The effect of condensation on sporulation ofHelminthosporium solani on potato tubers infected with silver scurf and held in simulated store conditions

Summary Chambers were designed to simulate environmental conditions present in commercial potato stores and were used to introduce condensation to tubers infected with silver scurf (Helminthosporium solani). The electrical resistance measured across the skin of tubers from the top of the chambers was consistently lower than that from the bottom of the chamber, and this was reflected in an increased number ofH. solani spores produced by the top tubers. An increase in the length of the condensation period resulted in the skin resistance remaining lower for a longer period and also resulted in higher number of spores. Tubers held at relatively high ambient temperatures (15 °C) required a shorter period of condensation to cause an increase in spore count. This investigation highlighted the need for close monitoring of store environments so that condensation events can be quickly identified and the tubers dried.     

Fungicide treatment of seed tubers infected with thiabendazole-resistantHelminthosporium solani andPolyscytalum pustulans for controlling silver scurf and skin spot on stored progeny tubers

Summary Seed tubers, cv. Désirée, derived from stocks treated annually with thiabendazole were infected with thiabendazole-resistant strains ofHelminthosporium solani andPolyscytalum pustulans. Samples of seed tubers were either untreated or immersed for 5 min in fungicide suspensions of thiabendazole, imazalil or thiabendazole plus imazalil and planted on four farms in 1988 and 1989. After harvest, tuber samples from each treatment were treated with thiabendazole and stored for 6 months. Applying imazalil or thiabendazole plus imazalil to seed tubers decreased the severity of silver scurf and skin spot on stored progeny tubers. Thiabendazole applied to seed tubers or to progeny tubers after harvest did not affect the severity of either disease, but post-harvest treatment decreased the incidence of black scurf after storage.     

Comparison of resistance toFusarium spp. of glasshouse-and field-grown tuber progenies of potato

Summary Field-grown tubers of 22 progenies ofSolanum tuberosum L. generated in a crossing programme involving seven parents differing in resistance toFusarium coeruleum Lib. ex Sacc. andF. sulphureum Schlect. (=F. sambucinum Fuckel, teleomorphGibberella pulicaris (Fr.) Sacc.) were wound-inoculated with a cornmeal + sand culture of each pathogen. Parental genotypes were also included. The mean lesion size of each progeny was compared in 2 years of tests, as well as with published data on glasshouse-grown tubers. ForF. coeruleum there was a high correlation between years as well as with the glasshouse data, but no such correlations were apparent withG. pulicaris. Furthermore, parental and GCA values, as well as progeny means and mid-parent scores, also correlated highly forF. coeruleum but not forG. pulicaris. Glasshouse-grown tubers of 11 wildSolanum spp. were also inoculated with both pathogens. Some resistance to one or other, or both, was apparent, particularly inS. chacoense.     

Causes of differences in growth pattern,yield and quality of potatoes (Solanum tuberosum L.) in short rotations on sandy soil as affected by crop rotation,cultivar and application of granular nematicides

Summary A crop rotation experiment was carried out on a light sandy soil in 1979–1986 to study the effects of the frequency of potato cropping on yield, quality and on the occurrence of soil-borne pathogens other than potato cyst nematodes. Tuber yield decreased markedly with increasing cropping frequency, but also depended on what crops were grown in rotation with potato. Growth during the early part of the season, as well as the length of the growing period, were reduced in short rotations. The fungiVerticillium dahliae andRhizoctonia solani, and root-knot nematodes (Meloidogyne spp.) were the most important yield reducing pathogens. The effects of rotation depended on the cultivar used. The percentage of mis-shapen tubers increased with increasing cropping frequency and after application of granular nematicides, but the incidence of common scab (Streptomyces scabies) was not affected.     

