Erwinia carotovora contamination of Finnish seed potatoes and the prevalence of bacterial subspecies and serogroups Pirkko Harju and Jyri Kankila Harju, P. & Kankila, J. 1993. Erwinia carotovora contamination of Finnish seed potatoes and the prevalence of bacterial subspecies and serogroups. Agric. Sci. Finl. 2: 345-352. (Dept. Plant Biology, FIN-00014 University of Helsinki, Finland.) Symptomless contamination with the rot-inducing bacterium Erwinia carotovora was detectable by the tuber incubation method in 82% of the commercial seed potato stocks surveyed. E. carotovora subsp. atroseptica (Eca) was more common than E. carotovora subsp. carotovora (Ecc ) among the tuber contaminants. In a four-year survey of ten meristem-based seed stocks, recontamination with both Eca and Ecc occurred typically during the second field generation, but three stocks remained free of detectable contamination throughout the survey period. The first blackleg symptoms occurred typically duringthe third field generation. The serogroup distribution of Finnish Eca isolates was different from that reported from other countries. The predominant serogroup, I, constituted only 74% of all Eca isolates, since serogroups XXXV and XLI occurred relatively frequently. Serogroup I was more common among isolates from diseased stems than among those from latently contaminated tubers. The results also suggest that serogroup I is more dominant in the southern than in the northern parts of the country. Key words: blackleg, soft rot, serology Introduction Potato blackleg and soft rot are caused by the bac- teria Erwinia carotovora subsp. atroseptica (van Hall) Dye (Eca) and E. carotovora subsp. caroto- vora (Jones) Dye (Ecc). Both subspecies are com- monly associated with tuber soft rot, but rot in the basal part of the stem (blackleg disease) is usually caused by Eca (PÉROMBELON and Kelman 1980, PÉROMBELON et al. 1987). In warm climates, black- leg is frequently caused by E. chrysanthemi (Pérombelon and Kelman 1980). This species has not been found to infect potatoes in Finland, and atypical Finnish E. carotovora strains were also confirmed as not belonging to E. chrysanthemi (Harju, unpublished). Inoculum for the blackleg and soft rot diseases is primarily transmittedfrom season to season in seed tubers (PÉROMBELON 1974). Studies in Scotland (PÉROMBELON 1972, 1973), Germany (FICKE et al. 1973) and the USA (De Boer and Kelman 1975, Nielsen 1978) have shown that most seed potato stocks are contaminated with E. carotovora. Also, recontamination of tubers derived from pathogen- free stem cuttings occurs soon in the multiplication process (Copeman et al. 1977, Sampson 1977, De Boer 1983). In Finland, the level of blackleg incidence has decreased markedly after 1976 (O. Ulvinen, pers. commun.) when the Seed Potato Centre was estab- lished to produce healthy seed potatoes by the mer- istem culture method (Pietarinen and Seppänen 345 Agric. Sei. Finl. 2 (1993) https://www.c-info.fi/en/info/?token=TT9jUSziIY6AdmUF.imbkMcWzbpj__xWOvv8ixQ.d544JEigpse_c6uYS4IV9Imtz70hKd0EsTJ9Oty11nJ59fKP4Y7vYEgxxUYpvXPLSEUmkcvXnUajvASTCWLwtdEm_ffFL608vryO4lZe3M0Y8TcVZ7Zvbl0SKC9SVW5qT1165pjsBt1KKtKzrnWq9MkG8FQ7OpyvHmSGx6nFkEkjmSQlDNXD1SevwYyLQIKxm1YZdcVuv-SUP6uMpYRPUH1U-353GAT06heEd_Q6vb_tWBCBlgD8l6NFm6Stbkm8Ue-Lw4jTJK6cniUeRlL7qtEh6i0 1981). Nevertheless, serious blackleg problems have occurred during some exceptionally wet growing seasons. The object of this study was to investigate the contamination by E. carotovora in commercial seed potato stocks and the recontamination in stocks derived from meristem-cultured plants dur- ing the multiplication process. The prevalence of the two subspecies was also investigated. Further- more, serogroups of Eca were determined in order to get basic knowledge for the development of se- rological diagnostics of the blackleg pathogen. Preliminary results of this study have been pub- lished elsewhere (Harju and Kankila 1987, De Boer et al. 1987). Stem samples Stems were collected from potato plants exhibiting typical blackleg symptoms. Stem samples for the serogroup survey were collected from seed potato fields in the surroundings of the Seed Potato Centre and from ware potato fields of several farmers in Southern Finland (less than 200 km from Helsinki). After surface sterilizing the stem with ethanol, an approx. 