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.     

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 irrigation strategies during tuber initiation on marketable yield and development of common scab disease of potato in Russet Burbank in Tasmania

Summary Management of soil moisture deficit during tuber initiation and early development was important for the optimisation of net and marketable tuber yields of cv. Russet Burbank on the typical red ferrosol cropping soils of Northern Tasmania, Australia. An effect of reduced water input during the tuber initiation period on increasing incidence of common scab disease was noted in one trial. In subsequent trials disease inoculum levels were not sufficient to show differences between treatments despite a history of disease at the site. For optimisation of marketable yield and probable assistance in the management of common scab disease it is recommended that irrigation scheduling to known soil moisture deficits be adopted during this period of growth on this soil type.     

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.

     

Evaluation in pot and field trials of resistance of potato cultivars and breeding lines to common scab caused byStreptomyces scabies

Summary The resistance to common scab of nine cultivars and eight advanced breeding lines of potatoes, fiveSolanum andigenum Juz. et Buk. clones, and three potato breeding lines selected for resistance to common scab, was investigated in a field and a pot trial. In the pot trial potaroes were planted in inoculated coarse river sand in plastic pots (150 mm in diameter) placed on irrigated subsoil in a glassfibre house. In a field trial potatoes were planted in an artificially infested field. The harvested tubers were scored individually for scab incidence according to two scales, viz. a 1–5 point scale for percentage surface covered with scab and a 1–3 point scale for lesion type. A scab index value was calculated for each tuber. A significant correlation coefficient of 0.82 was obtained for the results of the two tests.     

The relationship between common scab severity and reducing sugar contents in the peel of potato tubers

Summary Total sugar, reducing sugar, glucose, fructose, and sucrose of scabbed (Streptomyces scabies) and scab-free harvested tubers of several potato cultivars were analysed in four portions (pell and tube flesh separtely, from basal and apical halves), and also in small whole growing tubers. The peel of scabby harvested tubes had higher reducing sugar (glucose and fructose) contents than that of corresponding scab-free tubers. The sugar contents in the flesh and in small growing tubers did not show significant difference beween the scabbed and scab-free state. Tubers of scab-susceptible cultivars had more scab and a higher reducing sugar content than those of resistant cultivars. The results indicate that the contents of reducing sugar (glucose and fructose) in tuber peel are positively correlated with scab severity.     

A comparison of laboratory and field reactions of a range of potato cultivars to infection withSynchytrium endobioticum (Schilb.) Perc.

Summary A comparison of reactions of potato cultivars toS. endobioticum indicated that the degree of susceptibility observed in laboratory tests correlated well with the reaction of cultivars in field tests. Some cultivars found to be less resistant (Resistance Grade 2) in laboratory tests produced winter spores in the field test, and 10% of those derived from cv. Ausonia germinated in distilled water. Compost from the field plot was estimated to contain 14–18 apparently viable spores/g. and this resulted in 100% infection of the highly susceptible cv. Arran Chief. The implications of these findings for descheduling sites previously scheduled due to wart infestation are discussed.     

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.     

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.     

Decreased common scab incidence after foliar sprays of daminozide

Summary In four small trials in two fields infested withStreptomyces scabies, potato foliage (cv. Maris Piper) was sprayed with solutions of the growth retardant daminozide in May–June. The sprays roughly halved the amount of scab in one field but were less effective in the other. The decrease in scab was not influenced by changes in daminozide concentration (3–10 g litre), by varying the timing of single sprays (mainly in June) or by spraying more than once. Yields of tubers were unaffected.     

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

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

Formation of microsclerotia ofVerticillium dahliae Kleb. on various plant parts of two potato cultivars

Summary Potato plants of cvs Element and Mirka were artificially infected withV. dahliae in two greenhouse experiments. Leaf blade, petiole, aerial stem, subterranean stem, stolon and root mass were separately harvested both when the canopy was still green and at maturity. After 4 weeks incubation, the plant tissue was air-dried and the numbers of microsclerotia per mg tissue and per plant were determined. The highest numbers of microsclerotia were observed in the haulm when harvest took place at maturity. Cultivar Element yielded significantly more microsclerotia in the haulm than cv. Mirka, whilst there were no cultivar differences in the microsclerotial production on subterranean parts. The petiole and the aerial stem contributed most to the total microsclerotial production, whereas roots were much more important for formation of microsclerotia than stolons.     

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     

Field trials of foliar sprays of 3,5-dichlorophenoxyacetic acid (3,5-D) against common scab on potatoes

Summary Foliar sprays of 3,5-dichlorophenoxyacetic acid (3,5-D) in June or early July decreased the incidence of common scab by about 30% in four field trials on cv. Maris Piper and cv. Désirée; however, significant decreases in yield and in mean tuber weight were also common. The effects of 3,5-D on potatoes resembled those of the growth retardant daminozide in some ways, but 3,5-D was not persistent enough to have a useful effect on scab in the field.     

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.     

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.     

Influence of ambient forced-air ventilation rate on potato storage wastage

Summary Potatoes were stored in three insulated bins having ventilation rates of 0.0190, 0.0306 and 0.0417 m³/s·t. The results showed that in the climate of northern Italy potatoes can be stored successfully at ventilation rates of not less than 0.0306 m³/s·t resulting in product weight loss of less than 2.0% after 100 days' storage. The incidence of diseases and of sprouting was very low at rates of 0.0306 and 0.0417 m³/s·t, but greater at 0.019 m³/s·t because of the higher tuber temperature. Laboratory tests simulating ambient conditions confirmed the validity of a thermodynamic model simulating air-potato heat exchange.     

Progeny testing for resistance to potato virus Y: a comparison of susceptible potato cultivars for use in testcrosses with resistant parents

Summary For potato breeding or genetical research purposes, the number of copies of a dominant major gene for resistance to potato virus Y in a parent clone can be determined by test-crossing with a susceptible parent and observing the segregation ratio of resistant to susceptible seedlings in the progeny. In a comparison of different susceptible cultivars for use in test-crosses for this purpose, their progenies differed in the clarity of symptoms and parental phenotype proved an unreliable guide. Cvs Maris Piper, Arran Peak and Dr MacIntosh were found to be suitable tester parents, Pentland Squire less so and cv. Désirée was confirmed as having a major gene conferring incomplete resistance. It was found advisable to test-cross with more than one susceptible parent.