Differential diagnosis ofVerticillium dahliae in potato with antisera to partially purified pathogen-produced extracellular antigens

Summary An extracellular protein-lipopolysaccaride antigen (PLP) was purified from culture fluids ofVerticillium dahliae. Antiserum produced in rabbits to the PLP detected the antigen in homogenates of tubers, stems and leaves ofV. dahliae-infected potato plants but not in extracts of healthy potato tissue or extracts of potato plants infected by other fungal pathogens. The antigen was not detectable in extracts of potato isolates ofV. tricorpus, V. nigrescens andV. nubilum or various other fungi. The antigen was shown to be different from cross-reactive antigens detected by antisera to mycelial antigens. When used as a tool for specific diagnosis ofV. dahliae infection in potato, antiserum to PLP was more reliable than that prepared against fungal body antigens. Publication of the Agricultural Research Organization No 245-E.     

Using chemical and biological treatments to reduce the potential of potato leak caused byPythium aphanidermatum in Tunisia

Summary The effect of fungicides and the antagonistic fungusTrichoderma harzianum (Th) on the potato leak pathogenPythium aphanidermatum (Pa) was investigated in vitro. Rot was reduced by 73% to 89% when tubers inoculated withPa were treated by immersion in solutions (0.5% w/v) of the fungicides Ridomil MZ58. Dithane M45. Manebe 80 and Tachigaren 360. Complete protection was achieved by dusting inoculated tubers with Ridomil MZ58 diluted in kaolin at a final concentration of 0.1% (w/w). The biological treatments of dipping inoculated tubers in a conidial water suspension ofTh at 10⁸ conidia/ml or of dusting with a mixture of a barley culture ofTh and kaolin to a final concentration of 2×10⁹ conidia/g were as effective as the chemical treatments. Control was still effective when inoculated tubers were treated with Ridomil MZ58. Tachigaren 360 or the conidial suspension ofTh after 24h incubation at 25°C.     

Effect of a continuous hot water treatment of potato tubers on seed-borne fungal pathogens

Summary The viability of five pathogens was decreased by treatment with hot water when tested in vitro.Polyscytalum pustulans was most sensitive andRhizoctonia solani least sensitive. Potato tubers were exposed to 55°C for 5 min in a commercial continuous hot water treatment plant using naturally contaminated seed tubers and tubers which had been inoculated by dipping in comminuted cultures. The frequency of eyes colonised byP. pustulans, Helminthosporium solani, andR. solani was reduced to virtually zero and the effect persisted on tubers subsequently stored at 4°C and at 15°C for up to 16 wk. Results withColletotrichum coccodes were inconclusive. Treatment suppressedPenicillium spp. which, however, rapidly recolonised the eyes during storage, leading to higher contamination levels in the treated than in the untreated tubers. With tubers inoculated withPhoma foveata, good control was achieved when the incubation period before treatment was 10 d but not when the period was 42 d.     

The effect of temperature on the development of wilting and on progeny tuber infection of potatoes inoculated with South African strains of biovar 2 and 3 ofPseudomonas solanacearum

Summary Potato plants, after inoculation with a South African strain of biovar 2 or a strain of biovar 3 ofPseudomonas solanacearum were grown in growth chambers at temperatures ranging from 14/16°C (dark/light) up to 25/30°C. When grown at 14/16°C or higher, plants inoculated with the biovar 2 strain developed wilting symptoms and internal symptoms on progeny tubers and the disease was transmitted by the progeny tubers. When the plants were inoculated with the biovar 3 strain, wilting symptoms developed from 18/20°C, internal symptoms on progeny tubers from 20/22°C and the disease was transmitted by progeny tubers from plants grown at 16/18°C or higher.     

