Effects of contamination byErwinia carotovora subsp.carotovora andE. carotovora subsp.atroseptica of potato seed tubers and of cultivar resistance on blanking or nonemergence and blackleg development in Valencia,Spain

Summary High grade potato seed tubers of three cultivars were inoculated in Scotland by vacuum infiltration in suspensions ofErwinia carotovora subsp.carotovora and subsp.atroseptica in different combinations, and transported to Valencia, Spain. They were cut into two to three pieces and planted in January 1984, 1985 and 1986 in fields furrow irrigated before and after emergence. High incidences of blanking and blackleg were associated with seed inoculated with high numbers ofE.c. atroseptica alone or together withE.c. carotovora and not with high or low numbers ofE.c. carotovora alone.E.c. atroseptica predominated at emergence in March in rotting seed pieces which failed to produce a plant, regardless of the seed treatment, and in diseased stems in April but it was replaced byE.c. carotovora in May. The levels of blanking and blackleg after inoculating 16 cultivars with both erwinias were not correlated between years nor were they significantly different within years among most of the cultivars.     

Transgenic potatoes expressing anErwinia pectate lyase gene — results of a 4-year field experiment

Summary Four potato lines of cv. Désirée that express the pectate lyase (PL) isoenzyme 3 ofErwinia carotovora subsp.atroseptica were examined in a 4-year field experiment with respect to plant development, tuber yield and resistance of tuber tissue toErwinia soft rot. The PL3 degrades plant cell wall pectin into unsaturated oligogalacturonates eliciting plant defence responses. In one line, enzyme expression was controlled by the CaMV 35S promoter (C) and in three lines it was driven by the potato patatin B33 promoter (D). Plant development of the D-lines in field plots was not distinguishable from that of the non-transgenic counterpart. Also tuber yield was not too different. By contrast, plants of the C-line were smaller than those of the nontransformed counterpart and also showed reduced tuber yield. There were no significant differences in dry mass, starch and protein content of tuber tissue between PL transgenic and non-transgenic potatoes. But compared with the latter, field-grown tubers expressing the PL displayed an enhanced resistance toErwinia soft rot. Thus, average rotting caused byEc-bacteria was diminished in tubers of PL-transgenic lines by 34.1%. The resistance of tubers toEc soft rot was significantly correlated with the PPO activity in tuber tissue.     

Effect ofErwinia carotovora subsp.atroseptica on yield of potatoes from cut and uncut seed tubers in Valencia

Summary The effect on the development of blackleg symptoms and on yield of inoculating seed tubers of 15 cultivars withErwinia carotovora subsp.atroseptica (Eca) was investigated. Tubers were planted either whole or as pieces. Eca inoculation reduced the mean yield per plant by 8% for whole tubers and by 12% for cut tubers. There was no correlation between blackleg symptoms and yield loss.     

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

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

Resistance toErwinia carotovora subsp.atroseptica in second backcross populations of somatic hybrids betweenSolanum tuberosum andS. brevidens

Summary Eleven clones obtained from a cross between cv. Katahdin and fusion products betweenSolanum tuberosum and the non tuber-bearing speciesS. brevidens, were backcrossed again withS. tuberosum (clone AR80-127-5). Small tubers harvested from 583 seedlings of these second backcross (BC2) populations were screened for tuber tissue resistance toErwinia carotovora subsp.atroseptica under aerobic conditions. After multiplication in the field. BC2 clones again were screened for soft rot resistance under aerobic as well as anaerobic conditions. In general, the resistance of the BC2 populations was reduced in comparison with the BC1. Variation for resistance was found within the BC2 populations, but there was no correlation between the results of the three tests performed. Four BC2 populations were also screenned for resistance to blackleg in the field. Significant differences were found between populations for mean percentage of diseased plants, but these differences could not be explained by the resistance of the parental clones.     

Lightning damage to potatoes

Summary Two distinct round patches were observed in a potato field where the haulm showed symptoms like those of blackleg. The remainder of the field was in vigorous growth. Affected stems had long cracks andErwinia carotovora var.atroseptica was isolated from some. This had probably developed subsequently on damaged tissue and no rotting was apparent below soil level. Most of the tubers were decayed but noErwinia and only green pseudomonads were detected. Local meteorological records showed that thunderstorms occurred on only one day (1 August) in the six weeks before observation of the field symptoms. It was concluded from all evidence that the condition described was caused by a lightning strike on that day.     

