viral coat protein. Even so, corn, incomparative experiments, afforded moreextractable virus particles (ie, eight timesmore than previously reported wherewheat and acid-heat treatment were used)with the purification procedure adoptedhere. The chloroform procedure was alsosimpler to perform and required fewersteps than the acid-heat method.

ELISA tests have been used to detectand differentiate virus serotypes (10). Inour WSMV-ELISA tests with relativelylow titer antiserum, however, the ease ofdetecting different virus isolates (eventhough WSMV type strain was a distinctbiotype) as well as a number of wheatisolates recently collected in the field (J.K. Uyemoto, unpublished), suggests thatthe isolates are closely related anti-genically. Hence, in contrast to virus

groups that contain known serotypes andrequire antiserum with broad cross-reactivity, a single source of WSMVantiserum apparently can be used forroutine virus diagnosis.

LITERATURE CITED1. BRAKKE, M. K. 1971. Wheat streak mosaic

virus. No. 48. Descriptions of Plant Viruses.Commonw. Mycol. Inst., Kew, Surrey, England.4 pp.

2. BRAKKE, M. K., and E. M. BALL. 1968.Purification and antigenicity of wheat streakmosaic virus. Phytopathology 58:963-971.

3. FINLEY, A. M. 1957. Wheat streak mosaic, adisease of sweet corn in Idaho. Plant Dis. Rep.41:589-591.

4. HOW, S. C. 1963. Wheat streak mosaic virus oncorn in Nebraska. Phytopathology 53:279-280.

5. McKINNEY, H. H., M. K. BRAKKE, E. M.BALL, and R. STAPLES. 1966. Wheat streakmosaic virus in the Ohio Valley. Plant Dis. Rep.

50:951-953.6. MOORHEAD, E. L. 1959. Serological studies of

viruses infecting the cereal crops. II. Theantigenic characteristics of wheat streak mosaicvirus as determined by the complement fixationtechnique. Phytopathology 49:151-157.

7. PFANNENSTIEL, M. A., and C. L. NIBLETT.1978. The nature of the resistance of agrotricumsto wheat streak mosaic virus. Phytopathology68:1204-1209.

8. SHEPARD, J. F. 1972. Gel-diffusion methodsfor the serological detection of potato viruses X,S, and M. Mont. Agric. Exp. Stn. Bull. 662.72 pp.

9. SHEPARD, J. F., and T. A. SHALLA. 1970. Anantigenic analysis of potato virus X and of itsdegraded protein. I. Evidence for and degree ofantigenic disparity. Virology 42:825-834.

10. UYEMOTO, J. K. 1980. Detection of maizechlorotic mottle virus serotypes by enzyme-linked immunosorbent assay. Phytopathology70:290-292.

11. UYEMOTO, J. K., and R. G. GROGAN. 1969.Chemical characterization of tobacco necrosisand satellite viruses. Virology 39:79-89.

Evaluation of Radish Cultivars for Resistance to Clubroot (Plasmodiophorabrassicae) Race 6 for Midwestern United States

RANDALL C. ROWE, Associate Professor, Department of Plant Pathology, Ohio Agricultural Research andDevelopment Center, Wooster 44691, and Ohio State University, Columbus 43210

ABSTRACTROWE, R. C. 1980. Evaluation of radish cultivars for resistance to clubroot (Plasmodiophorabrassicae) race 6 for midwestern United States. Plant Disease 64:462-464.

Sixty-eight radish cultivars and breeding lines were tested in the greenhouse for resistance toclubroot caused by Plasmodiophora brassicae, race 6. Although most of the American radishestested were moderately to highly susceptible, all of the Japanese and many of the Dutch cultivarswere completely resistant. Further field tests of 16 Dutch radishes indicated that the cultivarsNovitas, Roem van Zwijndrecht, Flevo, Robijn, Saxafire, Scharo, and Verano were resistant andhorticulturally suitable for commercial production in the midwestern United States.