The control of silver scurf and development of thiabendazole resistance inHelminthosporium solani as affected by the rate of fungicide applied to potato seed tubers

Summary Seed tubers were immersed in suspensions of thiabendazole (Storite) or thiabendazole plus imazalil (Extratect) at different concentrations and planted in a field experiment. During the following 2 years samples of the produce were planted after treatment with the same fungicide formulation used on the seed. Silver scurf on daughter tubers decreased as concentrations of fungicide increased and Extratect gave better disease control than Storite applied at the same thiabendazole concentration. Isolates ofH. solani resistant to thiabendazole were found on seed tubers 6 weeks after treatment with Storite, and also on daughter tubers after the first application of the higher rates of Storite; their incidence increased with further annual treatments. In 3 years no thiabendazole-resistant isolates were found after treatment with Extratect. Chlamydospores ofH. solani developed when sensitive isolates were subcultured onto agar containing thiabendazole. Isolates sensitive to thiabendazole were slightly more sensitive to imazalil than resistant ones.     

Patterns of spatial arrangement of early potatoes

Summary Yield comparisons were made at three stages of maturity between half and whole tubers ofUlster Prince planted equidistant triangularly (row width 33 cm), and whole tubers planted at twice that row width at the same seed rates which averaged 4.5 and 9.0 tonne/ha. At the same seed rate narrow rows gave no increase in total yield at the first lift, but about 1.3 tonne/ha more at lifts made 10 and 20 days later. The higher seed rate outyielded the lower by about 4.0, 6.0 and 7.5 tonne/ha in total yield for the three lifts respectively. In yield of tubers under 32 mm, there was a large response to increased seed rate and a small response to reduced row width. Single experiments withUlster Premier andArran Pilot, showed responses which differed in some respects from those ofUlster Prince.     

The role of phenols in potato tuber resistance against soft rot byErwinia carotovora ssp.carotovora

Summary Increasing doses of nitrogenous fertilizer increased storage rots in six potato cultivars of which cvs Kufri Sindhuri and Kufri Lalima rotted less and were found to contain high amounts of total phenols compared to the more susceptible cultivars Kufri Jyoti and Kufri Badshah. Nine phenolic acids were identified in the peel and the pulp of tubers and four of these and of other phenolic extracts from tubers suppressed the growth ofErwinia carotovora ssp.carotovora. Research Publication No. 5811, Experiment Station, G.B.P.U.A. & T., Pantnagar 263 145, India.     

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.     

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.     

Detecting resistance to the root-knot nematodesMeloidogyne hapla andM. chitwoodi in potato and wildSolanum spp.

Summary Various methods of screening for resistance to root-knot nematodes were compared and evaluated. Seedling populations ofSolanum spp., grown in clay pots and plastic tubes with silver sand and inoculated with juveniles ofMeloidogyne chitwoodi andM. hapla, showed large differences in the number of egg masses on roots 7 weeks after inoculation. The differences were reproducible when re-testing was done with cuttings and plants from tubers. No resistance toMeloidogyne spp. was observed with ten potato cultivars when grown in clay pots, plastic tobes or closed containers. Plants from tubers in growth pouches developed a large two-dimensional root system, and after inoculation with juveniles the infection process could be observed over 8 weeks. A method of infecting potato tuber tissue withMeloidogyne is described, using tuber slices in Petri dishes as a potential screening test for tuber resistance.     

Growth and yield of potatoes as affected by severity of stem canker (Rhizoctonia solani)

Summary Different amounts ofRhizoctonia solani inoculum were applied to sprouted and non-sprouted seed tubers cultivar Désirée during planting in field experiments at Rothamsted in 1984 and 1985. Severity of stem canker and stolon infection increased with increasing amounts of inoculum; with small or medium amounts these infections were most severe on plants from sprouted seed. In 1985 plants grown from non-sprouted, seed had most stem canker when large amounts of inoculum were applied. Severe infection following high inoculum levels delayed shoot emergence, increased the variability in stem height and decreased the mean height of stems and weight of foliage but did not affect the number of stems per plant or final plant populations. Tuber initiation was delayed and tuber numbers slightly decreased by severe infection and at harvest the size distribution of tubers was altered. Tuber yields from severely infected plants were decreased by 15% (sprouted seed) and 19% (non-sprouted seed) in 1984 and by 17 and 28% respectively in 1985.     

