Screening potato cultivars for common scab of potato in a naturally infested field

Summary Twenty seven cultivars of potato were screened for common scab grown in a commercial field in two successive years (1996–97 and 1997–98). Eight cultivars were least susceptible and the others ranged from medium susceptible to very highly susceptible. None of the cultivars was resistant. Most showed a stable resistance reaction in both years.     

Accumulation of mineral elements in tuber periderm of potato cultivars differing in susceptibility to common scab

Summary Concentrations of Ca, P, K, Mg, Al, Fe, Mn, Cu and Zn were determined in healthy tuber peelings of cultivars less (Karin, Santé and Symfonia) and more (Agria, Désirée and Tomensa) susceptible to common scab when grown at two sites that differed in the level of scab incidence. The accumulation of some elements was significantly influenced by site, year, cultivar, maturity and the age of tuber periderm. At both sites, Ca and P in periderm tissue declined but Mg increased during the growing season. The Ca/P ratios in tuber periderm of all cultivars greatly decreased 83 days after planting. Concentrations of mineral elements measured at harvest may not reflect conditions present during the infection period, and consequently may not be related to scab incidence or severity.     

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.     

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.     

Evaluation of dihaploid populations from potato varieties and breeding lines

Summary Dihaploid populations from 20 varieties and 11 breeding lines ofSolanum tuberosum differed greatly in the occurrence and frequency of mutant phenotypes, in tuberization, flowering, pollen stainability, 2n-pollen production and resistance toGlobodera rostochiensis pathotype Ro-1. One population contained up to 61.7% lethal mutants: 39.1% of the total of 5377 dihaploids obtained did not tuberize, and 32.4% of 825 vigorous dihaploids did not flower. Of 825 vigorous dihaploids, 26.9 and 3.9% respectively had a pollen stainability higher than 20 and 60%. Among 222 dihaploids with a pollen stainability higher than 20%. 23.0% produced some 2n-pollen and none of these had a stainability higher than 60%. Among the dihaploid populations investigated the proportion of resistant to susceptible dihaploids ranged from 8:26 to 62:8.     

Inheritance of resistance toFusarium sulphureum in crosses betweenS. tuberosum potato cultivars measured on field and glasshouse grown tubers

Summary Solanum tuberosum potato cultivars with differing levels of resistance to dry rot caused byFusarium sulphureum were hybridised in a half diallel crossing programme. Glasshouse grown tubers from true seedlings and field-grown tubers of the resulting progenies were assessed for their resistance toF. sulphureum. Statistical analysis revealed differences between the progenies forF. sulphureum resistance on both field and glasshouse grown tubers. On the field grown tubers, differences between progenies were due to differences in the general combining ability (GCA) of the parents. Parental phenotypes and GCA values were highly correlated. Overall. GCA was consistent for resistance or susceptibility, and glasshouse grown progenies showed a good correlation for GCA between years (r=0.85^***), and with the results of the field grown progenies (r=0.61^* in 1998 and r=0.69^** in 1999). It appears that resistance toF. sulphureum is heritable, and that this heritability is better assessed on field grown rather than glasshouse grown tubers, although the two are largely in agreement.     

Ring rot symptom development on potato cultivars and lines in southern Alberta

Summary Plants were grown in the field from seed pieces of potato cultivars injected with ring rot bacteria (Corynebacterium sepedonicum). Symptoms were produced with both 18 and 180 colony forming units (cfu) on cvs Norchip and Red Pontiac and with 180 cfu on cv. Belrus plants. Tuber symptoms were detected in all cultivars except Belrus and Teton. Tuber progeny produced plants that developed top symptoms on all cultivars except those of cv. Nooksack, Russet Burbank, and Teton. Tuber progeny of the second crop produced both plants and tubers with symptoms developing only in the cv. Nooksack. In another 3-year experiment, variability in the disease response of selected cultivars and lines was examined following knife-inoculation of tuber seed with a high level of ring rot bacteria. Significant correlations between top and tuber symptoms were detected, but they were not high enough to make unnecessary the examination of both top and tuber symptoms in ring rot disease selection studies. Contribution no. 3879111     

Relative resistance of the potato cultivar krantz to common scab caused byStreptomyces scabies as determined by cluster analysis

Twenty potato breeding selections and five potato cultivars (25 clones) were evaluated in replicated field plots for their resistance to common scab caused byStreptomyces scabies, at Presque Isle, ME and Cranesville, WV in 1993. At harvest, all tubers in each plot were individually scored for the surface area covered (0=0% to 5>75%) and for lesion type (0=no lesions to 5=pitted lesions). The individual tuber scores for either surface area covered or lesion type for each plot were totaled and divided by five times the number of tubers to create an index of surface area covered (SAI) or lesion type (LI), respectively. Clonal mean SAI ranged from 0.05 to 0.96 in Maine and 0.21 to 0.89 in West Virginia. Clonal mean LI ranged from 0.06 to 1.0 in Maine and 0.48 to 1.0 in West Virginia. There were significant differences among clones for SAI and LI. Clones were clustered on mean SAI and LI in ME and WV. The clones clustered into four groups. The resistance of Krantz and two breeding selections, B0348-2 and B0339-1, was similar to Ontario.     

