Maataloustieteeilinen Aikakauskirja Vol. 63: 415—434, 1991 Occurrence of clubroot and Plasmodiophora brassicae Wor. races in Finland ANNIKKI LINNASALMI and ANNELI TOIVIAINEN Agricultural Research Centre, Institute of Plant Protection, Plant Pathology Section Jokioinen, Finland Abstract. Examination of clubroot in cruciferous vegetables in Finland in 1974—1978 re- vealed the disease in 81 % of the 101 communes inspected. The disease was most common in southern and central Finland, but was also discovered in the northern parts of the region in which cruciferous crops were cultivated (66 —67° N. lat.). Clubroot was found in 65 % of the 375 plant samples collected. It occurred in 68 % of the samples of the most commonly cultivated vegetable, cabbage (56 % of the material), in 63 °Jo of the cauliflower samples (22 % of the material), in 56 % of the samples of other cole species (13 % of the material) and in 64 % of the samples of cruciferous root crops (10 % of the material). P. brassicae race determinations were performed on 90 samples. The classification system of Williams (1966) was applied. The races that were isolated were 1,2, 3,4, 6 and 7. Race 2 was by far the most common, being found in 32 communes; races 3,4, 6 and 7 were each found in 9—12 com- munes; race 1 only in one commune. No clear differences in the occurrence of the races in the various parts of the country could be observed. A comparison is made between Williams’ and the ECD (Buczacki et al. 1975) classification systems. In addition, the pathotypes in clubroot material from Norway and Iceland were determined. Index words: clubroot, Plasmodiophora brassicae Wor., pathotypes, races, cruciferous vegetables Introduction The records on the occurrence of clubroot disease in Finland date back to the 1860s (Jamalainen 1936). It is assumed that the dis- ease came to Finland from the east, from Rus- sia (Rainio 1930). In his studies on the cause of crucifer hernia, Woronin (1878) mentions that cabbage plant materials used by him came from the regions of St. Petersburg (Leningrad) and Wiborg 1 in Finland. According to the in- formation collected by the Department of Plant Pathology, Agricultural Research Centre, clubroot had spread all over the country, in- cluding the western regions, by the 1930s 1 Since 1944 that area has been part of the Soviet Union. 415 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=f3Hy96L6m_J1Lg-7.NlIb9Cdw90eN58Spriay7Q.0FcwEZ5EIqgHvXn-ctTkAz6Qix6wVA8Ib4AEFZu5xFpixSpvXIfm2e4Thn9FdjLLvYDhK_QHA82R7eoRmkPUpncBXt8p0jSRP2O_dLVU2lMsFQRxU3deF-CrSrzwIzAPYbXy3YDGo5bCwlvJ-HQchUAwoZRI6WUVBUGoGyxsYjLXsCEMeBak_N96Q5DH6DDuETXDdTjOblZC5JA7hWkNp2twnBYXi2CjfBKNQnebeY4LvEPYT8Jru1T-VQ (Rainio 1930, Jamalainen 1936). In the 1930s the susceptibility of cruciferous vegetable cul- tivars and wild Brassicaceae plants to the fun- gus Plasmodiophora brassicae Wor. was also studied at the Research Centre. In addition, experiments for chemical control of the dis- ease were carried out (Jamalainen 1936). The efficiency of new pesticides was studied in 1944—1977 (Linnasalmi 1948, 1952, 1959; Linnasalmi and Tiittanen 1960; Murtomaa and Uoti 1972; annual reports of the Depart- ment of Plant Pathology of the Agricultural Research Centre 1960—1977, mimeographs). A preliminary study on P. brassicae races in Finland had been carried out in the years 1971—1972 (Linnasalmi and Palonen 1974). In this study, the occurrence of clubroot and the pathotypes of P. brassicae in crucifer- ous vegetables in Finland in 1974—1978 were examined. In addition, the pathotypes in clubroot material sampled in Norway and Ice- land were determined. The study is a part of the Nordic clubroot project, NKJ project 27 1974—1977, of the Nordic Joint Committee for Agricultural Re- search: Breeding for clubroot resistance, Plas- modiophora brassicae Wor. races, and the ef- ficiency of new pesticides. Materials and methods Collection of the basic material Most of the material for studying the dis- tribution of clubroot was collected by inspect- ing cruciferous vegetable fields throughout Finland during the growing season, from June to October. A small part of the material, about 2 %, was sent in by agricultural research stations, advisers of agricultural information organizations and farmers. The inspections chiefly focused on large cole and root crop farms, but a considerable number of small farmers’ crucifer fields were also inspected. The research material collected as described above can be assumed to give a general pic- ture of the distribution of clubroot disease and the P. brassicae races in the period under study in the 1970 s in Finland. Samples of galls from the various cultivars affected withclubroot were taken for the anal- ysis of P. brassicae races. Propagation ofP. brassicae club materialand preparation of the inoculum The young galls of the original samples were washed thoroughly with running water. They were crushed to prepare a water suspension, which was used to inoculate a steamed (at 100°C for 1 h) soil-peat medium. From the samples for race analysis additional club ma- terial was grown on the original plant species and, if possible, on the original cultivar or on a cultivar known to be susceptible to P. bras- sicae, such as cabbage cv. Ditmarsk and cauliflower cv. Erfurter. To check and ensure the viability of the