The effect of irrigation and nitrogen on powdery scab and yield of potatoes

Summary Powdery scab, caused by the fungusSpongospora subterranea, has developed in the Cappodocia region of Turkey because of changes in agronomic practices, such as excessive irrigation and nitrogen fertilizer use and growth of highly susceptible cultivars. The most suitable irrigation and nitrogen application levels were established to maintain powdery scab at minimum levels without affecting potato yield. Three irrigation levels were used in 1998 (673, 897, 1121 mm) and five in 1999 (329, 494, 658, 823, 987 mm). Nitrogen levels were applied 150, 300, 450, 600 kg N ha⁻¹. In 1998, least powdery scab occurred with 673 mm irrigation, which yielded 41 t ha⁻¹ potatoes. The lowest disease severity occurred in 1999 with 494 mm irrigation and 150 kg N ha⁻¹. At these levels, tuber yield was 34 t ha⁻¹ which was not significantly different from the highest yield measured. In 1999 and at two of the three irrigation levels inl998, nitrogen increased disease severity.     

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.     

Assessing the resistance of potatoes to powdery scab (Spongospora subterranea (Wallr.) Lagerh.)

Summary The reaction of nine cultivars to infection bySpongospora subterranea (powdery scab) was studied in two successive years of trials in naturally infested field soil and in artificially-inoculated field soil contained in brick-sided beds. The incidence of infection was recorded on a 1–9 scale, with complete absence of the disease scoring 9. The correlation coefficient (r) between scores from bed and field was 0.84 (1985) and 0.81 (1986). The field test gave satisfactory discrimination between resistant and susceptible cultivars in only one year, but the bed test gave good results in both years and is suggested as a small-scale alternative to field trials for this disease.     

Resistance to powdery scab of seedling progenies ofSolanum tuberosum

Summary Tubers from individual seedlings of nineteen progenies involving eleven parental cultivars were tested for resistance to powdery scab over 2 years in artificially infested soil beds. There was a significant correlation between the mean resistance of a progeny and the phenotypic resistance of its parents.     

Some properties of soils affecting russet scab and silver scurf of potatoes

The relationship of soil ammonia, nitrates, pH, total bacteria,Streptomyces sp., temperature and moisture on the incidence of russet scab and silver scurf of potatoes was studied. The soil properties were changed by the addition of various levels of alfalfa, wheat straw and green rye residues. Within the limits of the changes caused by these residues, the most consistent relationship was found between the nitrate level in the soil and russet scab, and to a lesser extent, silver scurf. In both instances there was a general decrease in russet scab and silver scurf with increasing levels of nitrates. Also, in the only comparison made, an increasing total bacterial count in the soil resulted in a decreasing level of infection from silver scurf. In some instances the alfalfa, rye and straw residue applications caused small changes in soil pH, soil temperature, and soil moisture. These changes were not great enough to show any consistent significant relationship to russet scab or silver scurf, except for a significant negative correlation between soil moisture at the 6-inch level and silver scurf in both years.     

Potato powdery scab, caused bySpongospora subterranea f. sp.subterranea, in Costa Rica

Summary Seven samples of potato powdery scab like-lesions were used to determine and confirm the occurrence ofSpongospora subterranea f.sp.subterranea in Costa Rica. Light microscope observations of spore balls and zoosporangia in tomato bait plants identified the organism asS. subterranea. Positive DAS-ELISA test results from all seven samples and PCR specific amplification products from five samples confirmed its identity. The geographical location of the samples source plantations suggests wide distribution of the plant pathogen across Costa Rica main potato growing areas. Different lesion types may indicate host-pathogen or environment-pathogen interactions. The symptoms differences and PCR amplification results among specific primer pairs suggest that some degree of genetic variation among theS. subterranea samples may exist; however, further testing is required.     

