Comparison of resistance evaluation in potato variety assessment

Summary Results of varietal assessment in France, Germany, Great Britain. The Netherlands and Poland have been compared for two characters: resistance to potato virus Y (PVY) and tuber resistance toPhytophthora infestans. In total 75 varieties were evaluated in more than one country. All evaluations were expressed or could be transformed into a 9-grade resistance scale. Evaluations did differ considerably due to two main reasons: countries differed in severity of evaluation (mean resistance level ascribed to varieties) and the relative resistance level ascribed to individual varieties depended on the country. Differences between countries in severity of the evaluation could exceed two grades in resistance to PVY and three grades in tuber resistance toP. infestans. In the relative resistance ascribed to individual varieties the differences between countries were larger, reaching up to 6 grades in evaluation of resistance to PVY and up to 8 grades in evaluation of tuber resistance toP. infestans. To achieve consistent variety assessment results from different countries, it will be necessary to use agreed standard varieties and more consistent evaluation methods. Professor Dr. K.M. Świeżyński passed away on 27th July 2000     

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.     

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.     

Testing the resistance of potato genotypes to tuber late blight

Summary The method hitherto used in Norway for testing for tuber resistance to late blight, inoculating newly harvested unwounded tubers, is described. Superficial wounding of the tubers prior to inoculation increased the level of infection. Tests on wounded and unwounded tubers ranged the cultivars in the same order, but the significance level was higher for wounded tubers. When the tubers were harvested 16 days before inoculation instead of the same day, unwounded tubers of susceptible genotypes were less infected, but using the mean for wounded and unwounded tubers, the distinction between susceptible and resistant genotypes was still clear. Tubers inoculated 7 days after harvest were most infected when wounded shortly before inoculation, but tubers wounded two days before inoculation were infected more than unwounded tubers, and the separation between susceptible and resistant genotypes was more distinct than for unwounded tubers.

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Zusammenfassung Die derzeit in Norwegen verwendete Methode zur Prüfung der Knollenresistenz gegen Braunf?ule wird beschrieben. Die Knollen werden vorsichtig geerntet und auf feuchten Filterpapier in Styropor-Boxen (‘Fischboxen’) gelegt. Am gleichen Tag werden die Knollen mit einer Zoosporen/Sporangien-Suspension vonPhytophthora infestans besprüht. Nach Inokulation werden die Boxen bei 15°C gelagert und dabei 20 Stunden lang zwecks Erhaltung hoher Feuchtigkeit gestapelt. Danach k?nnen die Knollen abtrocknen. In Untersuchungen erh?hte sich die Zahl der Infektionen nach Rollen der Knollen über 10 Nagelspitzen, 2 mm hoch und von einer h?lzernen Basis aufw?rts weisend (Tabellen 2 und 3). Bei verwundeten und nichtverwundeten Knollen waren die Rangfolgen der Sorten nicht signifikant unterschiedlich. Für die getesteten Sorten jedoch war die Unterscheidung zwischen resistent und anf?llig, wie derF-Test aus der Varianzanalyse zeigte, bei verwundeten Knollen besser (Tabelle 3). Das Vorkommen von Braunf?ule in unverwundeten Knollen anf?lliger Sorten wurde reduziert, wenn die Knollen 16 Stunden vor der Inokulation anstatt am gleichen Tag geerntet wurden (Tabellen 1 und 2). Bei Verwendung des Durchschnitts bei verwundeten und nichtverwundeten Knollen war die Unterscheidung resistenter Sorten von anf?lligen Sorten noch klar m?glich (Tabelle 1). 7 Tage nach der Ernte inokulierte Knollen wurden am st?rksten infiziert, wenn sie kurz vor Inokulation verwundet wurden (Tabelle 4). Zwei Tage vor Inokulation verwundete Knollen zeigten zudem noch st?rkere Infektion als unverwundete Knollen. Für die in dieser Studie verwendeten Knollen war die Unterscheidung der Sorten etwas besser bei einem Tag vor Inokulation verwundete Knollen m?glich als bei Knollen, die am gleichen Tag oder zwei Tage vor Inokulation verwundet worden waren. Nichtverwundete Knollen von. Pflanzen aus 3 verschiedenen Pflanzterminen waren nahezu gleich anf?llig (Tabelle 5). Es liess sich kein signifikanter Effekt der Knollenreife auf die Resistenz feststellen.