Comparaison de trois méthodes d'identification deCorynebacterium sepedonicum dans les tubercules de pomme de terre

Résumé Au cours de la campagne d'importation de pommes de terre de consommation d'Amérique du Nord (hiver 1976–1977), près de 300 tubercules ont été analysés séparément par 3 méthodes vis-à-vis du flétrissement bactérien (Corynebacterium sepedonicum): Gram in situ. immunofluorescence (IF) in situ et isolementcaractérisation. L'isolement est une technique peu sensible et les colonies croissent beaucoup trop lentement pour un test de détection. Le Gram in situ se montre presque aussi sensible que l'IF et plus simple à effectuer. Néanmoins son manque de spécificité gêne considérablement la détection, en particulier sur des tubercules décomposés. L'IF ne pose pas de problème de spécificité sérologique avec la flore des tubercules, exception faite d'une réaction commune et réciproque avecErwinia carotovora var.atroseptica. Ne pouvant pas déterminer le niveau de tolérance, ce qui ne peut se réaliser qu'en pays contaminé. nous avons fixé un seuil arbitraire de positivité du test IF, supérieur au bruit de fond de la méthode.

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Summary During the winter 1976–1977, ware potatoes were imported from North America. About 300 tubers with and without visible symptoms were tested for the presence ofCorynebacterium sepedonicum, using 3 methods: Gram staining in situ, immunofluorescent staining (IF) in situ and isolation-characterization. In each case the heel end of the tuber was peeled and the net of vascular bundles removed and cut into 1-mm long pieces in sterile water. Three drops of extract from the same tuber were tested comparatively. Isolation was made on an enriched medium (21 C, 15 to 20 days) and the bacteria characterized on the basis of cultural features. Gram staining. behaviour on Hugh and Leifson glucose medium and hypersensitive reaction on tobacco. IF staining was performed by the indirect method. withC. sepedonicum anti-serum (INRA-CNBP 1154=NCPPB 1499) diluted to 1–200. The result of a smear was based on the estimated number of fluorescent bacteria in 200 microscope fields. Isolation was by far the least sensitive method for the detection ofC. sepedonicum (22 postitive tubers (Table 1)) and the colonies were too slow growing for a detection test (10 to 21 days). Gram staining was roughly as sensitive as IF (41 positive tubers (Table 2)) and easier to perform. Nevertheless, its lack of specificity prevented reliable detection, particularly in degraded tubers invaded by numerous Gram-positive coccoid bacteria (Lactobacillaceae). IF staining showed high serological specificity against other tuber flora (200 bacteria tested), except for a reciprocal serological reaction withErwinia carotovora var.atroseptica found in 9 tubers. In tuber tissue,C. sepedonicum appeared much smaller thanErwinia and coccoid: IF staining was less intense with both heterologous sera. For routine detection. absorption of the common antibodies may be necessary. Regarding visible symptoms: some apparently healthy tubers yieldedC. sepedonicum (Gram. IF, even isolation), whereas many tubers showing brown vascular fibers were free ofC. sepedonicum, typical symptoms being a broad shiny yellow vascular ring, then, in an ultimate phase, detachment of the cortex of the tuber with complete degradation of vascular tissues often accompanied by a corky reaction. Unfortunately, facilities for eggplant bioassay were not available at the time. Without knowing the tolerance level (number of bacteria in the seed tuber below which the disease does not appear even under optimal conditions), a threshold that could only be determined in a country in which the disease was endemic, we settled for an arbitrary level of positive response to the IF test (if in 200 microscope fields, a positive smear showed more than 1 fluorescent bacterium per field on average), higher than the background of the method. We noticed that in the majority of positive tests (even without visible tuber symptoms) the smears showed from 100 to 500 fluorescent bacteria per microscope field. The IF test may be successfully used to detectC. sepedonicum in seed tubers. During that winter of 1976–1977, 7 consignments of ware potatoes were intercepted at French ports. 6 were shipped from Maine (USA) and 1 from New Brunswick (Canada) (Table 3).