1 cm piece was cut from the rotten tissue, adjacent to the border of healthy tissue, and macer- ated with a glass rod or forceps in a drop of sterile water. The suspension was streaked onto the isola- tion medium, which was further treated as de- scribed above for tuber samples. Material and methods Tuber samples In 1981-83, tuber samples of commercial stocks were collected from the stores of two starch facto- ries in Southern Finland and those of the Seed Potato Centre, Tyrnävä. Randomly selected stocks belonged to different varieties and to the seed classes ofElite seed and Commercial seed. Further- more, ten clones derived from meristem cultured plants were selected in 1981 for a four-year survey ofErwinia spp. recontamination. Tubers were washed individually, pricked with sterile toothpicks and wrapped in wet paper towels. The wrapped tubers were placed in polyethylene bags and incubated for 3-4 days at +22°C (De Boer and Kelman 1975, Pérombelon 1979). Small pieces of rotting tissue were taken from every le- sion in the tuber and suspended in a drop of sterile water on a glass plate. The suspension was streaked on crystal violet pectate (CVP) medium (Cuppels and Kelman 1974) or on Stewart’s medium (Stewart 1962). Pectolytic colonies typical of E. carotovora were restreaked and purified on Stewart’s medium. Single colonies were then trans- ferred to nutrient agar (Oxoid CM3) on which they were maintained for further examination. All cul- tures on agar media were incubated at +2B°C for 48 h. Determination of subspecies Eca was differentiated from Ecc by production of reducing substances from sucrose and acid from alpha-methyl glucoside (Graham 1972). Serology The strains which had been biochemically identi- fied as Eca were serotyped using the Ouchterlony double-diffusion method (De Boer et al. 1979). The antisera were kindly provided by H. Vruggink (Institute ofPhytopathological Research, Wagenin- gen, the Netherlands) or prepared according to the method described by Vruggink and Maas Geesteranus (1975) (see Table 1). Two antisera against serogroup I, obviously having similarprop- erties, were used because of limited availability of individual antisera. Table 1. E. carotovora subsp. atroseptica strains used for antiserum production. Serogroup Strain Origin I 161 MaasGeesteranus Netherlands I 169 Harju Finland XXXV 64 Harju Finland XLI 281 Lapwood UK XLIII 10 Harju Finland 346 Agric. Sei. Finl. 2(1993) Table 2. Tuber contamination with E. carotovora in stored commercial seed potato stocks during a three-year survey. Number of stocks Contaminated Seed contaminated No. of tubers No. of Eca Year class tested with Erwinia tubers % isolates % 1981 Elite 13 13 160 69 237 74 Comm. 18 18 200 77 408 78 1982 Elite 11 8 481 25 210 49 Comm. II 9 401 30 237 70 1983 Elite 13 9 480 12 260 60 Comm. 5 1 180 4 18 17 Total number 71 58 1902 1370 Mean per cent 30 67 Results E. carotovora contamination was detected in 58 of the 71 commercial seed potato stocks surveyed in 1981-83. Contamination was most widespread in the crops harvested in 1981, which was the year of highest precipitation within the period; none of the stocks surveyed in 1981 was found free of E. caro- tovora, and more than 70% of the total number of tubers were contaminated (Table 2). In contrast, several stocks surveyed in 1982 and 1983 did not harbor detectable levels of E. carotovora. The blackleg pathogen, Eca , was more common than Ecc among the tuber contaminants during the wholeperiod of 1981-83. An average of67% ofthe isolates were identified as Eca. The proportion of Eca among the isolates was slightly higher after the rainy summer of 1981 than during the rest of the period. The recontamination of healthy, meristem-de- rived seed stocks was surveyed by subjecting ten different stocks to the tuber incubation test after each of the four first propagations (Table 3). The first propagation took place in a glasshouse, the following ones in the field. A total of 100 glass- house-grown tubers were tested, and one of them was found contaminated by