inocula, the black mustard (Brassica nigra (L.) W.D.J. Koch) breeding line Sv. 72-6842 of the Swedish Seed Associ- ation, Sweden, which was very susceptible to clubroot, was also used as a host plant both during the. propagation of club materials and later in identification tests of P. brassicae races. During the initial stage of the study, the in- oculation of the host was repeated two or three times as five-week growth periods. As the sampling technique improved, the club material from the first propagation could be used as basic material to grow callus cultures and small-club material. The material was stored at —lB°C. The callus cultures were prepared from young galls, I—21 —2 cm long, which were surface-disinfected with ethanol (C2H S OH, 94 %) and mercuric chloride (HgCl2, 1 %) and rinsed with distilled water. Small pieces, ca. 2 mm in diameter, were cut from the in- side of the galls, treated with ethanol (99.5 %) for a few seconds and rinsed with distilled wa- ter. The pieces were placed in Erlenmeyer flasks or test tubes on an agar medium. The nutrient solution used was a modification of the solution of Murashige and Skoog (1962), 416 in which 3-indoleacetic acid had been replaced Isolation and identification of races with 1-naphthylacetic acid. Coconut milk (100 ml/1) was added because our tests had shown that it increased the growth rate of callus tis- sue considerably. The cultures were incubat- ed in the dark at ca. 22°C. Small pieces of cal- lus tissue were transferred every 7 to 10days to a fresh culture medium, the passages to- talling three to four. The callus material was stored at —5°C. Small-club inoculummaterial was made di- rect from young fresh or frozen (—lB°C) galls, which were washed with running water and rinsed with distilled water. For preparing the inoculum, the callus and small-club materials were crushed mechanical- ly, suspended in distilled water and filtered through a nylon filter cloth. The filtrate was centrifuged (2400 g, 7 min.) three to four times. Before each test, the suspension used was diluted to a concentration of 108 spores per ml, using a hemocytometer. Method The isolation and identification of races was carried out according to the system of Wil- liams (1966), a method based on selecting four Brassicaceae plants as differential hosts and studying their resistance or susceptibility to P. brassicae. The test plants were cabbage (Brassica oler- acea L. convar. capitata (L.) Alef.), cultivars Jersey Queen and Badger Shipper, and swede {Brassica napus L. var. napobrassica (L.) Rchb.), cultivars Laurentian and Wilhelms- burger. On the basis of 16 possible interac- tions, 16 races can be isolated and classified according to the following scheme (Williams 1966). The use of genetically uniform, homozy- gous differential host material is a prerequi- site for the reliability of the race determina- tions. The seed material used in this study was Possible host reactions to infection by races of Plasmodiophora brassicae. Differential Race 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Cabbage: Jersey Queen + + + + + + + + Badger Shipper + + + + + + + + Rutabaga; Laurentian + + + + + + + + Wilhelmsburger + + + + + + + + + Indicates a susceptible host reaction; indicates a resistant host reaction. supplied by NKJ project member Dr. R. JOnsson, and produced or acquired by the Swedish Seed Association (nowadays Svalof AB), Svaldv, Sweden. Testing The tests were carried out in a glasshouse, mean temperature 22°C (20 —24°C). When necessary (in winter), Osram HQL 400 W/R mercury high-pressure lamps were used for supplementary lighting. The growing medium, Enhetsjord K (AB W. Plantin & Co, Oxie, Sweden), was a fully fertilized, fine-grained mixture of clay and peat, pH 6.3 ± 0.2. It was steamed at 100°C for 1 h. Seeds of differential hosts were sown in 9 x 9 cm pots of thermosetting plastic, 10 seeds/pot, four replicates. Each pot had a plastic tray of its own. Inoculation with the spore suspension, 10ml/pot, was performed after the sowing and the seeds were covered with a 0.5 cm layer of the growing medium. Throughout the growing period, the plants were watered by pouring water into the trays to keep the moisture even and to prevent the possibility of cross-contamination. The growing period was from four to six weeks. The galls were well developed, the 417 average size being 0.5 to 1.0 cm 3 . They were light in colour and usually contained a great number of spores. To conclude the test, the roots were washed with running water and rinsed with distilled water. The degree of clubroot infection of each plant was assessed on a scale of o—3,0 —3, and the clubroot index was calculated according to Williams (1966): clubroot index lQxn,+ 60xn2+ 100xn3 no+n, + n2+n3 n 0 = no clubs n, = a few small clubs on the secondary roots n 2 = considerable clubbing on the lateral roots n 3 = severe clubs on the primary and secon- dary roots This was the primary analysis. The clubs from the replicates of each differential host were combined to make a spore suspension, which was used to re-inoculate all four differential host plants, in accordance with the following scheme: If the result of this cross-testing was simi- lar to that of the primary analysis with respect to both degree of infection and clubroot in- dex, the race determination could be