The development of mature plant resistance in four potato cultivars against aphid-inoculated potato virus YO and YN in four potato cultivars

Summary Field-grown potato plants of cvs King Edward, Record, Maris Piper and Désirée were inoculated on seven different dates during the growing season of 1987 and 1988 with either potato virus Y^O (PVY^O) or PVY^N, using three viruliferous peach-potato aphids (Myzus persicae) per plant. In each cultivar, the proportion of progeny tubers infected with PVY^O or PVY^N was high in plants inoculated during the four weeks following emergence, the proportion declining to zero or close to zero in the subsequent 4–6 wks.     

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.

     

Reduced fungicide inputs to controlPhytophthora infestans in potato cultivars with high levels of polygenic resistance

Summary Final disease ratings, apparent infection rates and areas under disease progress curves were used to calculate lower fungicide rates for potato cultivars with high levels of polygenic resistance toPhytophthora infestans. Areas under disease progress curves offered more reliable indications than final disease ratings or apparent infection rates. Control of epidemic development was achieved using 20–80% of the recommended application rates of mancozeb on plots of cvs Brodick. Cara and Torridon compared with full application rates on cv. Maris Piper.     

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.     

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.     

Assessment of a glasshouse test for measuring the resistance of potato cultivars to common scab

Summary Resistance to common scab was assessed on 19 cultivars in a glasshouse in pots containing naturally-infested sandy soil, diluted or not with river sand. Disease assessments by directly scoring the produce of the pots on a 1–9 scale of increasing resistance was as satisfactory as calculating the percentage of infected tubers after estimating the amount of common scab on each tuber. Tubers from plants growing in sandy soil diluted to 50%, as described, gave a wider variation in scab score between cultivars and a reduced error variance compared with undiluted sand. The data from the 50% mixture also correlated more closely with scab resistance scores provided by other sources. It is calculated that the use of 12 replicate plants would permit the detection of a difference in mean resistance of 1.3 points on a 9-point scale.

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Zusammenfassung Die Resistenz gegenüber Kartoffelschorf (Streptomyces scabies) wurde in mehr als 2 Jahren im Gew?chshaus nach der Methode von Bjor & Roer (1980) bestimmt. Dafür wurden die Saatknollen von 9 (1982) bzw. 19 (1983) Sorten in 10 cm-T?pfe mit natürlich befallenem sandigen Boden gepflanzt, der entweder mit gleichem Volumen Flussand versetzt war oder ungemischt blieb. Die T?pfe wurden in Beete mit Pflanzerde gesetzt, die durch Tr?pfchenbew?sserung versorgt wurden, so dass der Boden in den T?pfen trocken blieb. Die Wurzeln der Pflanzen wuchsen durch den Topfboden in die darunter liegende Erde. 1982 wurden 12 Wiederholungen und 1983 6 Wiederholungen pro Sorte für jedes der beiden Beete mit den entsprechenden Sandtypen angesetzt. Die Bonitur des Krankheitsbildes erfolgte bei der Ernte sowohl durch direkte Einstufung der Knollen eines jeden Topfes in eine 1–9 Skala (mit zunehmender Resistenz) als auch an Hand der prozentualen Anteile infizierter Knollen (definiert als Befall auf mehr als 1/32 der Oberfl?che). 1983 ergaben sich gesicherte Unterschiede bei der direkten Einstufungsmethode zwischen den Beeten mit dem selben Sandtyp, der gr?sste Teil der Unterschiede war jedoch zwischen den Beeten mit unterschiedlichen Sandtypen zu beobachten (Tab. 1, a). Wechselwirkungen zwischen Beeten und Sorten wurden nicht gefunden, was anzeigt, dass sich die Sorten in beiden B?den gleichartig verhielten. Die Einstufung der infizierten Knollen (die Werte wurden einer Winkeltransformation unterzogen, Tab. 1, b) ergab das gleiche Bild, obgleich die Wiederholungen zusammengefasst worden waren und nur die Einzelwerte der Beete und der Sorten verglichen werden konnten. Die in beiden Jahren jeweils getrennt vorgenommenen Untersuchungen der 2 Sandtypen (Tab. 2) weisen darauf hin, dass bei beiden Einstufungsmethoden die 50%ige Mischung im Vergleich zum ungemischten Sand eine gr?ssere Variabilit?t bei der Einstufung von resistenten und anf?lligen Sorten, aber einen geringeren Zufallsfehler ergab. Die Resistenzeinstufung mittels der Sandmischung korrellierte auch besser mit den Sch?tzwerten aus einem Feldversuch mit gleichem Sandboden, dem das Inokulum aus dem Gew?chshaustest zugesetzt worden war und ausserdem mit den Werten aus den amtlichen Quellen (Tab. 3). Zur Bestimmung der Schorfresistenz von Züchterklonen wird die Verwendung gemischter befallener Felderde und Anwendung der einfachen direkten Einstufung als ein brauchbarer Test empfohlen. Die Tab. 4 zeigt die notwendige Anzahl von Wiederholungen (d.h. T?pfe), um einen belegbaren Unterschied in der Anf?lligkeit aus der 1–9 Skala zu begründen; z.B. 12 Wiederholungen würden eine Differenz von 1, 3 Punkten in der Skala ergeben.