Ecc. However, neither of the subspecies could be detected in the crop of the first field generation (305 tubers tested). After the second field generation, five of the ten stocks were contaminated, and both subspecies were found among the contaminants, but no blackleg or soft rot symptoms were seen at this stage. The first blackleg cases were observed during the third field generation in cultivars Pito and Satuma, but not in cv. Record. Furthermore, three of the four cv. Re- cord stocks were also free of E. carotovora con- tamination of the tubers. Results of serotyping a total of2330 Eca isolates are presented in Table 4. The majority of stem as well as tuber isolates belonged to serogroup I. How- ever, in each survey year, serogroup I was more frequently found among stem isolates than among tuber isolates. Furthermore, the percentage of sero- group I among the stem isolates from Southern Finland was always close to 90%, whereas the stem isolates from Northern Finland (seed production area) had a more diverse serogroup spectrum (Table 5). Among the other serogroups, XXXV and XLI were the most frequent ones, and XLIII was rarely found. 1.5% of the isolates did not produce a homologous reaction with any of the four antisera. It was typical of the isolates to react with only one antiserum, but occasional cross-reactions were also observed. Ecc was occasionally isolated from blackleg stems collected in Southern Finland. In each case, however, Eca was also isolated from the same stem. During the serogroup survey, no single stem was found colonised by more than one serogroupof Eca (up to ten isolates per stem). 347 Agric. Sei. Fin!. 2 (1993) 348 Table 3. Contamination of ten seed potato stocks with E. carotovora during a four-year multiplication in 1981-84. Stock Number Tubers Number and of contaminated of Eca blackleg variety Yeara tubers % isolates % incidence b 1. Pito 1 10 0 0 0 2 30 0 0 0- 3 60 30 109 100 4 30 57 89 75 + 2. Pito 1 10 0 0 0 2 30 0 0 0- 3 60 5 7 29 4 30 30 52 81 + 3. Pito 1 10 0 0 0 2 35 0 0 0 - 3 60 12 66 59 4 30 50 159 81 + 4. Satuma 110 0 0 0 - 2 30 0 0 0 - 3 60 0 0 0 - 4 30 17 25 32 + 5. Satuma I 10 0 0 0 - 2 30 0 0 0 - 3 60 13 50 76 4 30 27 79 88 + 6. Satuma 1 10 0 0 0 2 30 0 0 0 - 3 60 13 59 17 4 30 33 82 100 + 7. Record 1 10 0 0 0 2 30 0 0 0 - 3 60 0 0 0 4 30 0 0 0 - 8. Record 1 10 0 0 0 2 30 0 0 0 - 3 60 0 0 0 4 30 0 0 0 - 9. Record 1 10 0 0 0 2 30 0 0 0 - 3 60 0 0 0 - 4 30 0 0 0 - 10. Record 1 10 10 6 0 - 2 30 0 0 0 - 3 60 12 35 91 4 50 6 23 52 - a Year 1, tubers raised from stem cuttings in a glasshouse. Years 2-4, tubers grown in the field. b Presence (+) or absence (-) of blackleg plants according to field inspection records. 349 Table 4. Frequency of serogroups among E. carotovora subsp. atroseptica isolates. a) Tuber isolates Survey Number of Isolates by serogroup, % year isolates I XXXV XLI XLIII unidentified 1983 318 27 8 64 0 I 1984 397 78 6 11 3 2 1985 16 13 81 00 6 1987 44 75 5 9 2 9 Total 775 55 9 32 2 2 b) Stem isolates Survey No. No. Isolates by serogroup, % year stems isolates I XXXV XLI XLIII unidentified 1983 69 313 90 8 2 0 1 1984 111 654 92 5 0 2 1 1985 35 346 56 28 16 0 0 1987 41 111 92 5 3 0 0 1988 97 131 82 3 6 2 7 Total 1555 83 II 4 1 1 Table 5. Prevalence of serogroup I among stem isolates of E. carotovora subsp. atroseptica from different geographical regions. Southern Finland Northern Finland Sampling No. Total % No. Total % year ser. I no. Eca ser. I ser. I no. Eca ser. I 1983 281 312 90 1984 597 650 92 1985 193 346 56 1987 102 111 92 1988 74 85 87 33 45 73 Total 1054 1158 91.0 226 391 57.8 Discussion Recontamination of healthy seed potato stocks with E. carotovora occurred typically during the second field propagation. This rapid recontamination is in accordance with reports from other countries (Copeman et al. 1977, Sampson 1977, De Boer 1983) and leads to the conclusion of Erwinia-free seed potatoes being very difficult to produce under field conditions, at least until further knowledge is gathered of the primary sources of bacterial inocu- lum. This study did not attempt to reveal the inoculum sources, but insects (Graham et al. 1976, Har- rison 1985), atmospheric aerosols (Graham 