Serogroups ofErwinia carotovora involved in systemic infection of potato plants and infestation of progeny tubers

Enrichment techniques and serological strain typing methods were used to determine the source of stem and progeny tuber populations ofErwinia carotovora (Ec) in 1980 and 1981. Plots were established in a field that had been planted with potatoes since 1972. Seed tubers of cultivars Russet Burbank and Norgold Russet were assayed forErwinia carotovora pv.carotovora (Ecc) andErwinia carotovora pv.atroseptica (Eca) by tuber peeling and direct plating and plating following enrichment in a pectate-based broth. Levels of seed tuber infestation ranged from undetectable to 2.4 × 10⁴ colony-forming units per g peel tissue. Prior to planting Ecc was detected in soil samples from field plots by enrichment in a pectate-based broth. Ec was isolated from symptomless plants with increasing frequencies from 8 weeks following planting regardless of the level of Ec populations in seed tubers. At harvest progeny tuber recontamination was low and not related to levels of seed tuber Ec populations. In 1980, serological typing of representative Ecc isolates indicated serogroups III, XXIX, and unknown strains isolated from the soil but not the seed tubers prior to planting were isolated from stems and progeny tubers. Serogroup XXIX was isolated from soil, seed, stems and progeny tubers in 1981. Throughout both years, Ecc was isolated more frequently than Eca from seed, soil, stems, and progeny tubers. These data emphasize the potential for reinfestation of seed stocks and infection of plants by populations of Ecc that overwinter in soil under Wisconsin conditions.     

Preliminary selection of antagonists ofErwinia carotovora subsp.atroseptica (Van Hall) Dye for application during green crop lifting of seed potato tubers

Summary Green crop lifting (GCL) for haulm killing was developed in The Netherlands and offers ideal conditions for controlling the blackleg pathogenErwinia carotovora subsp.atroseptica (Eca) by antagonists. Based on the use of mini-tubers or young tubers from field crops, two bioassays for wound protection were developed. GCL was simulated by artificially skinning or wounding tubers, inoculating the damaged skin with Eca, treating with antagonists and incubating in either potting compost or outside in field soil. Mainly fluorescent pseudomonads, pre-screened for in vitro antagonistic activity on agar and high soft rot reducing ability on tuber slices, were tested in the mini-tuber bioassay. Strains giving the highest degree of wound protection were further tested individually and in combination under field conditions in the young tuber bioassay. One individual strain and two combinations, resulting in reduction of contamination levels on skinned surfaces of 85% and between 60% and 70%, respectively, show good potential for biological control of blackleg.     

Effect of applying nitrogen fertilizer to a potato seed crop on the susceptibility of the daughter plants toErwinia carotovora subsp.atroseptica

Summary Seed potatoes of cvs Record and Bintje were grown with 30, 80 or 130 kg N/ha. In the subsequent growing season the seed tubers, which were inoculated or not with two concentrations ofErwinia carotovora subsp.atroseptica (Eca), were subjected to the same fertilization treatment. The incidence of blackleg and the level of seed tuber decay increased from the uninoculated treatment to the higher inoculum level, but the degree varied considerably from one year to another. Cv. Bintje was more susceptible to blackleg and yield loss caused by Eca than cv. Record. In 1990, when disease incidence was high, the proportion of blackleg stems increased with increasing doses of nitrogen fertilizer which had been applied to the seed crop, but was not significantly affected in 1988 and 1992. Seed tuber decay was retarded by the lowest nitrogen dose. The yield of the subsequent crop was not affected by the nitrogen dose applied to the seed crop.     

Effect of acetylsalicylic acid on soft rot produced byErwinia carotovora subsp.carotovora in potato tubers under greenhouse conditions

Summary The efficiency of acetylsalicylic acid (ASA) in inducing localised acquired resistance against infection byErwinia carotovora subsp.carotovora was evaluated by treating potato tubers with ASA at three concentrations. Three days after treatments, tubers were inoculated withE. carotovora subsp.carotovora by wound inoculation or by irrigation with a bacterial suspension. Experiments were performed for two consecutive years. Statistical analysis revealed that treatment of tubers by immersion in ASA solutions at low concentrations induced a significant reduction in the soft rot incidence. Wounding of the tubers was the most effective inoculation method and ASA at concentration of 0.0125% (w/v), pH 7 was more efficient than at 0.025 or 0.05%. No phytotoxicity of such treatment was observed.     

