possible that the disease has been spread from infected imported lettuce.
206
The results showed that virulence factors vl to 11 are ali represented in the Finnish B. lactucae population but there were dif-ferences in their frequency. The fact that factor v2 was present in every isolate is probably a result of R2 being present in cultivars, e.g. Amanda, Larganda and Noran, commonly grown at the time the population was sampled. By comparison, vl, 4 and 6 were ali present at lower frequencies than have been recorded in some other surveys (Dixon and Wright 1978, Wellving and Crute 1978, Crute and Dixon 1978).
The resistance factor R1 primarily charac-terised by the resistance of cv. Blondine is rarely recorded as effective against B.
lactucae isolates. In other words, vl, the virulence factor matching R1 and rendering it ineffective is very frequent and widely distributed throughout the pathogen popula-tion. In the UK, vl was shown to be present at a frequency of 1,00 (i.e. in a survey carried out between 1976 and 1978 ali isolates tested were pathogenic on cv. Blondine) (Crute and Dixon 1978). The best documented isolate which Blondine is resistant to is NL3 which has the virulence phenotype v: 5, 6, 7, 8, 10 (Crute and Johnson 1976, Blok and van der Schaaf-van Waadenoyen Kernekamp 1977).
At NVRS, a UK isolate (IM 43) which has the same virulence phenotype has been studied.
Since these two isolates also lack v2, 3 and 4, any cultivar which is resistant to these isolates by „virtue of R2, 3 or 4 or any com-bination of these factors (proved following experiments with other isolates) may also carry R1 undetected (see also footnote e, Table 4 in Crute and Johnson 1976). The reaction patterns of the isolates of B. lactucae
from Finland suggest that this may have oc-curred. Since R1 is rarely effective against B. lactucae in Europe at least, these findings have little practical relevance. They dem-onstrate, however, that the presence of R1 previously undetected in certain commercial lettuce cultivars, may have resulted in selec-tion for vl and hence explain more readily its frequent occurrence in the population.
In common with other regions of Europe where studies have been conducted, factor v11 was found to be present at the lowest frequency. The relatively new factor, R11 particularly combined with other R-factors, R4, 6 or 10, is therefore most likely to pro-vide protection from the disease in Finland.
Variation for the field susceptibility of lettuce cultivars was demonstrated when a range of cultivars was inoculated with the multivirulent isolate SF6. However, when cultivars resistant by virtue of R11 were eliminated from the analysis there was no correlation between disease severity on seed-lings and that on adult plants. There have been comparatively few studies on the rela-tive field susceptibility of lettuce cultivars, but those reported confirm that variation exists which could possibly be exploited both by plant breeders and growers (Dixon et al.
1973, Crute and Norwood 1981).
Acknowledgements. — K. 0. would like to thank Di. Ake Wellving (Swedish Seed Association, Svalöv, Sweden) for advice on techniques; 1Vliss Ida Blok (IPO, Wageningen, Holland), Oy Hortus Ab and other seed firms for providing seeds used in tests and Mrs. Elli Laine for technical as-sistance. The financial support of the Academy of Finland and of the Finnish Association of Academic Agronomists is gratefully acknowledged.
207
REFERENCES Blok, I. & Schaaf van der - Waadenoyen
Kerne-kamp van, K. 1977. Problemen rond Bremia in sla. Zaadbelangen 31: 57-58.
Crute, I. R. & Davis, A. A. 1976. New virulence gene combinations in British isolates of Bremia /actucae. Ann. Appi. Biol. 83: 173-175.
& Dixon, G. R. 1978.. Specific virulence in the British Bremia lactucae population. Abstr. 3rd Intern. Congr. Pl. Path. Milnchen. p. 293.
& Johnson, A. G. 1976. The genetic relation-ship between races of Bremia lactucae and cultivars of Lactuca saava. Ann. Appi. Biol.
83: 125-137.
& Norwood, J. M. 1978. Incomplete specific resistance to Bremia /actucae in lettuce. Ann.
