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.     

A method of assessing potato seedling progenies for resistance to the white potato cyst nematode

Summary The resistance expressed by progenies of potato seedlings to the white potato cyst nematode (Globodera pallida) has been assessed on bulk sowings of seedlings and on individual plants raised from tubers, and a positive correlation between the seedling and plant tests has been established. The use of such a seedling test as an addition to other methods of screening is discussed.     

The effect of soil pH on yield losses caused by the potato cyst nematodeGlobodera pallida

Summary The effect of soil pH on yield losses due to potato cyst nematodes (Globodera pallida) was investigated in experiments in the field and in a semi-controlled environment. Through annually liming a reclaimed peat soil with about 22% organic matter, plots with pH levels of 4.5, 5.0, 5.5, 6.0 and 6.5 were obtained. In 1990 potato yields in these plots decreased from 45 t/ha at pH 4.5 to 33 t/ha at pH 6.5, whereas the nematode densitites decreased from about 18 to 9 juveniles per g soil. In a container experiment a strong interaction was recorded between soil pH and the presence of nematodes. Tuber yields were about 11% lower at pH 6.5 than at pH 4.5 in the absence of nematodes, but about 44% lower when an initial population of 27 juveniles per g soil was present.     

Inheritance of tolerance ofGlobodera pallida and the relationship between tolerance and resistance toG. pallida in potatoes

Summary The tolerance of and resistance toGlobodera pallida of two potato progenies which segregated for these characteristics were assessed in pots in the glasshouse. The level of tolerance varied widely between genotypes within the progenies, with several genotypes being significantly more tolerant than the intolerant parent. The levels of resistance and tolerance in the progenies were not significantly correlated. No indications were obtained that the ranking of genotypes for tolerance is influenced by the density ofG. pallida in the soil. The results confirm that selection for tolerance in aG. pallida resistance breeding programme is useful and feasible.     

Extreme resistance to infection by potato virus X in genotypes of wild tuber-bearingSolanum species

Summary Clones derived from thirty-one different accessions (nineteen of Argentine origin) belonging to eightSolanum species were screened for resistance to infection by potato virus X strain cp (PVX cp) by mechanical inoculation of plantlets that had been micropropagated in vitro. Estimates of PVX multiplication obtained by enzyme linked immunosorbent assay and slot blot nucleic acid hybridization allowed the identification of resistant clones derived from five accessions belonging toS. commersonii S. oplocense, S. sparsipilum andS. tuberosum andigena. Resistant genotypes supported PVX concentrations 5 to 15 times smaller than did the susceptible control cultivar Spunta. Graft inoculation test confirmed the presence of extreme resistance similar to that conferred by the ‘immunity’ gene X¹ (also called RX_act).     

Relationship between potato cyst nematodes and their principal host. IV. Tolerance of potato cultivars and the effect of potato cyst nematode on initial plant growth

Summary Greenhouse experiments on the effects of white potato cyst nematode infestations (Globodera pallida Stone) on initial growth and development of a series of potato cultivars are compared with the results of field experiments on sandy and sandy-peat soils on the effect of nematode density on tuber yield. A simple greenhouse test, assessing root growth response to potato cyst nematode infection, provided a good insight into a cultivar's tolerance performance in the field early in the growing season. As a very limited number of plants is needed for the greenhouse test, screening for tolerance can be conducted in the early stages of a breeding programme.     

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.     

Bacillus thuringiensis cry1Ab gene confers resistance to potato againstHelicoverpa armigera (Hubner)

Summary Helicoverpa armigera is one of the important insect pests adversely affecting the yield of potatoes in India. A synthetic gene encoding the insecticidal crystal protein (Cry1Ab) ofBacillus thuringiensis (Bt) has been introduced into five genotypes of potato usingAgrobacterium tumefaciens. Southern analysis of DNA from transgenic plants confirmed the integration and copy number of the transgene. Double-antibody quantitative sandwich ELISA analysis demonstrated high levels of Cry1Ab protein expression in transgenic plants. Insect bioassays on the leaves of transgenic plants showed considerable protection against the larvae ofH. armigera in terms of leaf area consumed and larval weight reduction.     

Fingerprinting Indian potato cultivars by random amplified polymorphic DNA (RAPD) markers

Summary Random amplified polymorphic DNA markers were used to distinguish and characterize 20 Indian potato cultivars. A total of 198 scorable fragments were amplified using 10 random primers, only two of which were monomorphic. Similarity values among the cultivars ranged from 0.33 to 0.80. A primer having resolving power above 7.4 was sufficient to distinguish all 20 cultivars. Wide variations in band profiles were observed when the same template DNA was amplified using Taq DNA polymerase from four different sources. No significant difference in profile complexity was observed at 40°C annealing temperature with a primer having 70% GC content. Prior restriction of template DNA resulted in band profiles whose complexity was similar to or higher than that of unrestricted template. However, multiplex RAPD with cleaved template DNA could not increase fingerprint complexity. Publication No. 1666, CPRI. Shimla     

Effect of haulm treatments on the formation of microsclerotia ofVerticillium dahliae Kleb. on potato

Summary In four pot experiments, potato plants of cv. Element were artificially infected withV. dahliae. At an early and a late harvest haulms were killed chemically, by burning or by various other treatments, including cutting them into pieces of different lengths and keeping the debris on the soil surface or covering with soil. After 4 weeks the plant material was air-dried and the number of microsclerotia per mg was determined. At the early harvest, in two experiments, the chemical treatment yielded more microsclerotia than the cutting treatments. Covering colonised haulm tissue with non-sterilised soil was effective in inhibiting microsclerotia formation. Shorter haulm pieces led to fewer microsclerotia at the later harvest if the material was kept on the soil surface. The variation in microsclerotial yield and in treatment effects among the different experiments was large.     