Factors affecting the accuracy of the tuber incubation test for the detection of contamination of potato stocks byErwinia carotovora

Summary Erwinia carotovora in potato stocks can be detected more efficiently by the tuber incubation method in which the test tubers are first induced to rot under anaerobic conditions (followed by detection isolation of the bacteria on a selective growth medium or by serological techniques) than by direct lenticel sampling. However, the efficiency of detection is affected by both the extent of decay at sampling time and the incubation temperature. Detection ofE. carotovora is poor from extensive lesions developed at temperature of 25 C or above. Furthermore, detection ofE. carotovora var.atroseptica is more frequent than that ofE. carotovora var.carotovora at temperatures below 22 C in tubers contaminated by both organisms, whereas at higher temperatures the reverse is true. Only within a narrow temperature range, close to 22 C is there an equal chance of detecting both varieties.     

Soft-rot resistance combined with other tuber characters in long day-adaptedSolanum phureja

Summary Offspring were produced from a cross between two long-day-adaptedSolanum phureja clones which carried resistance to tuber soft rot (Erwinia carotovora subsp.atroseptica). In tests carried out on the produce of field-grown plants raised from tubers, over fifty per cent of the 173 offspring were found to be highly resistant. Assessments were also carried out of tuber yield, mean tuber weight, tuber number, shape, regularity, flesh colour, texture of the steamed flesh, fry colour, after-cooking blackening, sprout length after storage and overall dormancy. There were statistically significant differences between clones for all characters (P<0.001). Twelve of the clones were selected on the basis of high resistance, yield, tuber weight, regularity of shape and absence of after-cooking blackening. The value of resistant long-day-adapted diploid material for commercial breeding is discussed.     

Storage of seed potatoes in pallet boxes. 1. The role of tuber surface moisture on the population of Erwinia bacteria

Summary Three stocks of seed potatoes contaminated with soft rot Erwinias were stored for 172 days in a commercial pallet box store. Three treatments were applied: naturally ventilated boxes, forced air ventilated boxes and single tubers on trays. Skin resistance values were 2.3, 9.4 and 19.3 megohm respectively. Rewetting events numbered 4, 7 and 15.Erwinia carotovora pv.carotovora (Ecc) andErwinia carotovora pv.atroseptica (Eca) declined in all stocks and treatments. Ecc declined significantly faster in the forced air ventilated treatment. The more frequent rewettings in the trays reverse the benefit of their drier skins. In the earliest sprouting stock, the naturally ventilated, but not the forced ventilated boxes, showed a decrease in skin resistance and corresponding increase in Ecc as spring approached. Forced ventilation in the first 14 days after harvest halved the level of silver scurf compared to the naturally ventilated treatment.     

Resistance to blackleg and tuber soft rot in sexual and somatic interspecific hybrids with different genetic background

Diploid interspecific hybrids from crosses betweenSolanum tuberosum haploids andS. canasense, S. multidissectum, andS. tarijense, along with tetraploid and hexaploid somatic hybrids S.tuberosum (+)S. commersonii, were screened for resistance to blackleg and tuber soft rot byErwinia carotovora subsp.atroseptica andE. carotovora subsp.carotovora. Among the diploid hybrids, those involving clone mlt la of S.multidissectum and tar 2b ofS. tarijense yielded the highest number of genotypes resistant to both blackleg and tuber soft rot. By contrast, all the hybrids involving clone tar 1lb ofS. tarijense were susceptible to both bacterial diseases. As far as resistance of somatic hybrids is concerned, the most interesting genotype was the tetraploid one which showed resistance to both blackleg and tuber soft rot Significant correlations were found betweenE. carotovora subsp.atroseptica andE. carotovora subsp.carotovora both in resistance to blackleg and in resistance to tuber soft rot, suggesting that resistance to the two subspecies is controlled by the same gene/s or that the relevant genes are linked. On the other hand, correlations between blackleg and tuber soft rot were never significant, indicating that different mechanisms may control resistance in tuber and stems. Alternatively, the resistance could be controlled by the same gene/s under a different spatial/temporal expression pattern. Twelve diploid sexual hybrids and one tetraploid somatic hybrid were selected for resistance to blackleg and/or tuber soft rot and for tuber characteristics.     

