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.     

Studies on natural repellents against potato tuber moth (Phthorimaea operculella Zeller) in country stores

Summary The effects of the leaves of five plant species, one from each of the generaAmbrosia, Anemone, Eupatorium, Eucalyptus andLantana, on potato tuber moth were investigated under indigenous storage conditions at the Central Potato Research Station, Shillong (1800 m above sea level). Their action was compared with that of a biological insecticide (spores ofBacillus thuringiensis), a chemical insecticide (carbaryl), and an untreated control. The data collected after six months storage on tuber damage, sprout damage and the rotting indicated that the leaves ofLantana aculeata provided most protection to the tubers, reducing damage from over 70% in the check to below 5%, and sprout damage from over 45% to below 3%. Next best wasEucalyptus globulus followed byB. thuringiensis. They may be used on tubers stored for table use or for seed as they had no adverse effect on germination or on the yield of a subsequent crop.     

Leaf effects of wild species ofSolanum and interspecific hybrids on growth and behaviour of the potato tuber moth,Phthorimaea operculella Zeller

Summary Four introductions ofSolanum berthaultii wild species and sevenS. tuberosumxS. berthaultii hybrids were evaluated by antibiosis and antixenosis laboratory tests with potato tuber moth. Some morphological and chemical traits related to the trichomes were analysed. Two clones of the wild speciesS. sparsipilum and five ofS. pinnatisectum, characterised by high tuber resistance, were tested for leaf resistance, both by antibiosis and antixenosis assays. The genotypes with high density of trichomes A and with high exudate PPO activity, showed a moderate negative effect on pupal weight and fecundity in the antibiosis tests. Trichomes B exudate showed a strong repulsive effect on oviposition of adult in the antixenosis tests. TheS. pinnatisectum clones revealed a good leaf resistance level, whereas theS. sparsipilum clones showed lower levels of antibiosis and antixenosis.     

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).     

Evaluation of diploid potato clones for resistance to tuber soft rot induced by strains ofErwinia carotovora subsp.atroseptica,E. carotovora subsp.carotovora andE. chrysanthemi

Summary Tubers of clones from a diploid hybrid population ofSolanum phureja andS. stenotomum were screened for resistance to tuber soft rot caused by strains ofErwinia carotovora subsp.atroseptica (Eca),E. carotovora subsp.carotovora (Ecc) andE. chrysanthemi (Ech). Significant, positive correlations between resistance to the different strains and species were observed, indicating that screening for resistance to tuber soft rot could be accomplished by using just one of the three pathogens. Strains of Ecc and Ech were found to be much more virulent than strains of Eca. Inoculating with Ecc and Ech resulted in more distinct differences among susceptible, moderately resistant and resistant clones than inoculating with Eca.     

Methods of assessing potato tubers for resistance to bacterial soft rot

Summary The susceptibility to soft rot caused byErwinia carotovora subsp.atroseptica of five potato cultivars and potato clones from the potato breeding programme at J?geva Plant Breeding Institute was assessed in a laboratory test. Three test methods were used: a single site inoculation, vacuum infiltration and slice inoculation. The severity of the disease varied widely with the different methods, which gave different cultivar rankings.     

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.     

Biological factors influencing the transmission of potato leafroll virus by different aphid species

Summary Macrosiphum euphorbiae, collected in the field from potato plants infected with potato leafroll virus (PLRV), transmitted the virus to fewer potato plants in a field trial than did laboratory-rearedMyzus persicae. In the laboratory,M. persicae was the only efficient vector of PLRV fromPhysalis floridana seedlings, potato sprouts or excised leaves toP. floridana. Two clones ofM. euphorbiae and one clone ofAulacorthum solani transmitted PLRV from infected potato plants toNicotiana clevelandii as effeciently asM. persicae but a clone ofAphis gossypii was an inefficient PLRV vector. An isolate of PLRV, whichM. persicae transmitted inefficiently from potato toN. clevelandii, was also transmitted inefficiently byM. euphorbiae andA. solani.     

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.     