consid- ered to be reliable. If the result was not simi- lar, the cross-testing was repeated three to four times.2 Comparison with the ECD classification system Comparison with the ECD classification system, developed by the International Club- root Working Group (Buczacki et al. 1975) 2 Purified and identified race material was sent to the Scandinavian members of thisproject from 1974 onwards to be used in their resistance breeding work. Since 1980 the type isolate material has been deposited in the race bank at the Swedish University of Agricultural Sciences, Department of Resistance Biology, Alnarp, Sweden. was performed with some of our P. brassicae isolates (totalling 12) classified by Williams’ system. The seeds of ECD differential hosts in Brassica campestris, B. napus and B. oler- acea groups (totalling 15 hosts) were received in 1976 from Dr. H. Toxopeus, Instituut de Haaff, Stichting voor Plantenveredeling, Wageningen, the Netherlands. The test arrangement and the growth con- ditions were the same as in the tests performed with Williams’ method (p. 417). Results and their evaluation Occurrence of clubroot Regional distribution Of the 192 farms, 61 %, and of the collect- ed plant samples, altogether 375, 65 ®/o, showed occurrence of clubroot (Table 1, Fig. 1). Thus the study revealed that clubroot was quite common all over Finland. Although the locations inspected were distributed over the country, the number of samples from the var- ious areas differed so much that it is impossi- ble to draw far-reaching conclusions about regional differences in the occurrence of the disease. It can, however, be noted that clubroot was fairly common in the old, dense- ly populated farming areas, Uusimaa (N), Varsinais-Suomi (Ab) and Etela-Hame (Ta), and also in Etela-Savo (Sa) and Etela-Karjala (Ka). The disease was more severe in eastern Finland than in western Finland. This was possibly partly caused by cultivation tech- niques, mainly by the fewer opportunities for crop rotation on the farms in eastern Finland. In these areas many farmers had to give up the production of cruciferous vegetables al- together because of clubroot disease in the 1960 s and ’7os. It also turned out that clubroot was often more common, and even more severe, on smaller than on larger farms, one probable reason being the limited opportunities for crop rotation. This was evident, for example, in gardens around population centres, wherevar- 418 Table 1. Distribution of clubroot and occurrence of Plasmodiophora brassicae Wor. races in Finland, 1974—1978. Biological province l Communes inspected Farms inspected Samples P. brassicae races 123467 I—7 with with with total clubroot total clubroot total clubroot no. % no. % no. % no. of isolates total Ab Varsinais-Suomi 13 100 19 100 46 100 0 6 1 2 2 1 12 N Uusimaa 6 100 11 100 28 100 0 3 1 2 0 1 76 100 11 100 28 100 0 3 1 2 0 1 7 Ka Etela-Karjala 4 100 9 56 14 71 0 3 1 0 0 0 4 St Satakunta 12 75 21 48 26 42 0 4 0 4 0 0 8 Ta Etela-Hame 20 95 37 73 88 77 0 11 0 1 8 3 23 Sa Etela-Savo 8 50 13 54 20 55 0 1 4 0 0 1 6 Oa Etela-Pohjanmaa 12 58 27 37 44 32 0 2 0 2 0 2 6 Tb Pohjois-Hame 9 56 25 24 46 24 0 1 0 1 0 2 49 56 25 24 46 24 0 1 0 1 0 2 4 Sb Pohjois-Savo 7 100 10 80 19 84 1 1 4 0 2 0 8 Kb Pohjois-Karjala 6 83 10 60 23 52 0 7 1 0 0 0 8 Om Keski-Pohjanraaa 2505606500 1 0 0 0 0 1 Ob Pohjois-Pohjanmaa 2 100 5 100 15 100 0000 2 1 3 total/mean 101 81 192 61 375 65 1 40 12 12 14 11 90 Heikinheimo and Raatikainen (1971). ious cole species and cruciferous root crops had been grown in the same fields for many years. Clubroot in different plant species Cruciferous vegetables are cultivated almost throughout Finland up to 66—67° N. lat., but the mainproduction takes place in the south- ern part of the country. The areas of the most commonly cultivated crops in 1974—1978 (mean) are presented below (Statistics of the National Board of Agriculture (Finland).) cabbage cauliflower swede Southern Finland 554 328 1164 Ab N Ka St Ta Sa Central Finland 118 23 733 Oa Tb Sb Kb Northern Finland 29 10 72 Om Ob Ok Total area ha 701 361 1969 In the 1980s, the areas of cabbage and cauliflower remained roughly the same, to- talling some 1000 ha. The area of swede de- creased markedly, from almost 2000 to 400 ha. The cultivation of Chinese cabbage in- creased to 600 ha. The total area of the other cole species was some 80 ha and that of rad- ish and turnip totalled some 40 ha. The data on the occurrence of clubroot in differentplant species are presented in Table 2. Clubroot was found in 68 % of the cabbage samples, in 63 % of the cauliflower samples, in 56 % of the samples of other cole species, and in 64 °7o of the root crop samples. The dis- tribution and number of samples from differ- ent plant species roughly indicate the fre- quency of cultivation of these crops. Plasmodiophora brassicae races and their occurrence As in practice it was not possible, within the limits of the study, to carry out a race deter- mination of every clubroot sample, the sam- ples for tests were chosen to represent as many communes as possible in each biological prov- 419 420 Fig. I. Location of inspected farms growing cruciferous vegetables and occurrence of clubroot in Finland, 1974 1978. Table 2. Occurrence of clubroot in different plant species. Species Samples Plant species total with % of % with