Resistance of tubers from different potato cultivars to soft rot caused byErwinia carotovora subsp.atroseptica

Mechanically harvested tubers of 14 potato cultivars grown on both loamy sand and silt loam soils were evaluated for resistance to bacterial soft rot caused byErwinia carotovora subsp.atroseptica. Cultivars were also assayed for calcium and dry matter content to determine possible correlations with soft rot resistance. Resistance of potato tubers to bacterial soft rot was assayed after harvest by inoculating tubers and determining percent surface area decayed after four days in a mist chamber. Tubers of the cultivars differed widely in resistance to bacterial soft rot. The ranking of the resistance of tubers from different cultivars to bacterial soft rot was similar for both soils, although disease severity in tubers from a particular soil type varied with the season. Of the cultivars tested, tubers of Butte, Russet Burbank, and Nor-gold Russet were ranked as the most resistant, whereas tubers of Redsen, Norchip, Norland and Belchip were rated as the most susceptible. Tubers of Rhinered, Red La Soda, Superior, Pontiac, Atlantic, and Nooksack were intermediate in resistance. Neither calcium concentration in medullar tissues nor dry matter of potato tubers alone could be correlated with ranking of the cultivars with respect to severity of bacterial soft rot. However, a multiple linear regression model incorporating both variables indicated that tubers with a high content of both calcium and dry matter were likely to be less susceptible to bacterial soft rot than were tubers with lower percentages of these variables.     

Hypoxic stress inhibits aerobic wound-induced resistance and activates hypoxic resistance to bacterial soft rot

Resistance to bacterial soft rot in potato tubers maintained aerobically was induced by wounding tubers 4 to 24 h prior to inoculation withErwinia carotovora subsp.carotovora. This wound-induced resistance phenomenon was blocked by hypoxic stress. An hypoxia-induced resistance mechanism also was detected: tubers acclimated to a hypoxic environment were resistant to rot when inoculated with aerobically grownE. c. subsp.carotovora and incubated in atmospheres made hypoxic by argon or nitrogen. With increased pretreatment, hypoxia-induced resistance approached the levels of resistance observed in tubers inoculated with aerobic-adapted bacteria and incubated aerobically. Hypoxia-adaptedE. c. subsp.carotovora overcame hypoxia-induced resistance.     

Effect ofErwinia carotovora subsp.atroseptica alone and withAlternaria solani orVerticillium dahliae on disease development and yield of potatoes in Israel

Summary Tubers of 17 cultivars were inoculated withErwinia carotovora subsp.atroseptica (ECA) and planted in 1986 and 1987 in the Negev in randomised plots in three blocks where eitherVerticillium dahliae orAlternaria solani were present or where both were absent. ECA reduced the yield in the control block and exacerbated the yield losses in the disease blocks, but the intensity of symptom expression did not always accord with the extent of yield loss. ECA had a greater effect on plants infected withA. solani than on those withV. dahliae. It was possible to categorise the cultivars as resistant, intermediate or susceptible with respect to ECA and early blight, but resistance toV. dahliae could not be differentiated from foliage maturity.     

Survival ofErwinia carotovora in Wisconsin soils

Erwinia carotovora var.atroseptica (van Hall) Dye andE. carotovora var.carotovora (Jones) Dye were detected in agricultural soils in Wisconsin using baiting and enrichment techniques. These soft rot bacteria could not be detected in soils of potato fields prior to planting of crops in the spring period using standard soil dilution plating techniques on a crystal violet pectate medium, however. A procedure involving incubation of samples in pectate enrichment broth followed by preparation of smears on slides and treatment with a fluorescent antibody stain specific forE. carotovora var.atroseptica was the most sensitive of the methods tested for detecting the blackleg pathogen. Erwinia carotovora was isolated more frequently during the spring from fields in which potatoes had been grown the previous year than from fields in which other crops had been grown. It was also isolated from potato tubers and stems that had overwintered in the field. The presence ofE. carotovora could not be detected in root zone samples of weed plants using the dilution plating method.     