Identification of potato interspecific hybrids resistant toVerticillium wilt and determination of criteria for resistance assessment

Summary Verticillium dahliae causes potato (Solanum tuberosum) early dying disease. Interspecific hybrids containingS. tuberosum, S. berthaultii, S. bukasovii, S. chacoense, S. gourlayi, S. sparsipilum andS. tarijense were identified with resistance toV. dahliae. Clones were grown in replicated trials for three years in a plot containing approximately 50 colony forming units (cfu)V. dahliae per g soil. Clones were evaluated for vine maturity, early dying symptoms and stem colonization. Five resistant 2x clones were identified with early maturity and low levels of stem colonization. A single maturity score and one disease score, each obtained in mid-August, were sufficient to distinguish among clones. An effective threshold level for considering a stem to be infected was determined to be 25 cfu/100 μl sap. Early dying symptoms did not correlate strongly with stem colonization. Numerous stems must be sampled when making resistance assessments. The average cfu/100 μl sap correlated strongly with the number of infected stems.     

Comparison of resistance toFusarium spp. of glasshouse-and field-grown tuber progenies of potato

Summary Field-grown tubers of 22 progenies ofSolanum tuberosum L. generated in a crossing programme involving seven parents differing in resistance toFusarium coeruleum Lib. ex Sacc. andF. sulphureum Schlect. (=F. sambucinum Fuckel, teleomorphGibberella pulicaris (Fr.) Sacc.) were wound-inoculated with a cornmeal + sand culture of each pathogen. Parental genotypes were also included. The mean lesion size of each progeny was compared in 2 years of tests, as well as with published data on glasshouse-grown tubers. ForF. coeruleum there was a high correlation between years as well as with the glasshouse data, but no such correlations were apparent withG. pulicaris. Furthermore, parental and GCA values, as well as progeny means and mid-parent scores, also correlated highly forF. coeruleum but not forG. pulicaris. Glasshouse-grown tubers of 11 wildSolanum spp. were also inoculated with both pathogens. Some resistance to one or other, or both, was apparent, particularly inS. chacoense.     

General resistance against potato virus Y introduced into a commercial potato cultivar by genetic transformation with PVYN coat protein gene

Summary Transgenic cv. Folva potato plants expressing the coat protein gene of potato virus Y strain N (PVY^N) were produced usingAgrobacterium tumefaciens mediated transformation. Forty independent transformants were selected for resistance screening. Four clones showed complete resistance to mechanical inoculation with all the five PVY isolates tested: the PVY^N isolate from which the coat protein gene was derived, two PVY^O isolates, and two PVY^NTN isolates. Two of the fully resistant clones contained only one copy of the transgene, demonstrating that it is possible by genetic engineering to obtain highly virus resistant potato clones that can also be useful in future breeding programmes.     

Biochemical and morphological characterization of potato clones differing in their resistance to late blight

Summary Morphological and biochemical parameters that could be involved in resistance to late blight were studied in non-infected and in infected potato hybrids resulting from a cross betweenSolanum phureja, resistant toPhytophthora infestans, and a susceptibleSolanum tuberosum. Some morphological differences between resistant and susceptible hybrids, indicating a positive correlation between stem diameter and phloem thickness in the stem and resistance toP. infestans, were observed. The lignin content in the leaves of the resistant hybrid rose upon infection byP. infestans. In the leaves of the susceptible hybrid, a diminution of the lignin content could be observed upon infection byP. infestans. In the same context, peroxidasic activity raised upon infection byP. infestans in both resistant and susceptible hybrids. Further characterization of the hybrid clones based on the polymorphism of peroxidases was attempted using isoelectric focusing.     

The susceptibility of tissue cell walls toErwinia enzymes differs among the potato cultivars

Summary In the cell wall matrix of higher plants, cellulose fibrils are embedded in a network of pectic substances, hemicelluloses and structural proteins. But plant pathogenicErwinia carotovora bacteria secrete a mixture of enzymes including pectinases, cellulases and proteases to degrade the cell walls of host plant tissue. Tuber tissue of 31 potato genotypes was exposed to a mixture ofErwinia enzymes before the degree of cell wall lysis was determined by a Neutral-red vital staining method. The data indicate that the resistance of cell walls to the action ofErwinia enzymes differed significantly among the potato cultivars and clones. The decrease in cell viability caused by the action of enzymes ranged from 7% for cv. Maxilla to >46% for cv. Arnika. An increased rate of N-fertilization coincided with enhanced susceptibility of cell walls to the action ofErwinia enzymes. There was a correlation between the amount of cell wall substances in tuber tissue and the resistance of cell walls to enzymic degradation.     