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Zusammenfassung W?hrend des Winters 1976–1977 wurden Speisekartoffeln aus Nordamerika importiert. Ungef?hr 300 Knollen, welche zum Teil sichtliche Symptome vonCorynebacterium sepedonicum aufwiesen, wurden nach 3 verschiedenen Methoden untersucht: Gram-F?rbung in situ. Immunofluoreszenz (IF) in situ und Bestimmung nach Isolierung. Vom gesch?lten Nabelende der Knolle wurde die Gef?ssbündelpartie entnommen und in sterilem Wasser in Fragmente von 1 mm geschnitten. Zum Vergleich wurden 3 Tropfen desselben Extraktes untersucht. Die Isolierung wurde auf einem angereicherten Milieu vorgenommen (21 C, 15 bis 20 Tage): dann wurden die Bakterien bestimmt aufgrund ihres Kulturzustandes. Gram-F?rbung. Verhalten in Hugh und Leifsonglukose und Hypersensibilit?treaktion auf Tabak. Die IF wurde nach der indirekten Methode untersucht, mit einem anti-C. sepedonicum-Serum (INRA-CNBP 1154=NCPPB 1499) auf 1–200 verdünnt. Die Ergebnisse eines Pr?parates wurden bonitiert durch Sch?tzung der Anzahl fluoreszierender Bakterien in 200 Mikroskopiefeldern. Die Isolierung ist eine schwach empfindliche Methode fürC. sepedonicum (22 positive Knollen, siehe Tabelle 1), und die Kolonien wachsen zu langsam für einen Befallsbestimmungstest (10 21 Tage). Die Methode der Gram-F?rbung in situ scheint gleich empfindlich wie die IF (41 positive Knollen, siehe Tabelle 1) und ist leichter durchführbar. Die Unspezifizit?t dieser Methode st?rt jedoch bei der Bestimmung, vor allem bei faulen Knollen in denen viele kokkenf?rmige Gram-positive Bakterien wachsen (Lactobacillaceae). Die IF-Methode bereitet keine Schwierigkeiten bezüglich der serologischen Spezifizit?t der Knollenflora (200 Bakterienkulturen wurden getestet) mit Ausnahme einer gemeinsamen und reziproken Reaktion mitErwinia carotovora var.atroseptica. In den Geweben istC. sepedonicum kleiner alsErwinia und kokkenf?rmig; die IF-Reaktion erscheint mit einer geringeren Intensit?t. Vermutlich ist für Routinebestimmung die Absorption dieser gemeinsamen Antik?rper notwendig. Was die sichtbaren Symptome betrifft. wurden alle Stadien beobachtet: scheinbare gesunde Knollen zeigtenC. sepedonicum (Gram, IF, manchmal Isolierung); viele Knollen mit braun gef?rbtem Gef?ssbündel waren jedoch frei vonC. sepedonicum: die typischen sichtbaren Symptome sind haupts?chlich ein verdickter, gelbgl?nzender Gef?ssbündelring, welcher sich in einem vorgerückten Stadium vom Rest der Knollen l?st, nach kompleter Zerst?rung des Gewebes der Gef?ssbündelpartie: manchmal wurden auch Korkbildungsreaktionen beobachtet. Leider konnte der biologische Test auf Eierpflanzen zu diesem Zeitpunkt nicht durchgeführt werden. Da die Toleranzgrenze nicht bekannt ist (Anzahl Bakterien in einer Pflanzknolle, unter welcher keine Welkung auftritt, auch unter optimalen Bedingungen) — diese kann nur in einem Lande mit Befall untersucht werden haben wir eine willkürliche Grenze der Positivit?t des IF-Testes angesetzt (Untersuchung von 200 Mikroskopiefelder, auf einem positiven Pr?parat befinden sich im Mittel mehr als 1 fluoreszierendes Bakterium) h?her als die Hintergrundfluoreszenz. Wir haben beobachtet. dass in den meisten positiven Pr?paraten (auch von Knollen ohne sichtbare Symptome) 100–500 fluoreszierende Bakterien pro Feld auftreten. Der IF-Test kann mit Erfolg für den Nachweis vonC. sepedonicum in Saatknollen angewendet werden. W?hrend des Winters 1976–1977 wurden 7 Schiffsladungen in den franz?sischen H?fen beschlagnahmt, 6 kamen vom Staat Maine (USA) und 1 von New-Brunswick (Kanada) (siehe Tabelle 3).