The spread ofErwinia carotovora subsp.atroseptica and subsp.carotovora from stem lesions and degenerating seed tubers to progeny tubers in soil

Summary In 1978 and 1979 field experiments were made in which serologically distinct isolates ofErwinia carotovora subsp.atroseptica and subsp.carotovora were inoculated on different dates into stems or seed tubers. The latter had been removed from emerging plants, inoculated and replaced on different dates. Samples of soil and progeny tubers were taken at intervals after inoculation to monitor spread. Progeny tubers were induced to rot and the bacterium responsible was identified by immunodiffusion tests. In 1978 no spread of subsp.carotovora from placement tuber lesions to progeny tubers was detected and spread on one occasion only from stem lesions was detected. Spread of subsp.atroseptica was detected from both inoculum sources. In 1979 spread of subsp.carotovora was detected on eight occasions during the season from placement tubers and on twelve from inoculated stems and for subsp.atroseptica on twenty two and nineteen occasions respectively.

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Zusammenfassung Im Jahr 1977 zeigten die Ergebnisse eines kleinen Feldversuches, dass nach Inokulation von Stengeln der Sorte King Edward mitErwinia carotovora subsp.carotovora der Organismus von den Stengell?sionen auf die Tochterknollen übertragen werden konnte. Um diese Beobachtung abzusichern, die H?ufigkeit solcher übertragung zu untersuchen und die relative Bedeutung von Stengell?sionen und verfaulenden Pflanzknollen als Quelle des Inokulums zu bestimmen, werden 1978 und 1979 Versuche mit serologisch identifizierbaren St?mmen der Subspeziescarotovora undatroseptica durchgeführt. 1978 wurden zwei Versuchspaare angelegt und in einem Versuch wurden Pflanzknollen (die vorher von den wachsenden Pflanzen weggenommen wurden), inokuliert mit subsp.atroseptica untergeschoben (siehe Lapwood & Harris, 1977) und Stengel der gleichen Pflanzen der Sorte Pentland Crown durch Injektion von Subsp.carotovora unter die Epidermis inokuliert. Im zweiten Versuch jedes Paares, vom ersten durch vier Leerreihen getrennt, waren die Inokulationsstellen umgekehrt. Die Versuchspaare waren von einander durch sechs Leerreihen getrennt und ein Paar wurde durch Beregnung mit ungef?hr 25 mm Wasser zu jedem der drei Termine w?hrend der Vegetationsperiode versorgt (17. Juli und 6. September; der für Mitte August geplante Termin entfiel wegen Regen). 1979 wurde nur ein Versuchspaar angelegt, die Beregnung wurde weggelassen. 