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Résumé La méthode utilisée jusqu'ici en Norvège pour apprécier la résistance des tubercules au mildiou est décrite. Les tubercules sont récoltés soigneusement et placés sur papier filtre dans des bacs à poisson en polystyrène. Le jour même, les tubercules sont soumis à la pulvérisation d'une suspension de zoospores et de sporanges dePhytophthora infestans. Après l'inoculation, les bo?tes sont placées à 15°C et empilées pendant 20 heures afin de conserver un niveau élevé d'humidité. Après ce temps, les tubercules peuvent sécher. Lors d'expériences de blessures superficielles des tubercules en les faisant rouler sur 10 pointes (de 2 mm de haut) de clous fixés sur un support de bois, on augmente le niveau de l'infection (tableaux 2 et 3). Le classement des cultivars n'est pas significativement différent entre les tubercules blessés et non blessés mais chez les cultivars éprouvés, la distinction entre les résistants et les sensibles est meilleure chez les tubercules blessés, comme le montre la valeur duF des analyses de variance (tableau 3). L'incidence de la maladie chez les tubercules non blessés de cultivars sensibles est réduite quand les tubercules sont récoltés 16 jours avant l'inoculation, comparée à une récolte réalisée le jour même (tableaux 1 et 2). En utilisant la moyenne des valeurs obtenues avec des tubercules blessés et non blessés, la distinction entre cultivars sensibles et résistants est encore claire (tableau 1). Les tubercules inoculés 7 jours après la récolte sont plus infectés quand ils sont blessés peu de temps avant l'inoculation (tableau 4). De plus, les tubercules blessés deux jours avant l'inoculation sont encore plus infectés que les tubercules non blessés. Avec les cultivars utilisés dans l'étude, la distinction entre cultivars est nettement meilleure chez les tubercules blessés un jour avant l'inoculation que chez ceux blessés le jour même ou deux jours avant. Les tubercules non blessés de récoltes issues de plantations différées toutes les 6 semaines possèdent quasiment la même sensibilité (tableau 5). Aucun effet de la maturité des tubercules sur la résistance ne peut être déterminé.

     

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.     

Field resistance to late blight of four indian potato cultivars

Summary The nature and extent of field resistance to late blight, and the effect of plant age and inoculum level on resistance components and host attributes, were studied on the Indian potato cultivars Kufri Chandramukhi, Kufri Jyoti, Kufri Badshah and Kufri Sherpa. Latent period, infection efficiency, colonization rate and sporulation were found to operate in cvs Kufri Jyoti, and Kufri Badshah. Based on component analysis cv. Kufri Jyoti was similar to or sometimes more resistant than Kufri Badshah. No correlation was observed between any of the host attributes and resistance to late blight, except that the erect canopy of Kufri Badshah was associated with its slow blighting rate. At high inoculum, colonization rate and total sporulation were increased but infection efficiency was decreased.     

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.     

Systemic induced resistance to late blight in potato by treatment with salicylic acid andPhytophthora cryptogea

Summary Tests for systemic induced resistance (SIR) to late blight were performed with different potato cultivars in field trials and in growth chamber experiments. The non-pathogenPhytophthora cryptogea (Pc) and salicylic acid (Sa) were used as inducer agents. In the field, plants were naturally infected byPhytophthora infestans while the indoor plants were inoculated. The degree of SIR obtained varied with the cultivar used and also with the type of inducer. For example in the field the susceptible cv. St. Cecilia had enhanced resistance with Pc whereas Sa caused increased susceptibility. Inbred resistant cvs acquired increased resistance after treatment with Sa. In a separate pilot experiment in a growth chamber, Sa at two different concentrations was injected into potato seed tubers before planting. The higher Sa concentration increased resistance in genetically resistant cultivars while the lower Sa concentration brought about increased susceptibility.     

The need for international research programmes on some potato pathogens

Summary Growers are being faced with a conflic between the effects of specialization on increasing pest and disease problems (particularly late blight, Colorado beetle, cyst nematodes and tuber moth), and public opinion that will force them to reduce the use of pesticides. For this disease and these pests international research programmes are needed to develop parental lines and cultivars with durable resistance that can be used world-wide. This account is based on ‘The potato in world perspective’, a paper read at the First World Potato Congress held on 8–10 July 1993 in Charlottetown, Prince Edward Island, Canada.     