     

Methods for assessing bacterial density on potato tubers

Summary Formulae which determined tuber surface area from tuber dimensions and weight were tested by using samples from three potato cultivars. Surviving populations of a labelled isolate ofErwinia carotovora ssp.atroseptica following planting, and populations of a fluorescent pseudomonad following dusting with a bacterial powder formulation were used to demonstrate the application of these formulae to the calculation of bacterial density on the tuber surface. Results obtained by washing whole tubers using the formulae to calculate the surface area were shown to be comparable in sensitivity with those obtained by dilution-plating of periderm discs of 1cm².     

Variation in tuber pathogenicity ofPhytophthora infestans in the Netherlands

Summary Variation in aggressiveness to tubers among isolates ofPhytophthora infestans sampled from three potato growing regions in the Netherlands was compared. Variation in the ability to infect tubers of cv. Bintje was found between isolates of each of the three regional populations. The most aggressive isolate of the old population matched the average level of the new population in its ability to infect tubers. As a consequence, the commonly used reference isolate VK 6C can no longer be considered to be representative of the present population ofP. infestans. Therefore it is recommended that testing tuber resistance for the official list of potato cultivars with this isolate should be discontinued. Tuber infection and subsequent spread of the fungus in the tuber tissues were not found to be correlated. The components of tuber pathogenicity studied were not correlated to pathogenicity factors in the foliage, as measured under growth chamber conditions.     

Inheritance of resistance toFusarium spp. in tuber progenies of potato

Summary Glasshouse-grown seedling tubers of 22 progenies from parents differing in resistance toFusarium coeruleum andF. sulphureum (Gibberella cyanogena) were wound-inoculated with a cornmeal + sand culture of one or other of these dry rot pathogens. WithF. coeruleum, differences between progenies were due entirely to differences in the general combining abilities (gca) of the parents. Parental and gca values were highly correlated, as were the mean resistance of a progeny and that of its parents. WithG. cyanogena the agreement between replicates was poor and differences between progenies were less clear. It is suggested that different resistance mechanisms operate against these two pathogens.     

Contamination of progeny tubers of potato plants by seed- and leaf-borneErwinia carotovora

Summary Marker strains ofE. carotovora var.carotovora and var.atroseptica spread from inoculated rotting tubers, buried at seed tuber level under potato plants in the field, to rhizosphere soil and progeny tubers late in the 1983 and 1984 growing seasons when weather conditions were wet. In 1982, tuber rotting was arrested in the dry soil conditions which prevailed after their burial and little spread was observed. Leaves, inoculated by spraying with bacterial suspensions, remained contaminated with low numbers of marker bacteria until the end of the season; the bacteria spread to rhizosphere soil and progeny tubers but only after multiplication in rotting leaf debris on the soil surface late in the season when the weather was wet. Bacteria from inoculated tubers and leaves also spread to plants in adjacent non-inoculated control plots, and contamination by airborne wild-type erwinias was detected, also late in the season. Over 90% of marker and wild-type bacteria isolated from all sources were var.carotovora.