1978 wurde an drei Terminen inokuliert (Tab. 1) und 1979 an neun (Tab. 2); die Daten der Probenahme sind ebenfalls in den Tabellen angegeben. Bei der Probenahme wurden aus jeder der drei Wiederholungen von jedem Inokulationstermin (und aus den nicht inokulierten Kontrollparzellen) 2 Pflanzen in nebeneinander liegenden Reihen mit einer, des-infizierten Grabgabel aufgenommen. Die in-okulierte (untergeschobene) Knolle wurde entfernt bevor Bodenproben genommen und fünf Tochterknollen jeder Pflanze geerntet wurden. Das die L?sionen enthaltende Stengelgewebe wurde zur Bestimmung und Isolierung von den Pflanzen abgeschnitten. Die Proben einer Parzelle wurden zusammengefasst und Bodenproben auf das Vorhandensein der Subspezies vonE. carotovora geprüft. Bei den Tochterknollen wurde durch Untertauchen in Wasser die F?ule provoziert und der dafür massgebende Erreger durch den von Vruggink & Maas Geesteranus (1975) beschrieben und von Lapwood & Harris (1980) modifizierten Immun-Diffusiontest identifiziert. Die 1978 mit subsp.carotovora inokulierten Knollen verfaulten sehr schnell, aber das Bakterium konnte weder in den faulenden Tochterknollen noch im die inokulierten Knollen umgebenden Boden gefunden werden, mit Ausnahme am 27. September (inokulier am 29. August) in den beregneten Versuchen. Nach der Inokulation der Stengel konnten die Bakterien schnell aus Stengeln isoliert werden, aus den Tochterknollen aber nur am 14. August (Inokulation am 26. Juli) und niemals aus dem Boden, gleichgültig, ob die Parzellen beregnet waren oder nicht. Die Ergenisse der Stengel-und Knolleninokulation mit subsp.atroseptica für die nicht beregneten Parzellen sind in Tabelle 1 zusammengefasst. Das Bakterium wurde mehrmals in Bodenproben aus der Umgebung der inokulierten Knollen gefunden, jedoch niemals im Boden nahe der Tochterknollen, gleichgültig, ob beregnet wurde oder nicht. Es konnte nur am 24. Juli (beregneter Versuch) und am 7. August (nicht beregneter Versuch) in induzierten F?ulen der Tochterknollen identifiziert werden, wobei die inokulierte Pflanzknolle das Inokulum lieferte, aber in vielen F?llen, wenn das Inokulum vom Stengel stammt. Das Muster der Verbreitung (Abb. 1) zeigt, dass das meiste w?hrend der nassen, kühlen Witterung Ende Juli und Anfang August geschah. 1979 konnte die Ausbreitung der subsp.carotovora w?hrend der Vegetationszeit achtmal von inokulierten Knollen und zw?lfmal von inokulierten Stengeln ausgehend beobachtet werden und bei subsp.atroseptica von 22 bzw. 19 (Tabelle 2); Abb. 1 zeigt, die Ergebnisse im Verh?ltnis zu den Witterungsbedingugen. Die Ergebnisse zeigen, dass in Stengel inokulierte Bakterien darauffolgend in Tochterknollen wiedergefunden werden k?nnen wahrscheinlich durch das Abwaschen und den Transport durch Regen. Stengell?sionen scheinen als Quelle des Inokulums ebenso wichtig zu sein, wie die Pflanzknollen, für subsp.atroseptica in beiden Jahren und für subsp.carotovora 1979. Die Stengell?sionen k?nnen für fast die gesamte Wachstumsperiode eine Quelle des Inokulums darstellen, w?hrend nassfaule Pflanzknollen, die schnell bis auf einen leeren Stengel reduziert sind, ihr gesamtes Inokulum bald entleert haben k?nnen. Zus?tzlich wird die Zerfallsrate der Pflanzknolle vielleicht auch durch das Eindringen von Sekund?r-Erregern beschleunigt, wodurch sowohl der Gehalt an Inokulum und die Dauer seiner Verfügbarkeit begrenzt wird, Bedingungen, die vielleicht weniger bei den Stengell?sionen auftreten.