Appi. Biol. 89: 467-474.
& Nondwood, J. M. 1981. The identification and characteristics of field resistance to lettuce downy mildew (Bremia lactucae Regel). Euphy-tica 30. (in press).
Dickinson, C. H. & Crute, I. R. 1974. Influence of seedling age and development on the infection of lettuce by Bremia tactucae. Ann. Appi. Biol.
76: 49-61.
Dixon, G. R. & Doodson, J. K. 1971. Assesment keys for some disease of vegetable, fodder and herbage crops. J. Nat. Inst. Agric. Bot. 12:
299-307.
& Doodson, J. K. 1973. Reaction of lettuce cultivars to Bremia tactucae Regel, and varia-tion within races of the pathogen. Hart. Res.
13: 89-95.
, Tonkin, M. H. & Doodson, G. K. 1973. Colo-nisation of adult lettuce plants by Bremia lac-tucae. Ann. Appi. Biol. 74: 307-313.
& Wright, I. R. 1978. Frequence and geo-graphical distribution of specific virulence fac-tors in Bremia /actucae populations in England 1973 to 1975. Ann. Appl. Biol. 88: 287-294.
Johnson, A. G., Crute, I. R. & Gordon, P. L. 1977.
The genetics of race specific resistance in let-tuce (Lactuca sativa) to downy mildew (Bremia lactucae). Ann. Appi. Biol. 86: 87-103.
, Laxton, S. A., Crute, I. R., Gordon, P. L. &
Norwood, J. M. 1978. Further work on the genetics of race specific resistance in lettuce (Lactuca sativa) to downy mildew (Bremia lac-tucae). Ann. Appl. Biol. 89: 257-264.
Lebeda, A. 1979. The occurrence of new races of Bremia lactucae in Czechoslovakia. Z. P11.-krankh. und Pfl.schutz 86: 729-734.
1980. Establishment of virulence phenotypes in races of Bremia lactucae CS1; N1N6 and N.
Phytopath. Z. 97: 289-294.
1981. Population genetics of lettuce downy mildew (Bremia lactucae). Phytopath. Z. (in press).
Norwood, J. M. & Crute, I. R. 1980. Linkage be-tween genes for resistance to downy mildew (Bremta lactucae) in lettuce. Ann. Appi. Biol.
94: 127-135.
Tjallingii, F. & Rodenburg, C. M. 1969. Onderzoek van slarassen op vatbaarheid your vier fysios van valse meeldaw (Bremia lactucae). Zaadbe-langen 23: 436-438.
Vanhanen, R. 197'7. Equipment for applying liquid fungicides to small amounts of seed of seed grain. Ann. Agric. Fenn. 16: 199-206.
Wellving, Ä. & Crute, I. R. 1978. The virulence eharacteristics of Bremia lactucae populations present in Sweden from 1971 to 1976. Ann.
Appi. Biol. 89: 251-256.
Wolf e, M. S., Barrett, J. A., Shattock, R. C., Shaw, D. S. & Whitebread, R. 1976. Phenotype-phenotype analysis: field application of the gene-for-gene hypothesis in host-pathogen re-lations. Ann. Appl. Biol. 82: 369-374.
Manuscript received April 1981 Kirsti Osara
Agricultural Research Centre Institute of Plant Pathology SF-01300 Vantaa 30, Finland Ian R. Crute
National Vegetable Research Station Wellesbourne
Warwick, U.K.
208
SELOSTUS
Salaatin lehtihomeen aiheuttajan Bremia lactucae saastutuskyvyn vaihtelusta
KIRSTI OSARA ja IAN R. CRUTE
Maatalouden tutkimuskeskus ja National Vegetable Research Station
Salaatin lehtihomeen (Bremia lactucae Regel) le--vinneisyyttä selvitettiin vuosina 1972-1976. Sa-laattinäytteitä saatiin eri puolilta maata, pääosan ollessa Etelä-Suomesta kerättyjä. Tautia tavat-tiin ainoastaan Uudenmaan läänin alueella 13 vil-jelmällä. Tutkituista näytteistä oli 38,5 Vo taudin-aiheuttajan saastuttamia. Tautia esiintyi avomaal-la syyskuussa ja kasvihuoneissa syys-lokakuussa.