The influence of 2,4-D and kinetin on leaf callus formation in different potato species

Summary Leaf segments ofSolanum tuberosum andS. acaule can proliferate callus. However, callus formation and growth for a specific species depends on different concentrations of 2,4-D and kinetin (6-furfurylamino purine) used in a medium. Large amounts of creamy white friable callus could be proliferated fromS. tuberosum when explants were cultured in media containing 3 mg of 2,4-D and 0.3 mg of kinetin per liter. WhenS. acaule explants were cultured in the similar medium, the segment tended to become yellow; however, when it was subcultured onto a similar fresh medium, friable and creamy white callus was then formed. In a medium containing 0.3 mg of 2,4-D and 1 mg of kinetin per liter, the compact callus could be directly proliferated fromS. acaule leaf segments. Segments incubated under light formed less callus than those incubated in darkness. Scientific Journal Series No 10559 of the Minnesota Agricultural Experiment Station. This research was supported in part by a research contract from the International Potato Center, Lima. Peru.     

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.     

The production of dihaploids and tetraploids with combined quantitative resistance to potato cyst nematode (Globodera pallida) and foliage blight (Phytophthora infestans)

Summary Dihaploids were produced from tetraploids which had resistance to foliage late blight (P. infestans) and had a range of levels of resistance to white potato cyst nematode (PCN) (G. pallida). Twelve of 33 dihaploids tested had useful levels of both resistances, 11 were resistant to blight alone and seven to PCN alone. There were no significant correlations between the resistances of the parents and the mean resistance of their dihaploids, for either character. There was also no correlation between levels of resistance to blight and to PCN. There were significant differences in mean levels of resistance to blight and to PCN between the tetraploid progenies of two dihaploids which possessed combined resistance.     

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.     

Testing the resistance to silver scurf in commercial potato cultivars under controlled environmental conditions

Summary Optimum storage conditions to identify resistance to silver scurf among commercially grown potato cultivars were determined in a series of experiments. Inoculation of field-grown tubers with a conidial suspension ofH. solani (concentration 10⁴ conidia per ml of water) and incubation at 15°C with 95% RH for 1 month followed by 2 months at 85% RH produced the greatest differences in disease severity between potato cultivars. Results with glasshouse-grown tubers were comparable with those from field-grown tubers, indicating that the test could be used to identify resistance in wild species of potato and to understand the inheritance of this resistance.     

The effects of (S)-(+)-carvone treatments on seed potato tuber dormancy and sprouting

Summary Potato seed tubers may suffer from premature sprouting during storage, thus limiting their suitability for cultivation. Commonly used sprout suppressant treatments negatively affect but viability and therefore a reliable method to inhibit bud development must still be found for seed tubers. The monoterpene carvone ((S)-(+)-carvone) was tested in small scale experiments. The vapour of this compound fully inhibited bud growth of tubers cv. Monalisa stored at 23°C without affecting bud viability throughout 6 months of treatment. The most effective range of carvone vapour concentrations was between 0.34 and 1.06 μmol mol⁻¹. With these qualities we can expect carvone to become a suitable sprout suppressant for seed tubers.     

Progeny tests to identify diploid potato clones homozygous at loci controlling resistance to PLRV

Summary Using potato parental lines homozygous at a locus or loci controlling resistance to potato leafroll virus (PLRV) can give advantages in the selection of resistant forms. In order to identify homozygous diploid clones their test-cross families were evaluated. All the clones that were test-crossed expressed resistance in primarily- and secondarily-infected plants and etiolated sprouts, and were derived from mating genotypes highly resistant to PLRV. Genotypes from test-cross families varied in resistance to PLRV, and one family was found which had only resistant genotypes, suggesting that the resistant parent of this progeny was homozygous at resistance loci. Evidence was gathered that resistance in some diploid clones may result from resistance to virus multiplication as well as restricted virus transport from leaves to tubers.     

Differential resistance to tuber late blight in potato cultivars without R-genes

Summary In 1983 and 1984 there were reports in Norway of more severe late blight infection than usual on tubers of cv. Pimpernel, a cultivar without R-genes known to be somewhat resistant to both haulm and tuber infection. Results from routine tests confirmed earlier experience that cv. Pimpernel shows stable resistance to tuber late blight, although the ranking of cultivars may vary between years. In experiments during three harvest seasons tubers of different cultivars were inoculated with isolates ofP. infestans obtained from stocks of cv. Pimpernel with severe tuber late blight and with other isolates. In these experiments the isolates from cv. Pimpernel caused more rot on tubers of Pimpernel than did other isolates.     

Correlation between resistance to late blight in foliage and tubers in potato clones from parents of contrasting resistance

Summary The relationship between resistance to late blight in foliage and tuber was examined on 50 clones of each of five progenies from crosses where one parent was resistant in both foliage and tuber and the other susceptible. Foliage resistance was assessed in a field trial and tuber resistance in a laboratory test on glasshouse-grown tubers. The genetical and environmental components of variance and the phenotypic and genetical correlations between foliage and tuber blight scores were determined for each progeny. Foliage and tuber resistance were correlated, indicating either that both are determined by the same genes or by different linked genes. The environmental component of variation was greater for tuber blight than foliage blight. We suggest that the most effective way of selecting for resistance to both aspects of the disease in a breeding programme is to select those resistant in the foliage and then screen them for tuber resistance.