number clubroot samples clubroot Cabbage 209 143 56 68 Cauliflower 82 52 22 63 Broccoli 21 10 ' Red cabbage 10 8 Brussels sprouts 8 4 Kohlrabi 3 3 13 56 Curly kale 3 1 Marrowstem kale 2 1 Chinese cabbage 1 0 Swede 28 16 -i F U HH iP , • t M 10 64Fodder turnip 1 1 I Radish 2 2 J 375 245 ince, altogether 69 communes, or 84 % of the communes where clubroot was found. Most determinations were performed on club sam- ples of the crucifers cultivated most frequent- ly: cabbage and cauliflower. Some addition- al determinations were made on samples tak- en from less common species. The number of P. brassicae isolates totalled 90. Races The races isolated were 1,2, 3,4, 6 and 7 (Appendix 1). The identification results for races 1,2, 4 and 7 were clear. The test results for races 3 and 6 showed some deviation from the classification scheme. Several isolates from both of these races showed slight contamina- tion in Badger Shipper, although according to the scheme this differential host should have been resistant to these races. As the identification tests with certain iso- lates of other races also showed more varia- tion in the clubroot indices in Badger Ship- per than in other differential hosts, it seems that the seed material of Badger Shipper may have had some genetic heterogeneity. It is also possible that the isolate materials classified as races 3 and 6 included some pathotypes that were variants of the main races. The possibil- ity of mutation in the differential hosts or in the purified P. brassicae race isolates must also be taken into account. A preliminary report on the occurrence of P. brassicae races determined in the present study was presented at the Brassica conference 1981 (Linnasalmi and Toiviainen 1981). Regional distribution The regional distribution of the races is shown in Table 1 and on the map in Fig. 2. The communes from which the samples were taken and data on the original host are given in App. 1. Of the six races isolated, race 2 was by far the most common. It was found in 32 communes, i.e. in 46 % of the com- munes with clubroot. Races 3,4, 6 and 7 were each found in about ten communes (9—12 communes, i.e. 14—17 %); race 1 occurred only once. Two different races were found in only a few communes: races 2 and 3 in Nur- mijarvi (N) and Joutseno (Sa), races 2 and 6 in Haukivuori (Sb), races 6 and 7 in Hartola (Ta), Kangasala (Ta) and Oulu (Ob). On the basis of this study, no clear prevalence of races can be demonstrated in different parts of Fin- land. All races were distributed fairly evenly over the country, with the exception of the 421 422 Fig. 2. Occurrence of Plasmodiophora brassicae Wor. races in Finland, 1974—1978. rare race 1. However, race 4 seems to show some prevalence in the western parts of the country, while race 3 occurs mostly in the east. In the preliminary studies conducted in 1971 —1972 on the occurrence of the P. brassicae races in Finland (Linnasalmi and Palonen 1974), the races 2,4, 6 and 7 were isolated. In two test places the races that were found in 1972 were, according to the present study, still the same, viz. race 4 in the test field of the Institute of Plant Pathology in Tikkurila (Vantaa) and race 7 in the test field of the In- stitute of Horticulture in Piikkio. Ten years earlier, in 1964, samples from these localities were sent to Prof. P.H. Williams. He iden- tified race 7 from Piikkio and race 2 from Tik- kurila (Williams 1966). Occurrence in different plant species The races displayed the following distribu- tion by different plant species: races 2,3, 4, 6 and 7 were isolated from cabbage (60 iso- lates) and cauliflower (17 isolates); races 2, 3 and 7 from broccoli (4 isolates); race 4 from red cabbage and marrowstem kale (1 isolate from each); races 1, 2 and 4 from swede (5 isolates); and race 2 from turnip (2 isolates). Of the cabbage cultivars grown in Finland, (cf. App. 2) only the Norwegian Resista and Respla (Weisaeth 1977) are partly resistant to some P. hrassicae races. These cultivars were accepted for marketing in Norway in 1973 and in Finland in 1975. In our infection tests (glasshouse tests) with some of the P. brassicae races occurring in Finland, both cul- tivars were found to have some resistance to races 2, 4 and 7 when compared with the se- verely contaminated Bldtopp cultivar: race (isol.) cultivar, clubroot index (0 —3) Resista Respla Bl&topp (control) 2 (F 118) 1.80 3 (F 147) 2.36 4 (F 155) 1.95 7 (F 171) 1.86 1.47 2.88 2.49 2.96 1.14 2.78 0.84 2.52 In some farms where these cultivars were grown in field sectors situated next to each other, severe contamination and consequent- ly weaker growth of Blatopp were evident, whereas Resista and Respla were only sparse- ly and slightly contaminated, and headed well. The occurrence of P. brassicae races determined according to Williams’ system in other countries Norway and Iceland According to the working plan of NKJ pro- ject 27, the determinationof P. brassicae races in Norwegian material was performed in Fin- land. The samples had mostly been taken from resistance breeding test fields in differ- ent parts of the country by the Norwegian pro- ject member, First Amanuensis G. Weisaeth. Half of the samples represented the breeding lines of cabbage. The test fields were located in 17 communes, and the number of samples totalled 38. The race distribution