1985), rain and irrigation water (Quinn 1985) have been indicated.The survival of E. carotovora, espe- cially Eca, in soil is limited, but may be longer in the rhizosphere of some non-host plants (PÉROMBE- lon and Hyman 1989).However, all studies on the epidemiology of E. carotovora have suffered from the insensitivity of detection methods. The prob- ably minute numbers of bacteria involved in the early stages of the recontamination process can be detected and quantified only after the development of Polymerase Chain Reaction (PCR) -based methods, now underway in several countries. It would seem logical that the pre-elite material surveyed here harbored some Erwinia bacteria al- ready during the first field propagation, but the detection methods used were too insensitive to reveal this. The one single case of Ecc among the glasshouse-grown tubers leads one to consider the possibility of the bacteria surviving, at hardly de- tectable levels, through the meristem culture pro- cedures. The results obtained by Weber and Schenk (1988) indicated that Eca may survive in meristem cultures without causing symptoms, and that the bacteria can be detected only after homoge- nization or enzymatic maceration of the plant tissue. It should be noted, however, that discovering the primary sources of inoculum will not be the final solution to the blackleg problem. Cultural and handling practices will maintain their importance, because some blackleg bacteria will probably al- ways be present in the environment, and poor culti- vation practice will certainly allow other soft rot- ting bacteria (e.g. some pseudomonads) to cause problems. Contamination withEca and Ecc was very com- mon in stored seed potato stocks classified as Elite seed or Commercial seed. This implies that the presence of Eca in the seed does not nearly always provoke blackleg symptoms, and stresses the im- portance of other health factors of the seed tuber and of the conditions in the field. The fact that contamination levels were highest after the rainy growing season of 1981, once again points out the importance of environmental factors in the whole blackleg problem. The results of the serogroup survey show that serogroup I, although making up the majority of Eca isolates, is not so dominant in Finland as it is in many other countries (De Boer et al. 1987). This anomaly is significant with regard to serological detection of the bacterium. Detection systems based on serogroup I antiserum are not suitable for the situation in Finland, because the contamination would remain undetected in many cases. Since the other serogroups also are pathogenic (with perhaps minor differences in aggressiveness), they cannot be ignored when determining the health status of seed potatoes. Approaches to these diagnostic prob- lems have been discussed elsewhere (Kankila 1990). The existence of the mentioned minor differ- ences in the aggressiveness of serogroups is sug- gested by the observed differences between the serogroup distributions of tuber isolates and stem isolates. Serogroup I may be a slightly more potent inducer of blackleg symptoms than the other sero- groups, since it was more common among stem than tuber isolates. Another observation of potential practical sig- nificance was the stronger dominance ofserogroup I in the southern than in the northern parts of the country. It should be kept in mind, however, that the sampling was not originally planned with an aim of detecting geographical differences, and a more thorough survey would be needed before drawing firm conclusions on this matter. Neverthe- less, these results (Tables 4 and 5) raise questions about the significance of bacterial lipopolysaccha- ride (LPS) in the ecology of Eca , since the sero- groups of Eca are mainly based on differences in the O-chains ofLPS molecules, which extend to the surface of the bacterial cell (De Boer et al. 1985). Factors accounting for the differential success of serogroups in different regions might include, for example, humidity of the environment, frequency of freezing and thawing of the soil, and altered physiology of the host in different temperatures and daylengths. In conclusion, E. carotovora is a very