüfung auf Resistenz gegenPhoma-Faule (Phoma exigua var.foveata) undFusarium-Trockenf?ule (Fusarium solani var.coeruleum) wurde die ‘scrape and paint’-Methode von Wiersema (1977a) mit einer ‘prick and paint’-Methode verglichen, wobei Kartoffeln vor Inokulation durch Rollen über eine Gruppe von Nagelspitzen, 1 cm auseinander und von einer h?lzernen Basis aufw?rtsweisend, besch?digt wurden. Für beide Krankheiten stieg der Krankheitsindex an, wenn die L?nge der Nagelspitzen von 2 auf 4 und 6 mm anstieg, war im Ganzen jedoch niedriger als bei geschürften Kartoffeln (Tabellen 2 und 4). Für die zehn für jeden Erreger in den Versuchen verwendeten Sorten korrelierten die Ergebnisse bei ‘prick’ (Rollen über Nagelspitzen) und ‘scrape’ (Schürfen) gleichermassen mit einem vermutlich besser zu verwirklichendem Feld-Inokulationstest (Bjor, 1980), dessen Ergebnisse in Tabelle 1 dargestellt sind. Die Unterscheidung von Sorten unterschiedlicher Resistenz war mindestens so deutlich beim ‘prick’ — wie beim ‘scrape’-Test, wie die F-Werte für Sorten in der Varianzanalyse zeigen (Tabellen 2 und 4). Darüberhinaus ist die ‘prick’-Methode betr?chtlich weniger arbeitsaufwendig als die ‘scrape’-Methode. Die ‘prick and paint’-Methode mit 4 mm-Nagelspitzen ergab Unterschiede zwischen 14 Standard-Sorten in Routinetests aus drei verschiedenen Jahren beiPhoma-F?ule, aber nicht so gut beiFusarium-Trockenf?ule (Tabelle 6). Die Korrelation zwischen den Ergebnissen nach der ‘prick and paint’-Methode und dem Feld-Inokulationstest bei den 14 Sorten warr=0,86 undr=0,71 beiPhoma-bzw.Fusarium-F?ule. Allerdings war die Variation innerhalb der Sorten bei Resistenz gegenPhoma-F?ule betr?chtlich gr?sser als die bei Resistenz gegenFusarium-Trockenf?ule.

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Résumé Le criblage au laboratoire pour la résistance à la gangrène (Phoma exigua var.foveata) et à la pourriture sèche fusarienne (Fusarium solani var.coeruleum) est réalisé selon deux méthodes. La méthode du ‘grattage et du badigeonnage’ est comparée à celle de la ‘piq?re et du badigeonnage’ où les pommes de terre sont blessées avant l'inoculation en les faisant passer sur un groupe de 10 clous distants de 1 cm, orientés vers le haut et fixés sur un support de bois. Pour les deux maladies, la note de maladie augmente quand la longueur de la pointe des clous passe de 2 à 4 et 6 mm, mais les tubercules blessés par piq?res sont moins malades que ceux ayant subi des égratignures (tableaux 2 et 4). Pour les 10 cultivars inclus dans les expériences pour chacune des maladies, les résultats des tubercules blessés par chacune des 2 méthodes sont également corrélés avec ceux d'un test ‘inoculation au champ’, plus s?r (Bjor, 1980) figurant au tableau 1. Le classement des cultivars de différents niveau de résistance est aussi clair à partir des tubercules blessés par piq?re que ceux blessés par grattage comme le montre la valeur du F pour les cultivars, des analyses de variance (tableaux 2 et 4). De plus, la méthode des piq?res est beaucoup moins fastidieuse que la méthode par égratignures. La méthode des piq?res avec des pointes de clous de 4 mm reproduit les différences entre 14 cultivars de référence inclus dans des tests de routine depuis 3 ans pour la gangrène, mais de fa?on moins nette pour la fusariose (tableau 6). La corrélation entre les résultats obtenus avec la méthode des piq?res et le test ‘d'inoculation au champ’ pour les 14 cultivars estr=0,86 etr=0,71, respectivement pour la gangrène et la fusariose. En outre, la variabilité des cultivars pour la résistance à la gangrène est considérablement plus grande que celle vis-à-vis de la fusariose (Figure 1).

     

Production of phytoalexins, glycoalkaloids and phenolics in leaves and tubers of potato cultivars with different degrees of field resistance after infection withPhytophthora infestans

Summary The kinetics of accumulation of phytoalexins, glycoalkaloids and phenolics was studied in two potato cultivars differing in their degrees of field resistance when infected withPhytophthora infestans. Tuber slices and leaves of cvs Pampeana INTA (high degree of field resistance, free of R genes) and Bintje (susceptible) were infected with race C (complex race 1, 3, 5, 7, 11) ofPhytophthora infestans. Phytoalexins and phenolics accumulated in tuber and leaf tissues which had been inoculated. The levels of these compounds in the susceptible cv. Bintje were relatively low and similar to those found before inoculation. Leaves of cv. Pampeana INTA had a very high glycoalkaloid content, suggesting that glycoalkaloids may play a role in protection of leaves against the fungus. However, we could find no correlation between resistance and glycoalkaloid content of tubers. Our results suggest a major role of phytoalexins, phenolics and glycoalkaloids in the complex mechanisms of field resistance.     

Verification of sap flow by heat balance method on three potato cultivars

Summary A heat balance method for measuring sap flow in actively growing plants was tested on three potato (Solanum tuberosum L.) cultivars Monona, Norchip and Atlantic. The method is based on applying a known heat source to the stem of an intact plant: by measuring the axial and radial heat dissipation the magnitude of sap flow can be quantified. The technique shows promise as 24 h accumulated sap flow measurements were highly correlated (r²=0.935, n=36) with gravimetric measurements of transpiration and produced an average root mean square error (RMSE) of 20 g d⁻¹. The accuracy of the method improved as daily accumulated flow increased. Under low daily flow rates overall RMSE increased, supporting the need for a heat storage term in the heat balance equation. Short term flow rates were less significantly correlated (r²=0.764, n=1572) and produced an average RMSE of 2.89 g h⁻¹. Identification of these short term flow rates during high flow periods may be difficult. The presence of water potential gradients and the likelihood of re-hydration during high flow events may have also affected the short term performance of the method. At low flow rates, many different errors may be introduced.