was as follows: race communes isolates host plants no. no. 1 5 6 cabbage, swede 2 3 5 cabbage, swede, rape 4 13 21 cabbage, cauliflower, kohlrabi, swede 7 35 cabbage 9 1 1 swede Detailed results from the race determina- tions of the Norwegian material, together with purified club material, were supplied annually to the Department of Vegetable Crops, Agri- cultural University of Norway, As, Norway. Some isolates from this material were used in our ECD test series (Table 3). A report on the occurrence of races 1, 7 and 9 in Norway, in the Trondelag region, is giv- en in the publication of Linnasalmi and Wei- saeth (1978). On the basis of our new sam- ple material (cf. data given above), it would seem that race 4 is more common than the others elsewhere in Norway. There were some 423 424 differences between the Finnish and Norwegian race spectra: race 9 was not found in Finland, and races 3 and 6, which are fairly common in Finland, were not found in the Norwegian material. However, data on races 3, 5 and 6, as well as races 1 and 9, are presented in some earlier studies on the Norwegian race spectrum (Weisaeth 1972). Two cabbage samples (Wei- saeth’s breeding lines) from Iceland were analysed. Only race 7 was found in both sam- ples (Linnasalmi and Weisaeth 1978). Sweden in connection with breeding work for clubroot resistance in cruciferous oil crops, R. JOnsson investigated the occurrence of P. brassicae races in Skane, in southern Sweden. Using the method of Williams, JOnsson (1971, 1972) concluded that in the populations col- lected from various localities there was the possibility of occurrence of several P. brassi- cae races, and definite occurrence of race 15 in one case. Using the callus technique, JOns- son isolated races 1,2, 3,4, 6 and 7 (JOnsson 1981). Other countries In addition to the Nordic countries, infor- mation about the occurrence of P. brassicae races has been published from the following countries: USA: races 6 and 7 (Williams and Walker 1963, Strandberg and Williams 1967) Canada: races 1,2, 3,4, 6 and 6A (Ayers 1972), races 2 and 6 (Chiang and CrEte 1972, Reyes et al. 1974); Japan: race 2 (Yoshikawa and Buczacki 1978); GDR: races 1,3, 4,6, 7 and 9 (Williams and Seidel 1968) Poland: races 2,4, 6 and 7 (Nowicki 1978); USSR: races 1, 6 and 7 (Krivchenko et al. 1982). Moreover, mention should be made of the race identifications made by Williams (1966) on materials from the fol- lowing countries: Czechoslovakia race 6; the UK race 1; FRG race 7; USSR race 2; Aus- tralia races 3, 6 and 7; New Zealand races 1, 2 and 4; Japan races 3 and 5. Comparison between the Williams’ and ECD classification systems In this study with the P. brassicae race ma- Table 3. Comparison between the Williams’ and ECD systems. Isolate Race Williams’ system ECD code Clubr. ind. in diff. hosts Diff. host groups JQ BS L W B.c. B.n. 8.0. N P74048 C 1 22 0 83 86 16 31 12 F 117 2 97 92 100 0 16 —o3 31 F 118 2 90 93 97 0 16 19 —3l FI 2 79 83 97 0 16 19 31 F 147 3 96 1 99 0 16 18 —3O F 155 4 89 68 94 80 16 31 —3l F 168 4 76 60 82 74 17 —3l —3l F 106 6 68 0 0 0 16 —oo 30 F 171 7 94 86 0 0 16 —oo 31 N P74046 7 94 88 0 0 16 02 31 I P74040 7 86 70 0 0 16 —o2 31 I P74043 7 91 69 0 0 16 —o3 31 Diff. host symbols cf. App. 1. B.c. Brassica campestris; B.n. Brassica napus; 8.0. Brassica cleracea; ECD codes according to Toxopeus 1974. Origin of isolates from Finland (F) in App. 1; from Norway (N) P74048 C Jersey Queen (Asker) and P74046 Badger Shipper (Stjordal); from Iceland (I) P74040 cabbage lines 666 Weisaeth and P74043 line 696 Weisaeth (Hruna- mannahreppur). 3 terial classified according to Williams’ sys- been reported from Canada by Chiang and tem (1966), a comparative test series was car- ried out using the ECD classification system (Buczacki et al. 1975). Pure isolates of races 1,2, 3,4, 6 and 7 (12 in total) were chosen for the tests. The results are shown in Table 3. In the Brassica campestris group, theresis- tance and susceptibility of all differential hosts to the various races are the same, code num- ber 16, with exception of one race 4 isolate, for which the code is 17 because of the sus- ceptibility of differential host 01. In the Brassica napus group, the differen- tial hosts show more diversity in their reac- tions to the different races, but there is also variability with regard to isolates of the same race. For example, the codes for race 7 are 00, 02 and 03. Number 03 is also the code of one of the isolates of race 2. For the isolates of race 2, the reactions of the ECD differential hosts are to a large extent borderline cases be- tween susceptibility and resistance, and there- fore the code can be either 03 or 19. The differential hosts of the group do not serve to separate races 1 and 4. All are 100 % suscep- tible, code 31. In the Brassica oleracea group, some differ- ences were found. The code numbers are 12, 30 and 31. Code 12 applies only to race 1, but code number 30 to both races 3 and 6, and code 31 to races 2, 4 and 7. Few results corresponding to those of our ECD test are found in publications from other countries. Code 16-31-12, which corresponds to race 1 in our material (isolate from Nor- wegian material), has been reported from France (Rouxel et al. 1983). Code 16-31-31, corresponding to one of our race 4 isolates, has been reported from the USA (Campbell et al. 1981) and from Scotland, UK (Broken- shire and Lewis 1981), and code 17-31-31, corresponding to the other race 4 isolate, also from the UK (Brokenshire and Lewis 1981, Dixon et al. 1981). The codes 16-02-31 and 16-03-31,corresponding to our race 7 (isolates from the Norwegian and Icelandic materials), were reported by Dobson et al. (1983) from the USA. Data on code 16-02-31 have also CrEte (1983), who, however, mention it as the ECD code of their race 2 (sensu Wil- liams). The survey of Toxopeus et al. (1986) of the ECD tests performed mostly in West- ern Europe up to 1982 includes code 16-31-12 (four occurrences), corresponding to our race I; code 16-19-31 (two occurrences), corre- sponding to our race 2; codes 16-31-31 and 17-31-31 (65 and 5 occurrences respectively), corresponding to our race 4; codes 16-00-31, 16-02-31 and 16-03-31 (3, 51 and 13 occur- rences respectively), corresponding to our race 7. In addition to the studies mentioned above, determinations of P. brassicae pathotypes by the ECD system have been reported in the fol- lowing publications: Toxopeus (1974) and Toxopeus and Jansen (1975) from the Nether- lands, Heyn (1981) from the FRG, Jones et al. (1982 a) from the UK, Naiki et al. (1984) from Japan and Lammerink (1986) from New Zealand. In comparing the advantages and disadvan- tages of Williams’ method with the ECD method for isolating and classifying P. bras- sicae pathotypes, it can be concluded that Williams’ system is a more time-saving meth- od. By the cross-testing technique described previously (p. 418), it was possible to detect the main pathotypes in a large area (approx. 3 000 ha) by means of only four differential hosts. On the other hand, it is evident that with such a restricted set of differential hosts it is not possible to identify all possible patho- types and pathotype variants with certainty. The ECD system, with its fifteen differential hosts, is much more laborious and the require- ment for growing space is many times greater than with Williams’ method. Even more seri- ous is the fact that in our comparative test se- ries, which was carried out with very clear and pure race isolates classified according to Wil- liams’ system, the ECD results showed uncer- tainties and contradictions, as can be seen in Table 3 and in the report of the results. Soon after introduction of the ECD system, other workers also began to draw attention to 425 the uncertainties of the pathotype determina- tions obtained by the method. The studies of Tinggal and Webster (1981) showed that P. brassicae populations identified by the ECD method and assumed to be pure isolates in fact contained several pathotypes. Similarly, Dixon et al. (1981) and Jones et al. (1982 a, b) found mixtures of pathotypes in their collec- tions. Toxopeus et al. (1986) mention uncer- tainties in coding P. brassicae populations by the ECD system. Worth mention is also the critical assessment of the limitations of the ECD method by Crute et al. (1980). Discussion Regardless of the classification system em- ployed, one of the most important require- ments for thereliable determination of patho- types is that a genetically uniform inoculum material can be obtained. One way to achieve this is the single spore technique starting from the resting spores of the P. brassicae fungus. Difficulties have been encountered in develop- ing the technique. In the studies of Buczacki (1977), Tinggal and Webster (1981) and Jones et al. (1982 b), with ECD population materials, in which Brassica napus and B. campestris varieties were used as test plants, the results have not been very promising; the infection rate remained low, 20—30 % at best (Tinggal and Webster). Moreover, Tinggal and Webster found that when two single spore isolates from ECD populations were tested further, roughly one half gave a result that corresponded with the original code, whereas four new races differentiated from one of the populations and two new races from the other. The single spore technique can obviously be improved, but judgingby what is known thus far about the multistage internal life cycle of P. brassicae and its development in the host plant (Ingram and Tommerup 1972, Ingram 1978), and about the microstructure of the fungus as revealed by electron microscopy (Dekhuijzen 1979, Ikegami et al. 1978, Buc- zacki et al. 1979), it may be difficult to achieve a homozygotic fungus material. Theo- retically there are many possibilities for differ- ent recombinations of differential pathogenic- ity genes (cf. Crute et al. 1980 with refer- ences, Tinggal and Webster 1981). The significance of the genetic properties of the host plants used in the P. brassicae infec- tion studies began to receive attention in the 19505. Macfarlane (1955) concluded that in certain cases the heterogeneity of the host plant population can cause variation in the in- fection results. In the 1960 s Williams (1966), among others, stressed the importance of ge- netically uniform differential host materials in pathotype determinations. Since our preliminary work on P. brassicae races (Lin- nasalmi and Palonen 1974), our aim has been to use homozygotic seed material of the differential hosts (cf. p. 417). This criterion was not always taken into account prior to the 1980s. Difficulties have been encountered in the production of