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Résumé Les résultats d'une expérimentation au champ, à petite échelle, à Rothamsted en 1977, montraient que lorsqueErwinia carotovora subsp.carotovora était inoculé dans les tiges de la variété King Edward, l'organisme pouvait être transmis à partir des lésions sur tiges, aux tubercules-fils. Pour confirmer cette observation, pour rechercher la fréquence de ce type de propagation et pour déterminer l'importance relative des lésions de tiges et des tubercules de semence pourris comme sources d'inoculum, des expérimentations ont été conduites en 1978 et en 1979 en utilisant une souche identifiée sérologiquement de subsp.carotovora et aussi de subsp.atroseptica. En 1978 deux séries d'expérimentations ont été mises en place et dans la première expérimentation de chaque série, des tubercules de semence (prélevés sur des plantes en culture), inoculés avec subsp.atroseptica ont été introduits (voir Lapwood & Harris, 1977) et les tiges des mêmes plantes de variétés Pentland Crown ont été inoculées avec subsp.carotovora par injection hypodermique. Dans la seconde expérimentation de chaque série, séparée de la première par quatre rangs non plantés, les sites d'inoculation étaient inversés. Les séries d'expérimentations étaient séparées l'une de l'autre par six rangs non plantés et la première série était irriguée par aspersion pour couvrir approximativement un besoin de 25 mm en trois occasions durant la saison (à savoir, 27 Juillet et 6 Septembre; celle prévue à la mi-ao?t n'a pas été appliquée à cause de la pluie). En 1979, seule une série d'expérimentations a été mise en place, l'irrigation ayant été omise. En 1978, il y avait trois date d'inoculation (Tableau 1) et en 1979, neuf (Tableau 2). Les dates d'échantillonnage sont également indiquées dans les tableaux. Pour l'échantillonnage, deux plantes de rangs adjacents sont arrachées avec une bêche désinfectée, à raison de 3 répétitions pour chaque date d'inoculation (et pour les parcelles témoins non inoculées). Le tubercule inoculé était enlevé avant de prendre les échantillons de sol et cinq tubercules fils sur chaque plante. Les tissus de tiges contenant la lésion étaient détachés de la plante pour détermination et isolement. Les échantillons de chaque parcelle étaient groupés. Les sols étaient examinés en ce qui concerne la présence des subsp. d'E. carotovora et les tubercules fils étaient mis à pourrir par immersion dans l'eau et l'agent causal était identifié par la méthode sérologique d'immunodiffusion décrite par Vruggink & Maas Geesteranus (1975) et modifiée par Lapwood & Harris (1980). En 1978, les tubercules inoculés avec subsp.carotovora pourrissaient rapidement mais la bactérie n'a pas été détectée dans les pourritures des tubercules fils induits et dans le sol entourant le tubercule inoculé sauf le 27 Septembre (inoculation du 29 Ao?t) dans l'expérimentation conduite en irrigation. Lorsque les tiges étaient inoculées, les bactéries étaient à nouveau isolables des tiges, mais elles n'étaient retrouvées dans les pourritures des tubercules fils que le 14 Ao?t (inoculation du 26 Juillet) et jamais dans le sol, que les parcelles soient irriguées ou non. Les résultats de l'inoculation des tiges et des tubercules avec subsp.atroseptica sont indiqués dans le Tableau 1 pour la condition non irriguée seulement. La bactérie a été retrouvée de nombreuses fois dans le sol entourant les tubercules inoculés mais jamais dans le sol environnant des tubercules fils que les parcelles soient irriguées ou non. Elle est retrouvée dans les pourritures des tubercules fils induits seulement le 24 Juillet (conditions irriguées) et le 7 Ao?t (condition non irriguée) lorsque le tubercule de semence inoculé était la source d'inoculum, mais dans de nombreuses occasions lorsque l'inoculum provenait de la tige. Le modèle indique (fig. 1) qu'il y avait plus de propagation par temps humide et frais de fin juillet et début ao?t. En 1979, la propagation de subsp.carotovora était mise en évidence à neuf reprises durant la saison à partir des tubercules inoculés et en 12 occasions à partir des tiges inoculées et celle de subsp.atroseptica dans 22 et 19 occasions respectivement (Tableau 2); les résultats sont en relation avec les param⪻tres climatique dans la fig. 1. Les résultats montrent que les bactéries inoculées aux tiges peuvent être retrouvées dans les tubercules fils, probablement à cause du lessivage et du transport par la pluie. Les lésions de tiges apparaissent comme était aussi importantes que le tubercule de la semence en tant que source d'inoculum pour subsp.atroseptica pour les deux années et pour subsp.carotovora en 1979. Les lésions de tiges peuvent être une source d'inoculum persistante durant la saison de culture du fait que les tubercules de semence sont atteints de pourriture molle. Ces lésions peuvent rapidement se limiter à une tige détruite et ainsi éliminer rapidement leur inoculum. De plus, le niveau de dégradation des tubercules de semence peut être accru par l'intervention d'organismes secondaires qui limitent à la fois le taux d'inoculum produit et la durée de sa disponibilité, conditions moins vraissemblables peut être d'apparaitre avec les lésions sur tiges.

     

The effect of green crop lifting on the contamination of seed potato tubers by pathogenicErwinia spp.

Summary In the Netherlands seed potato crops are harvested when still green, the haulm being destroyed before harvest. We compared the effect on the contamination of seed potatoes byErwinia carotovora subsp.atroseptica (Eca) andE. chrysanthemi (Ech) of the common method of haulm destruction by flailing and chemically destroying remaining stems, with the recently developed green crop lifting method. After chenical haulm destruction the levels of contamination with both Eca and Ech surviving cold storage were found not to differ significantly from those after green crop lifting. Losses due to watery wound rot (Pythium ultimum) were considerable in the warm and humid conditions during 1991, especially after green crop lifting.