A simple non-destructive technique for healthy and diseased leaf area assessment of potato plants

Summary A method for assessing the leaf area of potato plants was based on the number of leaves coinciding with imaginary vertical lines descending from a horizontal grid of points above the plant or crop canopy. Leaves seen through each vertically aligned pair of holes in a perforated double table, covering the midpoint of the exposed area, were counted and moved aside one by one, until the ground was seen. For both green and diseased surfaces the leaf area, or its horizontal component, was calculated as the product of sum of records and the grid cell size. Tested against a destructive method, the technique worked equally well for healthy and diseased leaf area of potato plants infected byPhytophthora infestans. The theoretic estimate of the standard error for a single measurement was derived as the geometric mean of the leaf area and the grid cell size. The method is also suitable forLAI of crop stands.     

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.     

Synergy of cymoxanil and mancozeb when used to control potato late blight

Summary Synergism between fungicides could be used to reduce the application dosage. Synergism between cymoxanil and mancozeb was studied using potted potato plants under field conditions. In three experiments the preventive efficacy of the fungicides and fungicide mixtures in the treatments were studied using a bio-assay. Potato plants were sprayed with the fungicides or fungicide mixtures. Leaves were picked from the potato plants with regular intervals after fungicide application. The efficacy of the fungicides was studied in a bioassay by inoculating the leaves withPhytophthora infestans zoosporangia. The Abbot method was used to show synergistic effects of the cymoxanil/mancozeb mixtures tested. In two of three experiments synergy of the fungicide mixtures was found.     

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.     

Testing the resistance of potato varieties to common scab

Summary Resistance to common scab was tested by growing potatoes in dry sand or soil in 3 l pots placed on irrigated subsoil. Inoculated coarse sand (0.5–0.005 Petri dish culture ofStreptomyces scabies per pot) proved more reliable than naturally infested soil. The incidence of scab on the seed tubers did not significantly affect the results. Tests in the greenhouse were most reliable, although those in open air also often gave satisfactory results. For screening purposes satisfactory results can be obtained by using only two or three pots per variety. Thirty-six tyrosinase positive isolates ofStreptomyces from potato and carrot from different parts of Norway were tested on four potato varieties and some of the isolates were tested on carrot. Large differences in aggressiveness were found, but no difference in virulence could be demonstrated, i.e. the same varieties were the most resistant to all isolates.