Effects of the nematicide 1,3-dichloropropene on weed populations and stem canker disease severity in potatoes

The soil fumigant 1,3-dichloropropene (1,3-D) has been used in the UK for the control of potato cyst nematodes (PCN), Globodera pallida (Stone) and Globodera rostochiensis (Wollenweber), but its potential herbicidal activity has not been extensively investigated in this country. Field and glasshouse studies were therefore conducted to evaluate the potential of 1,3-D for the control of weeds in potatoes, and observations were made on the severity of potato stem canker, caused by the fungus Rhizoctonia solani Kühn [teleomorph: Thanatephorus cucumeris (Frank) Donk]. Autumn application of 1,3-D at 211.5 L active substance (a.s.) ha⁻¹ significantly suppressed the number of germinating weeds and the percentage of weed ground cover by 83% and 79%, respectively, relative to controls. There were also species-specific significant decreases (field pansy, Viola arvensis, in particular) in the number of weed seeds germinating in field soil in the glasshouse post-1,3-D treatment. The effect of 1,3-D declined in time and single (autumn or spring) or combined application produced a slight, but not significant, reduction in the number of weeds germinated on potato ridges relative to those recorded in untreated soil. The severity of stem canker on potato plants was not significantly reduced by 1,3-D but both mean number and weight of stems per plant were significantly increased compared with plants from untreated plots. These studies demonstrated that 1,3-D, in addition to giving PCN control, has efficacy against weeds; implications are the potential for reduced herbicide input in the crop rotation with accompanying economic and environmental benefits.     

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.     

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.     

Incidence of potato virus Y and potato leaf roll virus in autumn potatoes in Israel

Summary Random sampling of autumn grown potatoes in Israel revealed potato virus Y incidences of 2.7, 2.5 and 3% during the years 1976, 1977 and 1978, respectively, figures only slightly higher than those found in spring-planted fields (2.3% in 1976), and which agree with visual estimates by the Inspection Service. Assaying random samples for potato leaf roll virus (PLRV) by aphid transmission to test plants, detected incidences as high as 38.8, 28.9, 36.7 and 33.3% during 1976, 1977, 1978 and 1979, respectively, although only minor levels were visable, indicating symptom masking in the autumn. Most of the infection was found to be secondary; ca. 25% of the local seeds for autumn planting, grown during spring, are PLRV-infected. The effect of the high incidences of PLRV on autumn yields is not known, but they are consistently lower than spring yields of the same varieties. Contribution No 315-E, 1980 Series, The Volcani Center, Bet Dagan, Israel.     

The relative importance of host and environment in determining the incidence and severity of lesions of common scab (Streptomyces scabies) on potato

Summary The relationship between the area of tuber surface covered with lesions of common scab and the severity of the scab type was examined by calculating regressions of cover on type for eight trials at two sites over two seasons. All the regression coefficients were highly significant and did not differ significantly from each other. This suggests that resistance to penetration and to colonization of tubers by the parasite are closely related. Differences in the position of regression lines were probably due to local environmental conditions. The interaction of the potato genotype with the environment can result in relatively few severe scabs or many superficial scabs.

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Zusammenfassung Die Knollen wurden auf ihre Anf?lligkeit für Flachschorf bestimmt, indem der Bedeckungsgrad gesch?tzt und der schlimmste Schorftyp auf den einzelnen Knollen notiert wurden. Der Bedeckungsgrad wurde anhand einer Skala bestimmt, wobei 0=kein Schorf, 1=bis zu 10% der Oberfl?che von Schorf befallen und jede weitere Einheit eine Zunahme von 10% bedeuten. Der Schorftyp wurde anhand einer Skala von 0–5 bonitiert: 0 = keine Schorfarten, 1 = wenig, oberfl?chliche L?sionen, 2 = gross, oberfl?chliche L?sionen, 3 = gross, rauhe L?sionen, 4 = grosse L?sionen, entweder aufgeworfen oder flachnarbig, 5 = grosse L?sionen, tiefnarbig. Die Mittelwerte für die Parzellen wurden errechnet. Das Verh?ltnis zwischen dem mit Flachschort befallenen Umfang der Knollenoberfl?che und der Schwere des Schorftypes wurde untersucht, indem für acht Versuche an zwei Orten über zwei Vegetationsperioden (Abb. 1) Regressionen für den Umfang pro Typ errechnet und die Homogenit?t der Regressionen durch Varianzanalysen (Tabelle 1) verglichen wurden. Alle Regressionskoeffizienten waren hoch signifikant und wichen nicht signifikant voneinander ab. Die Tatsache, dass der Schorfbefall sich im gleichen Mass wie der Schorftyp ver?ndert, weist darauf hin, dass — innerhalb der untersuchten Population von Genotypen — die Resistenz der Kartoffel gegen das Eindringen von sich entwickelnden Lentizellen und die Besiedlung von Knollengewebe eng zusammenh?ngen. Die Interaktion des Kartoffelgenotyps mit der Umwelt kann manchmal Knollen mit nur wenigen, tiefen L?sionen, wie im Versuch mit frühen Sorten in Trumpington 1975 (Abb. 1¹), oder mit zahlreichen oberfl?chlichen Schorfen, wie im Versuch mit mittelfrühen bis mittelsp?ten Sorten in Trumpington 1976 (Abb. 1⁷), bewirken. Es ist daher nicht angezeigt, nur aufgrund des Vorhandenseins von oberfl?chlichen oder normalen Schorfen eine Auslese auf Resistenz oder Anf?lligkeit zu treffen. Obwohl alle Regressionskoeffizienten hoch signifikant waren, gab es doch einzelne Abweichungen von den Regressionslinien. Ein Beispiel ist der in Abb. 1^5,6 mit × bezeichnete Klon, der eine kleinere Befallsfl?che aufwies als 1976 an beiden Orten zu erwarten gewesen w?re.