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Résumé La résistance à la gale commune (Streptomyces scabies) est évaluée pendant 2 ans en serre par une méthode similaire à celle de Bjor et Roer (1980). Des tubercules de semence de 9 (1982) et 19 (1983) variétés sont cultivés dans des pots de 10 cm, contenant un sol sableux naturellement contaminé, atténué ou non par un volume équivalent de sable de rivière. Les pots sont placés sur des billons de terre végétale avec une irrigation par goutte à goutte de fa?on à garder un sol sec dans les pots. Les racines traversent le fond du pot et se développent dans le billon. Douze répétitions par variété en 1982 et six en 1983 ont été retenues pour chacune des conditions composées par les deux types de sable. Les évaluations de la maladie sont données à la récolte par une notation directe pour chaque pot, selon une échelle de 1 à 9 de résistance croissante, ainsi que par un calcul de pourcentage de tubercules contaminés (définis comme ayant plus de 1/32 de leur surface contaminée) à partir de l'ensemble des répétitions par condition, pour chacune des variétés. En 1983, la méthode par notation directe a montré des différences significatives entre les billons d'un même type de sable (tab. 1, a), ce facteur (c.a.d. le niveau d'inoculum) expliquant la plus large part de la variation entre les billons. L'interaction entre billons et variétés n'est pas significative, ce qui indique que les variétés sont classées de fa?on similaire pour chaque billon. Le comptage du nombre de tubercules contaminés (valeurs indiquées dans le tabl. 1, b après conversion angulaire) a donné une image similaire, bien que seuls les billons et les variétés ont pu être examinés, les répétitions ayant été regroupées. En analysant les deux types de sable séparément au cours des deux dernières années (tabl. 2), il appara?t qu'avec les deux méthodes de notation, le mélange à 50% donne un plus grand écart entre les variétés sensibles et résistantes, l'écart type étant également plus faible, comparé à celui du sable non atténué. Les valeurs de résistance obtenues par le mélange sont également mieux correlées aux évaluations faites en essai de plein champ dans le même sol sableux ayant fourni l'inoculum pour le test en serre, ainsi qu'aux valeurs données par des sources officielles (tabl. 3). L'utilisation d'un sol contaminé atténué et d'une méthode directe de notation peut donc être suggérée comme test adapté aux mesures de résistance à la gale commune pour les cl?nes de sélection. Le tabl. 4 montre le nombre de répétitions (c.a.d. de pots), nécessaires pour détecter une différence sur l'échelle de 1 à 9 au niveau des seuils de sensibilité; par exemple 12 répétitions permettent de mettre en évidence une différence de 1,3 points sur l'échelle.

     

The effect of netted scab (Streptomyces spp.) andVerticillium dahliae on growth and yield of potato

Summary The effect ofStreptomyces spp. (netted scab) on the growth of potato was investigated in three pot experiments, in two of which the effect ofVerticillium dahliae was also assessed. The netted scab organisms attacked all underground plant parts of susceptible potato cultivars early in the growing season; the roots were especially seriously attacked, markedly reducing tuber yield and number but prolonging the duration of the growing season. The nematicide oxamyl had little effect on the incidence of netted scab. Repeated growing of the susceptible cv. Bintje greatly increased soil contamination with the netted scab pathogens. V. dahliae reduced haulm growth before wilt symptoms were evident and it reduced tuber yield but not number. Oxamyl delayed infection byV. dahliae by controlling parasiticPratylenchus nematodes (mainlyP. thornei).     

In vitro evaluation of resistance of potato cultivars toPhytophthora infestans

Summary The resistance of ten potato cultivars (Agria, Ajax, Désirée, Liseta, Kennebec, Majestic. Monalisa, Prima, Spunta and Tonda di Berlino) toPhytophthora infestans was analyzed in vitro using 8 fungal strains. An assay based on electrolyte leakage was used for screening leaves and tuber tissues with fungal culture filtrates. With almost all cultivars the resistance of leaves did not correlate with the resistance of tubers. Cv. Ajax appeared the least susceptible in both leaf and tuber tests, while the cv. Prima was the most susceptible in tuber tests.     

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.     

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.     

Effect of planting date and inoculum level on incidence and severity of powdery scab on potato

Summary Powdery scab incidence and severity was examined on six potato cultivars planted at two different dates in a naturally infested field. There were two sub-plots; one where seed was exposed prior to planting to spores ofSpongospora subterranea from scabby peels, which were then added to the furrow at planting, and another where seed was exposed only to natural inoculum within the field. The later planting data reduced incidence and severity, but there were no differences between sub-plots. All cultivars became infected, but ‘Rosa’ was the most susceptible and ‘Katahdin’ the most resistant. Contribution No. 1720, Department of Plant Pathology, The Pennsylvania Agricultural Experiment Station. Authorized for publication September 22, 1988 as Journal Series Paper No. 8007.     

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