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Zusammenfassung Es wurden Versuche angestellt, um das Verfahren von Wiersema (1970) für den lokalen Gebrauch anzupassen oder zu verbessern. Er prüfte die Resistenz gegen den gew?hnlichen Schorf durch Anbau von Kartoffeln in natürlich verseuchtem, trockenem Boden in T?pfen auf bew?ssertem Untergrund. In unseren Versuchen brachte die Verwendung von natürlich verseuchtem Boden unterschiedliche Ergebnisse (Tabelle 1), haupts?chlich wegen der kapillaren Aufw?rtsbewegung des Wassers aus dem Untergrund. Die Verwendung von inokuliertem, grobem Sand erwies sich als zuverl?ssiger (Tabellen 2 und 4). Drei Wochen alte Kulturen vonStreptomyces scabies auf PDA in 9 cm-Petrischalen wurden in Wasser zerkleinert (100 ml pro Topf) und gründlich mit grobem Sand vermischt. Für grosse Quantit?ten bei Routinetests wurde ein Zementmischer gebraucht. Der Sand wurde dann in Plastikt?pfe (3 1 Inhalt) abgefüllt. W?hrend des Knollenwachstums konnten relativ trockene Bedingungen in den T?pfen aufrechterhalten werden. Vor der Bonitierung auf Schorfbefall konnte der Sand leicht von den Knollen gewaschen werden. Wenn agressive Isolate gebraucht wurden, war die Schorfentwicklung über die ganze Skala der getesteten Inokulumkonzentrationen, von 0,5 bis zu 0,005 Petrischalenkulturen pro Topf, stark (Tabelle 2). Inokulierter Sand konnte in den folgenden Jahren in Mischungen mit grossen Mengen von nicht inokuliertem Sand wiederholt verwendet werden (Tabelle 3). Sowohl die Schwere der L?son (l = Oberfl?chenschorf; 2 = mittlerer Tief- oder Buckelschorf; 3 = Tiefschorf) als auch die relative Bedeckung der Oberfl?che (0–9) wurden bestimmt; auf Basis dieser Ergebnisse wurde ein Schorfindex errechnet (0–100) (Tabelle 5). Jede Knolle, die gr?sser als 2,5 cm Durchmesser war, wurde einzeln bonitiert. Der Genauigkeitsverlust war jedoch klein, wenn für alle Kartoffeln in einem Topf nur eine Bewertung gegeben wurde (Tabelle 4). Unterschiedliche Schorfgrade auf den Saatknollen beeinflussten die. Ergebnisse nicht signifikant (Tabelle 6). Die Untersuchungen im Glashaus waren meistens zuverl?ssig, aber es konnten auch im freien Feld befriedigende Ergebnisse erzielt werden (Tabelle 8), wo das geeignetste Verfahren darin bestand, die T?pfe fast bis zu ihrem Rand in D?mme einzugraben (Tabelle 7). Die erhaltenen Ergenissee waren über Jahre übereinstimmend, und für gut bekannte Sorten waren sie in gutem Einklang mit früheren Ergebnissen aus Feldversuchen und praktischer Erfahrung. Befriedigende Ergebnisse für Auslesezwecke dürften durch Verwendung von nur zwei oder drei T?pfen pro Klon erzielt werden. Mit der beschriebenen Methode wurden 36 tyrosinasepositiveStreptomyces-Isolate von Kartoffeln und Karotten aus verschiedenen Teilen Norwegens an vier Kartoffelsorten getestet, neun davon auch an Karotten. Bei Anwendung der Terminologie von Van der Plank (1968) wurden grosse Unterschiede in der Agressivit?t festgestellt, aber es konnte kein Unterschied in der Virulenz bewiesen werden, d.h. gegenüber allen Isolaten bestand gleichbleibende, sortenbedingte Resistenz (Tabellen 9 und 10). Zwei der Isolate Karotten.verursachten die st?rkste Infektion auf Karotten. Sie geh?rten auch gegenüber Kartoffel zu den am meisten pathogenen Isolaten. Die Ergebnisse weisen darauf hin, dass die Isolate vor Anwendung im Ausleseverfahren auf Schorfresistenz auf ihre Agressivit?t untersucht werden sollten. Als Alternative k?nnte eine Mischung von verschiedenen Isolaten benützt werden.