Effect of functional female and male fertilities on crossability in diploid potato breeding

Summary Functional female fertility (FFF) and functional male fertility (FMF) were studied in diploid potato clones, includingS. tuberosum haploids, F₁ hybrids between haploids and otherSolanum species, and in progenies from subsequent hybridisations. The results of 402 crosses between 91 diploid potato clones revealed that 60% of all genotypes had low levels of FFF as well as low levels of FMF regardless of their origin and degree of hybridity. Failed crosses (without any fruits at all) and those with a low level of success (with less than 3 seeds per pollinated flower) constituted 69% of all studied crosses. Most of these had resulted from parents with low levels of FFF and FMF, and these two characteristics were expressed independently. Functional female fertility affected crossability behaviour no less than FMF, therefore both FFF and FMF should be controlled in diploid potato clones involved in breeding. Reliable FFF and FMF estimates can be based only on crossing results. The coefficient of multiple correlation between seeds per pollinated flower in crosses and FFF and FMF estimates of the parental clones was 0.80 (P<0.01). Hence the compilation and analysis of the results of diploid potato crosses allow a reliable assessment of FFF and FMF and consequently the prediction of crossability with a degree of sufficient accuracy.     

Resistance to potato cyst nematodesGlobodera rostochiensis andG. pallida in the British Neo-Tuberosum potato population

Summary Surveys of Neo-Tuberosum clones have revealed continuous ranges of reaction to bothGlobodera rostochiensis andG. pallida with a few clones being highly resistant. Reactions of clones to the two species show some correlation but correlations between reactions to differentG. pallida populations are closer. Tests on progenies withG. pallida show the resistances to be heritable. Neo-Tuberosum populations have also been developed elsewhere.     

Late blight resistance in the British Neo-Tuberosum potato population

Summary Development of the British Neo-Tuberosum population included selection for resistance to late blight. High levels of foliage resistance are most often found in later-maturing clones but some early-maturing clones have good resistance. Resistance of foliage to Race 4 and to a complex race is associated. Some clones give variable results in tests for resistance in tubers. While those consistently displaying resistance are most frequent among those with resistant foliage, some have rather susceptible foliage. Tuber resistance appears most common among early maturing clones but this could be misleading, perhaps being due to tubers of later-maturing clones tending to be immature when tested (resistance often increasing as tubers mature). However some ‘lates’ are resistant, suggesting that genes for resistance in immature tubers occur. Selections are being used experimentally as parents in breeding for blight-resistant cultivars, and controlled breeding within Neo-Tuberosum to produce superior parents has commenced.     

Stability of late blight resistance in population B potato clones

Summary Increased virulence associated with fungicide resistance and variablePhytophthora infestans populations has been recorded in many potato growing regions with enormous economic effects. The current emphasis on disease management in East Africa includes the use of potato varieties with durable resistance to late blight. Seven promising clones from Population B with quantitative resistance (no R-genes), two advanced clones from Population A (with Rgenes) and three control varieties were grown for three cropping seasons in order to determine their reaction and stability of late blight resistance. Late blight occurrence was detected in all cropping seasons. The analysis of variance of disease data (AUDPC) for genotypes, locations x seasons was highly significant (P<0.001) indicating the differential response of the geno-types and the need for stability analysis. The Additive Main Effects and Multiplicative Interaction (AMMI) statistical model, showed that the most stable genotypes were 392127.256, 381471.18, 387121.4 (resistant) and 391049.255 (susceptible to late blight). Within environments, the ranking of genotypes was not consistent. The variety Kabale was ranked as the most susceptible genotype while clone 381471.18 and Rutuku were ranked the most resistant. Selective deployment of resistant and stable varieties is critical in minimizing economic loss and damage attributed to late blight in low input farming systems.     

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.