commonly occurring organism in the potato production sys- tem, and it would be very difficult, if not imposs- ible, to eradicate it, but the diseases caused by the bacterium can be kept in an acceptable level. After the initiation of this study, Finnish seed potato pro- duction has developed in many respects. Cultiva- tion and handling practices have been revised with 350 Agric. Sd. Pint. 2 (1993) an emphasis on blackleg control, and the number of field generations has been decreased.Still, blackleg problems occur in a few susceptible varieties. Ad- vances in diagnostics may finally make it feasible to classify seed lots on the basis of their blackleg potential. Furthermore, the current breeding efforts are expected to yield more resistant varieties. Acknowledgements. The authors appreciate the excellent technical assistance provided by Tuula Laine. The Seed Po- tato Centre and the Potato Research Institute are acknow- ledged for their co-operation. This study was financed by the Academy of Finland. References Copeman, R.J., Schneider, F.F. & De Boer, S.H. 1977. Erwinia carotovora recontamination of potato tubers produced by stem cutting derived plants. Proc. Amer. Phytopathol. Soc.4: 136-137. (Abstr.). Cuppels, D. & Kelman, A. 1974. Evaluation of selective media for isolation ofsoft rot bacteria from soil and plant tissue. Phytopathology 64: 468-475. De Boer, S.H. 1983. Frequency and distribution of Erwinia carotovora serogroups associated with potato in the Pem- berton Valley of British Columbia. Can. J. Plant Pathol. 5: 279-284. —, Bradshaw-Rouse, J.J., Sequeira, L. & Me Nauohton, M.E. 1985. Sugar composition and serological specificity of Erwinia carotovora lipopolysaccharides. Can. J. Mi- crobiol. 31: 583-586. —, Copeman, R.J. & Vruggink, H. 1979. Serogroups of Erwinia carotovora potato strains determined with dif- fusible somatic antigens. Phytopathology 69: 316-319. & Kelman, A. 1975. Evaluation of procedures for detec- tion of pectolytic Erwinia spp. on potato tubers. Am. Potato!. 52: 117-123. —, Verdonck, L., Vruggink, H., Harju, P., Bång, H.O. & De Ley, J, 1987. Serological and biochemical variation among potato strains of Erwinia carotovora subsp. atroseptica and their taxonomic relationship to other E. carotovora strains. J. Appi. Bacteriol. 63: 487-495. Ficke, W., Naumann, K., Shadow, K., Muller, H.J. & Zielke, R. 1973. Die Lebensdauer von Pectobacterium carotovorum var. atrosepticum (van Hall) Dowson auf dem Pflanzgut und im Boden, Arch. Phytopathol. Pflan- zenschutz 9; 281-293. Graham, D.C. 1972. Identification of soft rot coliform bac- teria. Proc. 3rd Int. Conf, Plant Path. Back, Wageningen 1971. p. 273-279. 1985. Spread ofErwinia bacteria in atmospheric aerosols. In: Rep. Int. Conf. Potato Blackleg Disease, Edinburgh 1984. p. 35-36. —, Quinn, C.E. & Harrison, M.D. 1976. Recurrence of soft rot coliform bacterial infections in potato stem cuttings: an epidemiological study on the central nuclear stock production farm in Scotland 1967-74. Potato Res. 19: 3-20. Harju, P. & Kankila, J. 1987. Contamination of seed potato stocks by Envinia carotovora in Finland. 10th Triennial Conf. EAPR, Aalborg, Denmark. Abstracts of Confer- ence Papers and Posters, p. 430. Harrison, M.D. 1985. Spread of Erwinia inoculum by in- sects. In: Rep. Int. Conf. Potato Blackleg Disease, Edin- burgh 1984. p. 33-34. Kankila, J, 1990. Methods for the detection of serologically heterogeneous populations of Erwinia carotovora subsp. atroseptica. In; Plant Pathogenic Bacteria. Proc. 7th Int. Conf. Plant Path. Back, Budapest, 1989. p. 889-894. Nielsen, L.W. 1978. Erwinia species in the lenticels of certi- fied seed potatoes. Am. Potato J. 55: 671-676. Pérombelon, M.C.M. 1972. The extent and survival of con- tamination of potato stocks in Scotland by Erwinia caro- tovora var. carotovora and E. carotovora var. alrosep- tica. Ann. appi. Biol. 71: 111-117. 1973. Sites of contamination and numbers of Erwinia carotovora present in stored seed potato stocks in Scot- land. Ann. appi. Biol. 74: 59-65. 1974. The role of the seed tuber in the contamination by Erwinia carotovora of potato crops in Scotland. Potato Res. 17: 187-199. 