homozygotic test plant lines by conventional methods. According to Crute et al. (1980), heterogeneity is apparent in some differential host lines of the B. oleracea group, possible in the B. campestris group and less likely only in the B. napus group, because the species is strongly inbreeding. However, new developments have improved the prospects of producing homozygotic test plant material that has the appropriate resis- tance/susceptibility qualities. For instance, the production of haploid plants from anther cul- tures via embryogenesis will allow rapid estab- lishment of pure lines. So far haploid plants have been obtained from various Brassicaceae species, e.g. B. napus, B. campestris, B. oler- acea var. italica (Keller and Armstrong 1977, 1979, 1983), B. oleracea var. gemmifera (Ockendon 1984) and B. oleracea var. capi- tata (Chiang et al. 1985). The rapid advance of plant molecular bio- chemistry and genetics offers new opportuni- ties for studying questions of resistance to dis- eases. One of the new possibilities already in sight is the application of gene technology to 426 modify the genom of a plant directly as desired. It remains to be seen how long it will take before the prerequisites exist for apply- ing these techniques in the breeding of clubroot resistant cruciferous vegetable culti- vars. Acknowledgements. This study was partly funded by the National Research Council for Agriculture and For- estry of the Academy of Finland, which we acknowledge with gratitude. We express our sincere thanks to the or- ganizations and persons who have given valuable help in our work. We are especially grateful for the skilful tech- nical assistance of Ms. Kirsti Nieminen. 427 428 Appendix 1. Clubroot (Plasmodiophora brassicae Wor.) races in Finland, 1974—1978. Differential plants: Jersey Queen JQ, Badger Shipper BS, Laurentian L, Wilhelmsburger W Locality Original host Clubroot index' Race sample no. ~ ~ ~ 777 species cultivar JQ BS L W Pohjois-Savo Sb 193 Virtasalmi swede Mustiala 18 0 81 67 1 Varsinais-Suomi Ab 97 Lohja cauliflower Erfurter 56 37 60 0 2 253 Masku cauliflower 87 65 88 0 2 162 Raisio cauliflower Flora Blanca 93 94 96 0 2 169 Rusko cabbage Amager, hog 99 94 99 0 2 11 Rymattyla cauliflower Flora Blanca 47 68 78 0 2 163 Sauvo cauliflower Flora Blanca 94 97 96 0 2 Uusimaa N 1 Hyvinkaa cauliflower Erfurter 79 83 97 0 2 76 Nurinijarvi cabbage Vasternordland 93 87 95 0 2 86 Nurmijarvi cabbage Kobenhavns Torve 90 57 77 0 2 Elela-Karjala Ka 154 Vehkalahti cabbage Ruhm von Enkhuizen 93 91 94 0 2 157 Vehkalahti swede Pandur 87 89 96 0 2 158 Anjalankoski cabbage Bldtopp Faales 94 94 99 0 2 Satakunta St 196 Fluittinen cabbage Futura 96 96 99 0 2 224 Kiikka marrowstem kale 78 94 68 0 2 227 Lappi T 1 cauliflower 91 83 85 0 2 221 Mouhijarvi broccoli Greenia 53 71 41 0 2 Eteld-Harne Ta 175 Asikkala cabbage Kobenhavns Torve 85 70 87 0 2 176 Asikkala cabbage Biatopp Faales 95 92 94 0 2 118 Hattula cabbage Ruhm von Enkhuizen 90 93 97 0 2 194 Kuhmoinen turnip 93 69 96 0 2 205 Kuorevesi turnip Guldbail 95 95 100 0 2 207 Lammi cabbage Bldtopp Faales 84 86 93 0 2 159 Orivesi cabbage 85 73 92 0 2 117 Sahalahti cabbage Bldtopp Faales 97 92 100 0 2 199 Sahalahti cabbage Bldtopp 94 89 99 0 2 209 Tuulos cabbage 93 95 94 0 2 126 Tampere cabbage 98 94 99 2 2 Eteld-Savo Sa 143 Joutseno cabbage Golden Acre 97 29 86 0 2 Etela-Pohjanmaa Oa 233 Jalasjdrvi cauliflower 74 71 67 0 2 236 Vahakyro cabbage 99 99 99 1 2 Pohjois-Htime Tb 214 Toivakka cabbage 95 56 99 0 2 Pohjois-Savo Sb 192 Flaukivuori cabbage Amager 81 68 94 0 2 Pohjois-Karjala Kb 63 Eno swede Ostgota 70 49 74 0 2 182 Pyhdselkd cabbage Bldtopp Faales 93 75 95 0 2 183 Pyhaselka cabbage Vasternordland 86 57 94 0 2 177 Raakkyla cabbage Kobenhavns Torve 95 28 97 0 2 178 Rddkkyld cabbage Bldtopp Faales 98 68 94 0 2 179 Raakkyla cabbage Ruhm von Enkhuizen 81 92 88 0 2 180 Raakkyla cabbage Golden Acre 93 95 98 0 2 Each index is the mean of four replicates. 429 Locality Original host Clubroot index 1 Race sample no. species cultivar JQ BS L W Keski-Pohjanmaa Om 243 Kalvia cabbage Biatopp Faales 86 78 89 0 2 Varsinais-Suomi Ab 80 Vihti cabbage Biatopp Faales 90 5 24 0 3 Uusimaa N 70 Nurmijarvi cabbage Biatopp Faales 88 6 90 0 3 ElelH-Karjala Ka 149 Kymi cabbage Biatopp Faales 95 9 93 0 3 Etelit-Savo Sa 146 Imatra cabbage Golden Acre 72 1 66 0 3 147 Imatra cabbage Vasternordland 96 1 99 0 3 142 Joutseno broccoli Greenia 83 2 63 0 3 144 Joutseno cabbage Ruhm von Enkhuizen 38 0 60 0 3 Pohjois-Savo Sb 58 Karttula cabbage 84 6 93 0 3 197 Kuopio cabbage Biatopp Faales 91 4 97 0 3 139 Maaninka cabbage 87 2 85 0 3 198 Suonenjoki cabbage Biatopp Faales 92 11 67 0 3 Pohjois-Karjala Kb 69 Joensuu broccoli Waltham 87 5 91 0 3 Varsinais-Suomi Ab 228 Laitila cauliflower 61 23 85 71 4 168 Turku cabbage Amager hog 76 60 82 74 4 Uusimaa N 246 Tuusula swede 49 34 82 69 4 155 Vantaa cabbage Biatopp Faales 89 68 94 80 4 Salakunla St 225 Eura cauliflower Flora Blanca 72 43 84 84 4 223 Kokemaki red cabbage 88 78 97 77 4 195 KOyliO cauliflower Igloo 81 54 98 97 4 229 Rauma swede 69 63 83 9 4 Etela-Hcime Ta 220 Loppi cabbage 85 58 95 81 4 Etelii-Pohjanmaa Oa 234 Ilmajoki cauliflower 49 31 90 71 4 230 Isokyro cabbage Marner Sepco 63 27 93 84 4 Pohjois-Hiime Tb 105 Jyvaskyla cabbage 82 30 87 85 4 Varsinais-Suomi Ab 21 Sammatti cabbage Golden Acre 47 0 0 0 6 22 Sammatti cabbage Golden Acre 91 5 0 0 6 EtelH-Hiime Ta 55 Hartola cabbage Igloo 26 0 0 0 6 122 Hauho cabbage Biatopp Faales 61 2 1 0 6 112 Kangasala cabbage Biatopp Faales 59 0 0 0 6 124 Kangasala cabbage Biatopp Faales 98 4 0 0 6 106 Palkane cabbage Vasternordland 68 0 0 0 6 107 Palkane cabbage 25 0 0 0 6 108 Palkane cabbage Biatopp Faales 52 1 0 0 6 170 Toijala cabbage 91 3 0 0 6 1 Each index is the mean of four replicates. 