Testikasvien avulla määritettiin sienieristyksis-tä kuusi virulenssifenotyyppiä SF1-6, joista ai-noastaan SF1 on aikaisemmin selostettu kirjalli-suudessa.
Virulenssitekijöiden, v1-11 lukumäärät vaih-telivat eri fenotyypeissä 2-10 tekijään. Tietyt tekijät esiintyivät toisia lukuisammin. Poiketen aikaisemmin Euroopassa julkaistuista tiedoista,
tekijä vl puuttui osasta isolaatteja, kun taas te-kijä v2 esiintyi kaikissa isolaateissa.
Täydellinen taudinkestävyys . on saavutettavissa viljelemällä lajikkeita, joilla resistenssitekijä R11 on yhdessä R4, 6 tai 10 kanssa.
Sirkkalehtiasteella olevilla taimilla määritettiin 126 salaattilajikkeen taudinkestävyys erikseen jo-kaista patogeenista fenotyyppiä vastaan. Testauk-sessa ilmeni, että eräät lajikkeet sisältävät ennen oletettujen resistenssitekijöiden ohella myös
teki-jän Rl.
Muovihuoneessa kasvatettujen salaattilajikkei-den kenttäkestävyydessä oli merkitsevä ero feno-tyyppiä SF6 vastaan, jossa on virulenssi-tekijät v1-10.
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ANNALES AGRICULTURAE FENNIAE, VOL. 20: 210-213 (1981) Seria PHYTOPATHOLOGIA N. 87— Sarja KASVITAUDIT n:o 87
RESEARCH NOTE
THE FIRST REPORT OF LETTUCE MOSAIC VIRUS IN FINLAND MARJA-LEENA LAHDENPERÄ
Lahdenperä, M-L. 1981. The first report of lettuce mosaic virus in Fin-land. Ann. Agric. Fenn. 20: 210-213. (Agric. Res. Centre, Inst. Pl. Path., SF-01300 Vantaa 30, Finland).
A virus disease occurring in some lettuce samples sent by growers in Finland proved to be lettuce mosaic. The virus was identified on the basis of the symptoms it induced in various test plants and by determining its thermal inactivation point and measuring the length of the virus particles in electron microscope.
Index word: Lettuce mosaic virus.
Samples of diseased lettuce grown under glass, cultivar unknown, were sent to the Institute of Plant Pathology in 1980 by a farmer in Varkaus in eastern Finland. Let-tuces with somewhat similar symptoms have been sent earlier, too, but the cause of the disease has not been diagnosed. The leaves of the plants were much smaller and thicker than usual. The veins, especially the mid rib, were big compared with the size of the leaf blade, which moreover was curly. The dis-ease was suspected to be caused by lettuce mosaic virus (LMV). The mosaic in young plants is easily recognized, but later the symptoms become indistinct and highly vari-able (Grogan 1980). Tests were consequently performed to find out the real cause of the disease.
Virus symptoms in lettuce plants It was necessary to make some inoculations to different lettuce cultivars, because the symptoms of lettuce mosaic vary greatly.
Five cultivars of lettude were used in infec-tion tests. They were butterhead lettuces
»America», »Nya Hilde» and »Ostinata», a crisphead lettuce »Grinlek» and a leaf lettuce
»Salad Bowl».
Sap transmission using phosphate buffer, pH 7,2, and carborundum powder as an abra-sive was made from the diseased lettuce plants to cultivars mentioned above. A pre-inoculation dark treatment was used to pro-mote the appearance of the symptoms.
The virus discussed was easily transmitted by sap. It produced somewhat similar symp- 210