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Résumé L’expérimentation a été faite dans le but d’améliorer ou d’adapter aux conditions locales la méthode de Wiersema (1970) qui consiste à tester la résistance à la gale commune par une culture de pommes de terre en sol sec contaminé naturellement, dans des pots placés sur un sous-sol irrigué. Dans nos essais, l’emploi de sol naturellement contaminé a donné des résultats irréguliers (tableau 1), d?s principalement à une remontée de l’eau du sous-sol par capillarité. L’utilisation de sable grossier contaminé artificiellement s’est a vérée plus viable (tableaux 2 et 4). Des cultures agées de 3 semaines deStreptomyces scabies sur milieu PDA en boites de pétri de 9 cm étaient broyées dans de l’eau et mélangées soigneusement au sable grossier (à raison de 100 ml par pot) en utilisant une bétonnière quand des quantités importantes étaient nécessaires pour les tests de routine. Le sable était ensuite mis en pots plastiques de 3 1. Durant la croissance du tubercule on pouvait maintenir dans les pots un milieu relativement sec. Avant d’effectuer les notations concernant la gale, on pouvait aisément laver les tubercules du sable qui y adhérait. Quand des souches agressives étaient utilisées, le développement de la gale était très important sur toute la gamme de concentration d’inoculum testée, de 0,5 à 0,005 contenu de boite de pétri par pot (tableau 2). Le sable contaminé pouvait être réutilisé plusieurs fois les années suivantes en mélange avec de grandes quantités de sable sain (tableau 3). Pour calculer un indice de gale (0–100), on a utilisé le produit de l’estimation de l’importance des lésions (1 = gale superficielle; 2=gale moyennement profonde ou en relief; 3=gale profonde) par l’estimation de la surface relative atteinte (0–9) (tableau 5). Tous les tubercules de plus de 25 mm ont été notés individuellement. Cependant, une notation globale de toutes les pommes de terre d’un pot n’entra?ne pas une grande perte de précisions (tableau 4). Différentes intensités de gale sur les tubercules de semence n’affectent pas significativement les résultats (tableau 6). Les essais en serre se sont avérés plus fiables mais on a pu obtenir des résultats satisfaisants en plein air (tableau 8), la meilleure méthode consistant alors à enterrer les pots dans des sillons presque jusqu’à leur bord supérieur (tableau 7). Les résultats obtenus sont logiques d’une année à l’autre et pour les variétés bien connues concordent avec les résultats antérieurs des essais en plein champ et de la pratique. Pour des essais de Screening, on peut obtenir des résultats satisfaisants avec seulement deux ou trois pots par variété. Par la méthode décrite, on a testé sur quatre variétés de pommes de terre 36 souches deStreptomyces de pommes de terre et de carotte ayant une réaction positive à la tyrosinase et provenant de différentes régions de Norvège. 9 d’entre elles ont été également estées sur carotte. En utilisant la terminologie de Van der Plank (1968), on a trouvé de grandes différences d’agressivité, mais on n’a pu mettre en évidence aucune différence de virulence, ce qui montre qu’il y a une résistance variétale uniforme à toutes les souches (tableau 9–10). Deux des souches de la carotte ont provoqué l’attaque la plus grave sur carotte et étaient également parmi les plus pathogènes pour la pomme de terre (tableau 10). Les résultats montrent que l’on doit d’abord déterminer l’agressivité des souches avant de les utiliser pour des Screenings de résistance à la gale. D’autre part, il faudrait utiliser un mélange de plusieurs souches.

     

The economic impact of potato late blight on US growers

Summary Potato growers have been able to control the fungus,Phytophthora infestans, that causes late blight with fungicides, but at an increasingly higher cost. A Delphi survey was conducted with thirteen experts to estimate the impact of late blight on potato yields, storage losses and fungicide use. It was estimated late-blight fungicides cost $77.1 million and lost revenue for US growers was an additional $210.7 million. These total costs, which average $507 per hectare, do not include non-fungicide control practices.     

Comparison of the NegFry decision suport system with routine fungicide application for the control of potato late blight in Ireland

Summary Field experiments between 1996 and 2000 compared the efficacy of fungicide programmes applied in accordance with the NegFry and Met éireann (Mé) decision support systems (DSS) for the control of late blight with 7- and 10-day routine fungicide programmes. The Mé DSS reduced fungicide use by 68% and 54% respectively while NegFry reduced fungicide use by 49% and 27% compared with the 7- and 10-day programmes. The NegFry DSS was similar to the 10-day routine programme in terms of late blight control, quality and marketable yield. A similar result was found when the NegFry DSS was compared with a 7-day routine mancozeb programme (included for two seasons only). Within the NegFry DSS the use of fluazinam resulted in improved yield, foliage blight and tuber blight control compared with mancozeb, but this benefit was significant for tuber blight only. The Mé DSS resulted in inferior disease control, yield and quality.     

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.     

Inheritance of resistance to late blight in foliage and tubers of progenies from parents differing in resistance

Summary TenSolanum tuberosum genotypes differing in resistance to late blight (Phytophthora infestans (Mont.) de Bary) in foliage and tubers were intercrossed to determine the inheritance of disease resistance in their progenies. Plots of 10–15 clones per progeny were established in each of 2 years and resistance assessed by field or laboratory tests. The parental genotypes were similarly tested each year. The parents differed in general combining ability (GCA) for both foliage blight (FB) and tuber blight (TB). The parental and GCA scores were significantly correlated for both aspects of the disease, but the correlations between foliage and tuber scores for parents and for GCAs were not significant. Three parental genotypes were highly resistant in both foliage and tubers, and the genotype with the highest GCA for resistance to both FB and TB (cv. stirling) is recommended as the best parent. There was no evidence of strong genetic correlation between both aspects of resistance, and it is suggested that both be selected for in a breeding programme.