430 Locality Original host Clubroot index 1 Race sample no. T ~ TT ~ ' ITT species cultivar JQ BS L W Pohjois-Savo Sb 191 Haukivuori cabbage Bldtopp Faales 57 2 3 0 6 188 Joroinen cauliflower Erfurter 48 1 0 0 6 Pohjois-Pohjanmaa Ob 67 Oulu cauliflower 84 6 0 0 6 250 Oulu cabbage 100 9 0 0 6 Varsinais-Suomi Ab 171 Piikkid cabbage TK 499 Weisaeth 94 86 0 0 7 Uusimaa N 88 Espoo cauliflower 97 19 0 0 7 Etela-Hame Ta 174 Hartola cabbage Toftegard 94 51 0 0 204 Langelmaki cauliflower Erfurter Dvarg 90 13 0 0 7 38 Kangasala cabbage Resista 100 27 0 0 ElelH-Savo Sa 148 Lemi cabbage Bldtopp Faales 100 24 2 0 7 Etela-Pohjanmaa Oa 231 Alavus cabbage 84 64 3 1 7 238 Laihia cabbage Futura 99 31 0 0 7 Pohjois-Hdme Tb 216 Jyvaskylan mlk broccoli 98 16 3 0 7 211 Keuruu cabbage Bldtopp Faales 100 21 0 0 Pohjois-Pohjanmaa Ob 248 Oulu cauliflower Erfurter Dvarg 97 57 3 0 7 1 Each index is the mean of four replicates. Appendix 2. The list of species and cultivars. Scientific nomenclature according to Zander Handworterbuch der Pflanzennamen (Encke et al. 1981). Brassica oleracea L. Cabbage convar. capitata (L.) Alef. var. capitata: Amager, Amager hog, Amager Stenhoved, Amager Toten, BlStopp, Bldtopp Faales, Dala, Dima, Ditmarsker, Futura, Golden Acre, Haloyen Lundes, Kobenhavns Torve, Marner Sepco, Resista, Respla, Ruhm von Enkhuizen, Septem- ber, Toftegdrd, Vasternordland Red cabbage Cauliflower /. rubra: Amager Tagenshus, Haco convar. botrytis (L.) Alef. var. botrytis: Erfurter, Erfurter Dvarg, Flora Blanca, Grandessa, Igloo, Stor dansk var. italica Plenek: Greenia, WalthamBroccoli Brussels sprouts convar. oleracea var. gemmifera DC.: Jade convar. acephala (DC.) Alef. Kohlrabi var. gongyiodesL.: Wiener glas var. sabellica L.: cv. unknown var. viridis L.: cv. unknown Curly kale Marrowstem kale Chinese cabbage Brassica pekinensis (Lour.) Rupr.: cv. unknown Brassica napus L. Swede var. napobrassica (L.) Rchb.: Messukylan lanttu, Mustiala, Pandur, Svensk gul, Wilhelmsburger, Ostgota Brass ica campestris L. var. rapifera Metz.Turnip syn. B. rapa L. subsp. rapa: turnip Guldbill, fodder turnip cv. unknown Raphanus sativus L. Radish var. radicula Pers.: cv. unknown 431 References Ayers, G.W. 1972. Races of Plasmodiophora brassicae infecting crucifer crops in Canada. Canad. Plant. Dis. Survey 52: 77—81. Brokenshire, T. & Lewis, S.J. 1981. Clubroot popula- tions in S.E. Scotland. Proc. Brassica Conf. 1981: 12—23. Buczacki, S.T. 1977. Root infections from single resting spores of Plasmodiophora brassicae. Trans. Brit. Mycol. Soc. 69: 328—329. —, Moxham, S.E. & Turner, R.H. 1979. Some morpho- logical features of the resting spore of Plasmodiophora brassicae. Trans. Brit. Mycol. Soc. 73: 343—347. —, Toxopeus, H., Mattusch, P., Jonston, T.D., Dixon, G.R. & Hobolth, L.A. 1975. Study of physiologic specialization in Plasmodiophora brassicae: proposals for attempted rationalization through an international approach. Trans. Brit. Mycol. Soc. 65: 295—303. Campbell, R.N., Greathead, A.S. & Myers, D.F. 1981. Clubroot in California. 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Ms received July 18, 1991 433 SELOSTUS Mohojuuritaudin ja Plasmodiophora brassicae Wor. rotujen esiintyminen Suomessa Annikki Linnasalmi ja Anneli Toiviainen Maatalouden tutkimuskeskus, Kasvinsuojelun tutkimus- laitos, Kasvitautien torjunnan tutkimusala Jokioinen, Suomi Tutkimuksessa selvitettiin mohojuuritaudin ja Plas- modiophorabrassicae Wor. patotyyppien esiintymista ris- tikukkaisissa (Brassicaceae) vihanneskasveissa Suomes- sav. 1974—1978. Tarkastetuista 101 kunnasta 81 %:ssa esiintyi mohojuurta.Tauti oli yleisinta maan etela- ja kes- kiosissa, mutta sita tavattiin myos ristikukkaiskasvien vil- jelyalueen pohjoisosissa (66 —67° Pohj. lev.). Kertyneis- ta 375 kasvinaytteesta 65 % oli mohojuurisia. Yleisim- min viljellyn kasvilajin, kerakaalin naytteista (56 % ai- neistosta) 68 %, kukkakaalinaytteista (22 % aineistosta) 63 %, muiden kaalilajien naytteista (13 % aineistosta) 56 % jaristikukkaisista juurikasnaytteista (19 % aineis- tosta) 64 % oli mohojuurisia.P. brassicae rotumaaritykset tehtiin 90 naytteesta. Kaytettiin Williamsin(1966) luoki- tussysteemia. Eristetyt rodut olivat 1,2, 3,4, 6 ja7. Ro- tu 2 oli ylivoimaisesti yleisin. Sita tavattiin 32 kunnassa, rotuja 3,4, 6 ja 7 kutakin 9—12 kunnassa, rotua 1 vain yhdessa kunnassa. Selvia eroja rotujen esiintymisessa maan eri osissa ei voitu todeta. Suoritettiin Williamsin ja ECD (Buczacki et al. 1975) luokitussysteemien vertai- !u. Williamsin systeemi todettiin nopeammaksi ja luotet- tavammaksi kuin ECD systeemi P. brassicae patotyyp- pien identifioimiseksi. Patotyypit maaritettiin myOs Nor- jassa ja Islannissa keratysta mdhojuuriaineistosta. Tutkimus on osa pohjoismaisesta mohojuuriprojektis- ta, NKJ-projekti 27 1974—1977: Mohojuurenkestavyys- jalostus, Plasmodiophora brassicae Wor. rodut seka uusien torjunta-aineiden tehotutkimus. 434