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Zusammenfassung Die Ausbreitung Antibiotika-resistenter Indikatorst?mme vonE. carotovora var.atroseptica und var.carotovora, von teilweise faulen Knollen, die in Knollenh?he vergraben waren, oder von mit Bakteriensuspensionen besprühter Bl?tter, wurde im Feld untersucht. Bakterien von faulenden plazierten Knollen wurden im Boden der Rhizosph?re (Tab. 3) und an Tochterknollen (Tab. 4) bei hoher Bodenfeuchtigkeit, welche sowohl das Faulen der Knollen wie auch die Bewegung der Bakterien durch den Boden begünstigte, entdeckt. Dies erfolgte 1983 und 1984 w?hrend des Septembers und Oktobers, also sp?t in der Wachstumsperiode, als reichlich Niederschl?ge fielen (Abb. 1), nicht jedoch im Jahre 1982, in dem nur geringfügige Ausbreitung beobachtet werden konnte, weil die Knollenf?ule durch trockene Bodenverh?ltnisse, die nach dem Vergraben der Knollen herrschten, verhindert wurde. Die Bl?tter blieben nach der Inokulation mit geringen Zahlen markierter Bakterien bis zum Ende der Wachstumszeit kontaminiert (Tab. 1). Die Bakterien breiteten sich im Boden der Rhizosph?re und an den Tochterknollen nur nach Vermehrung auf verrottenden Bl?ttern an der Bodenoberfl?che aus (Tab. 2). Dies konnte sp?t in der Vegetationszeit beobachtet werden, wenn die Regenf?lle h?ufiger waren und feuchte Bedingungen das Faulen der Bl?tterrückst?nde f?rderten (Abb. 1). Unter derartigen Bedingungen breiteten sich markierte St?mme von inokulierten Knollen und Bl?ttern zu benachbarten nicht-inokulierten Kontrollparzellen aus; Kontamination durch luftbürtige Bakterien vom Wildtyp (nicht markiert) konnte ebenfalls entdeckt werden. über 90% der markierten und der Wildtyp-Bakterien aus allen Herkünften geh?rten dem var.carotovora an (Tab. 5).

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Résumé La contamination de souches d'Erwinia carotovora var.atroseptica et var.carotovora, résistantes aux antibiotiques, est examinée au champ à partir de tubercules partiellement pourris et enterrés au niveau des tubercules de semence ou par pulvérisation sur les feuilles d'une suspension bactérienne. Les bactéries provenant des tubercules contaminateurs sont détectées au niveau de la rhizosphère (tabl. 3) et de la descendance (tabl. 4), pour des conditions d'humidité du sol élevées qui favorisent à la fois la pourriture des tubercules et la dissémination des bactéries dans le sol. Ces conditions furent réunies tardivement, en septembre et octobre 1983 et 1984, où la pluviométrie fut abondante (fig. 1); par contre peu de contamination fut observée en 1982, car les conditions sèches du sol ont stoppé la pourriture juste après enfouissement des tubercules contaminateurs. Les feuilles sont restées contaminées après inoculation avec un faible nombre de bactéries ‘marqueurs’ jusqu'à la fin de chaque période (tabl. 1). Les bactéries se disséminent dans la rhizosphère et sur les tubercules fils seulement après multiplication dans les débris de feuilles en décomposition à la surface du sol (tabl. 2). Ceci s'observe tard en culture lorsque les pluies sont plus fréquentes, les conditions humides favorisant la décomposition des débris de feuille (fig. 1). Dans ces conditions, les souches ‘marqueurs’ provenant de tubercules et de feuilles inoculés contaminent les plantes voisines noninoculées des parcelles-témoin; la contamination aérienne d'Erwinia type sauvage est également détectée. Plus de 90% des bactéries ‘type sauvage’ et résistantes aux antibiotiques appartiennent à la variétécarotovora (tabl. 5).