1979. Factors affecting the accuracy of the tuber incuba- tion test for the detection of contamination of potato stocks by Erwinia carotovora.Potato Res. 22: 63-68. & Hyman, L.J. 1989. Survival of soft rot coliforms, Erwinia carotovora subsp. carotovora andE. carotovora subsp. aP oseptica in soil in Scotland. J. Appi. Bacteriol. 66: 95-106. & Kelman, A. 1980, Ecology of the soft rot erwinias. Ann. Rev. Phytopathol. 18: 361-387. —, Lumb, V.M. & Zutra, D. 1987. Pathogenicity ofsoft rot erwinias to potato plants in Scotland and Israel. J. Appi. Bacteriol. 63: 73-84. Pietarinen, E. & Seppänen, E. 1981. Start of seed potato production in Finland. Ann. Agric. Perm. 20: 184-187. Quinn, C.E. 1985. Occurrence of Erwinia bacteria in surface and underground water, rain and snow. In: Rep. Int. Conf. Potato Blackleg Disease, Edinburgh 1984. p. 43-45. Sampson, PJ. 1977.Contamination with Erwinia carotovora and Verticillium albo-atrum during multiplication of pathogen tested seed potato crops, cultivar Kennebec. Am. Potato J, 54: 1-9. 351 Agnc. Sei. FinI. 2 (1993) Stewart, D.J. 1962. A selective-diagnostic medium for the isolation of pectolytic organisms in the Enterobacteri- aceae. Nature 195: 1023. Vrugoink, H. & Maas Geesteranus, H.P. 1975. Serological recognition of Erwinia carotovora subsp. atroseptica, the causal organism of potato blackleg. Potato Res. 18: 546-555. Weber, J. & Schenk, G. 1988. Symptomlose Ausbreitung des Naßfäuleerregers Erwinia carotovora subsp. atroseptica (van Hall) Dye an m-v/rro-Pflanzen der Kar- toffel. Arch. Phytopathol. Pflanzenschutz 24: 395-402. Manuscript received July 1993 Pirkko Harju Jyri Kankila Department of Plant Biology P.O. Box 28 FIN-00014 University of Helsinki, Finland Jyri Kankila Present address: Agricultural Research Centre of Finland Institute of Plant Breeding FIN-31600 Jokioinen, Finland SELOSTUS Suomalaisen siemenperunan Erwinia carotovora -saastunta sekä bakteerin alalajien ja seroryhmien yleisyys Pirkko Harju ja Jyri Kankila Helsingin yliopisto Perunan tyvi- ja märkämätää aiheuttava bakteeri Erwinia ca- rotovora esiintyi oireettomana 82 %:ssa perunateollisuuden ja Siemenperunakeskuksen varastoimista siemenperunaeristä vuosina 1981-83. Bakteerin alalajeista tyvimätäbakteeri (£. carotovora subsp. atroseptica, Ecä) oli yleisempi kuin märkä - mätäbakteeri (E. carotovora subsp. carotovora, Ecc). Meristeemiviljeltyjen, terveiden siemenperunaerien saas- tumista tutkittiin määrittämällä tyvi- ja märkämätäbakteerin yleisyys kymmenessä siemenerässä neljänä kasvukautena. Ensimmäinen kasvukausi oli lisäys kasvihuoneessa. Bakteeri - saastunta havaittiin useimmissa erissä toisen peltolisäyksen jälkeen ja ensimmäiset tyvimätäoireet kolmannen peltolisäyk- sen aikana. Kuitenkin kolmessa siemenerässä ei esiintynyt oireetontakaan saastumaa koko seurannan aikana. Eca: n seroryhmien yleisyyttä tutkittiin, jotta saataisiin pe ruslietoa siemenerien saastuntatasoa määrittävän serologisen testin kehittämiseen. Vuosina 1983-88määritettiin seroryhmä yhteensä 2330£ca-isolaatista. Seroryhmäjakauma osoittautui olevan Suomessa poikkeava monissa muissa maissa vallitse- vasta. Seroryhmä I:een kuului vain 74 % kaikista £ca-isolaa- teista, kun taas muualla sen osuus on ollut yli 90 % isolaateis- ta. Täten seroryhmä I:n antiseerumiin perustuvat kaupalliset tunnistussarjat eivät sovellu käyttöön Suomessa. Muita ylei- sesti esiintyviä seroryhmiä olivat XXXV jaXLI. Seroryhmien taudinaiheuttamiskyvyssä ei ole selviä eroja, mutta seroryhmä I oli kuitenkin yleisempi tyvimätäisestä var- resta kuin oireettomasta mukulasta eristettyjenbakteerien jou- kossa. Alustavien tulosten mukaan seroryhmä 1on myös ylei- sempi Etelä-Suomessa kuin siementuotantoalueella. Tuloksista voidaan päätellä, että E. carotovora esiintyy hyvin yleisesti perunassa ja perunan tuotantoympäristössä, mutta aiheuttaa tautioireita suhteellisen harvoin. Bakteerin täydellinen hävittäminen olisi erittäin vaikeaa tai mahdotonta. Diagnostisten menetelmien kehitys saattaa tulevaisuudessa mahdollistaa bakteerisaastunnan määrityksen kaikista sie- meneristä, mutta siemenen muu kunto sekä viljely- ja varas- tointitekniikka säilyttävät tällöinkin merkityksensä tyvimädän ja märkämädän torjunnassa. 352 Agric. Sd. Finl. 2 (1993)