Clubroot, caused by Plasmodiophorabrassicae Wor., is a disease of worldwideimportance in crucifer production(2,5,9,11,17). The fungus infects roothairs of susceptible hosts and causesdeveloping roots to form large distortedswellings and knobs (5,9). Severelyinfected plants are stunted and often wiltdue to reduced root function. The fungusforms resting spores within infected tissuethat are released into the soil when thetissue decays. Spores can remain viable insoil for at least 7 yr (5,9), and heavilyinfested fields may have to be abandonedfor crucifer production.

In recent years, clubroot has become alimiting factor in the production ofradishes (Raphanus sativus L.) on

Approved for publication as Journal Article No. 200-79 of the Ohio Agricultural Research andDevelopment Center, Wooster 44691.

00191-2917/80/05046203/$03.00/001980 American Phytopathological Society

462 Plant Disease/Vol. 64 No. 5

organic "muck" soil in Ohio and othermidwestern states (13). The diseaseaffects the base of the bulb and thetaproot (Fig. 1) and necessitates extensivegrading after harvest to remove infectedradishes. In severe cases, entire plantingshave been destroyed.

Chemical control of clubroot has beenonly partially successful. Root dipping oftransplants in fungicides containingpentachloronitrobenzene (PCNB) orbenzimidazole derivatives has providedcontrol in some cases (1,3,10,12,17) buthas been completely ineffective in mucksoils (6,8). A preliminary study weconducted confirmed that these com-pounds were ineffective in controllingradish clubroot when incorporated intomuck soil (14).

The main thrust of clubroot controlhas been based on developing resistantcultivars (9,11,17). Differences insusceptibility among radish cultivarswere observed quite early (7), but only afew studies have been made since then (9).

Breeding efforts with all crucifers havebeen frustrated by the development ofphysiological races of the pathogen(2,4,12,15-17). This report summarizesgreenhouse and field screening tests ofradish germ plasm for resistance to P.brassicae, race 6.

MATERIALS AND METHODSRadish bulbs infected with P. brassicae

were collected from a commercial muckfarm in north central Ohio. These weresent to Paul Williams, Department ofPlant Pathology, University of Wisconsin-Madison, for determination of thevirulence spectrum of this populationusing the concepts and standard set of 15European clubroot differentials (ECD)developed by Buczacki et al (4). Infectedbulbs collected at the same location werekept frozen at -18 C for use as inoculumthroughout this study.

Sixty-eight radish cultivars andbreeding lines from the United States,Europe, and Japan were evaluated in thegreenhouse for clubroot resistance duringJanuary and February 1979. Muck soilwas placed in 30 X 60 X 7 cm woodenflats, and radish lines were seeded insingle rows 30 cm long and 10 cm apart.Two replicates of one row each were usedfor each line. Resting spores of P.brassicae were extracted from frozenradish bulbs by comminuting the"clubbed" roots with a small volume ofsterile distilled water in a blender for Imin, filtering through cheesecloth, and

adjusting the final spore concentration toapproximately 1.7 X 106/ ml. About 30 mlof this suspension was poured uniformlyover each row of seeds before coveringwith soil. After I wk, seedlings werethinned to 10 per row. Plants were grownin the greenhouse at 20-25 C underfluorescent lights with a 12-hr photoperiodat 100-200 hlx. Soil was kept at or nearfield capacity by frequent watering. Sixweeks after seeding, mature radishes wereuprooted and examined for clubrootsymptoms and horticultural character-istics.

Cultivars resistant in greenhouse testswere further evaluated in field tests inAugust 1979 at two commercial muckfarms in northeast and north centralOhio. Identical plots were planted at eachlocation on naturally infested soil and athird was seeded on uninfested soil at thenorth central location. Seeds of eachvariety were put into separate hopperboxes of a commercial radish drill, andsingle rows about 100 m long wereplanted. Four weeks later, 100 matureradishes were harvested by hand fromeach row and evaluated for clubrootdevelopment and horticultural character-istics.