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Résumé On a étudié la sensibilité des tubercules à la gale commune par estimation de la surface atteinte et par notation du type de gale le plus grave sur chaque tubercule. On a établi une échelle de 1 à 10 en fonction du degré d'attaque, ou 0=pas de gale, 1=jusqu'à 10% de la surface atteinte. A chaque unité supplémentaire correspond une augmentation de 10%. Une échelle de 0 à 5 a été utilisée pour le type de gale; 0=pas de gale, 1=petites lésions superficielles, 2=grandes lésions superficielles, 3=grandes lésions rugueuses, 4=grandes lésions en relief ou en creux peu profonds, 5=grandes lésions en creux profonds. On a calculé des moyennes pour chaque parcelle. La relation entre la surface du tubercule couvert de lésions et la sévérité du type de gale a été examinée par calculs de régressions entre ces deux facteurs à partir de huit essais situés à deux endroits durant deux saisons (fig. 1) et par un test d'homogénéité des coefficients de régression par analyse de variance (tableau 1). Tous les coefficients de régression ont été hautement significatifs sans être différents de manière significative entre eux. Le fait que la surface des lésions change de manière uniforme à l'intérieur de la population des génotypes testés, conduit l'auteur à penser que la résistance des pommes de terre à la pénétration par les lenticules formées et la colonisation des tissue du tubercule sont intimement liées. L'interaction entre le génotype et l'environnement peut quelquefois produire des tubercules ayant seulement quelques lésions profondes, comme cela s'est produit dans l'essai récolté t?t de Trumpington en 1975 (fig. 1¹), où de nombreuses lésions superficielles, comme pour l'essai de Trumpington en 1976 (fig. 1⁷) récolté plus tardivement. Par conséquent, dans le cadre d'un programme de sélection, il est insuffisant de classer des plantes résistantes ou sensibles uniquement à partir de gales respectivement superficielles ou normales. Bien que tous les coefficients de régression soient hautement significatifs il y a des déviations individuelles à partir des courbes de régression. Un exemple est donné par le clone marqué d'une croix (×) dans la fig. 1^5,6 lequel avait une surface atteinte plus petite que ce que l'on pouvait expecter pour les deux endroits en 1976.

     

Improved diagnosis of powdery scab (Spongospora subterranea f.sp.subterranea) symptoms on potato tubers (Solanum tuberosum L.)

Summary Based on visual inspection, discrimination between common scab (Streptomyces spp.) and powdery scab (Spongospora subterranea) can be difficult. Inspections are performed on unwashed samples, incidentally supported by microscopic examination. During 1994–1996 surveys were performed in The Netherlands on tubers with symptoms resembling common scab. Under microscopic assessment nearly all samples showed the presence of structures resembling cystosori (sporeballs) ofS. subterranea. At that time confirmation using alternative techniques was not possible. In 2003 research was undertaken to clarify the situation with respect to scab on potato tubers in The Netherlands. One hundred and eighteen scab samples were extensively tested forS. subterranea. All samples were digitally photographed, microscopically examinated and tested with real-time PCR and DAS-ELISA. Use of these modern methods resulted in a clear picture of symptoms that can be attributed toS. subterranea. A lot of scab samples with structures resembling cystosori could not be confirmed as contaminated withS. subterranea.