RESULTS AND DISCUSSIONThe infection pattern of the population

of P. brassicae obtained from Ohioradishes on the 15 European clubrootdifferential hosts was ECD 16/6/30 (4).This response indicates that thispopulation varies only slightly from thosenormally found in the midwestern UnitedStates in fields planted to Brassicaoleracea crops such as cabbage, broccoli,cauliflower, and brussel sprouts. Thispopulation, commonly referred to as race6, gives an infection pattern of ECD16/2/30. The difference between thesetwo designations is that approximatelyhalf of the ECD No. 8 differential plants(Brassica napus) were suscep-tible while the others were resistant,indicating a buildup of virulence in thepopulation capable of infecting thisnormally resistant differential.

Greenhouse tests of radish cultivarsindicated that most of the Americanradishes were moderately to highlysusceptible to this population of P.brassicae, whereas all of the Japanese andmany of the European radishes wereresistant (Table 1). Sixteen resistantEuropean cultivars that were horticul-turally similar to the red bulb-typeradishes grown in the United States werefurther evaluated in the field along withstandard American cultivars. In thesetests, serious clubroot developed only onthe infested plot at the north central farmwhere 14 Dutch cultivars remained

completely resistant while the susceptiblestandards, Scarlet Knight and RedPrince, were 100 and 68% infected,respectively. At the other two locations,all cultivars were evaluated for desirablehorticultural characteristics as well as

marketable yield, which is reported indetail in a separate paper (13).

From these data the following resistantDutch cultivars (listed in approximateorder of preference) appear to be those

most suited to commercial radishproduction in the midwestern UnitedStates: Novitas, Roem van Zwijndrecht,Flevo, Robijn, Saxafire, Scharo, andVerano (Table 1).

Table 1. Response of 68 radish cultivars and breeding lines to Plasmodiophora brassicae race 6 in greenhouse andfield evaluations

Entry- -- i

Novitas'Roem van Zwijndrecht'FlevoeRobijn'SaxafireScharoVeranoKokardeKordaMiddle East GiantsAmericanoSarxkatraQumkaderRealScarabellePiggelmeeKaderMinitasRobinoKatraRotaRadarRS 1004RisenbutterPrinz RotenBolideSaxerreCrimson GiantNoviredCavalrondoCrimson KnightChampionCherry BellNK3NK8Red BoyScarlet KnightRed PrinceScarlet GlobeEarly Scarlet GlobeScarlet Globe Short TopStopliteCometCavalierNK7NKIONK13NK15FuegoFar RedRound Scarlet GiantLange Scharlaken RodeLong Scarlet GlobeFrench Breakfast18 daagse Halflange

Rode WitpuntHalfrood/ Half witRonde Rode KleinwitpuntSparklerRonde WitteIjskegelIcicle Short topOokuraShogoinWakayamaShikouNezumiHooryoShirokubi-Myashige-Maruziri

Radishtype'

2

II

222

2

3

333455666

6666

OriginHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandHollandFrance

United StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited StatesUnited States

JapanHolland

United StatesUnited States

Holland

HollandHolland

United StatesHollandHolland

United StatesJapanJapanJapanJapanJapanJapanJapan

Susceptibilityb

Greenhousec Fieldd

0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 10000000

230

222223 33 333333333333303332

3330220000000

I = red bulb type, 2 = elongated red bulbs, 3 = red bulbs with white tips, 4 = white bulb type, 5 = white icicle type,6 = large, elongated, white Japanese type.

b0 = resistant (no sign of infection), I = slightly susceptible (< 10% of bulbs with clubbed roots), 2 = moderatelysusceptible (10-30% of bulbs with clubbed roots), 3 = highly susceptible (> 30% of bulbs with clubbed roots).

'Test consisted of two replicates of 10 plants each.dEvaluation consisted of 100 plants pulled at random from about a 100-m row.e Resistant and horticulturally suitable for commercial production in the midwestern United States.

Plant Disease/May 1980 463