Paul H. WilliamsUniversity of Wisconsin, Madison

Black Rot:During the latter half of the 19th

century, immigrant farmers fromnorthern Europe brought with them tothe midwestern United States the love ofbrined and fermented cabbage. Over thewinter months, sauerkraut, as it wasknown, was savored along with roastedpork, wursts, and potato dumplings. Bythe late 1800s, extensive acreages ofcabbages were being grown from NewYork to Wisconsin and sauerkrautproduction was thriving. In the 1890s, aserious disease devastated the cabbagefields of southeastern Wisconsin, and theresident professor of the new science ofbacteriology at the University ofWisconsin, H. L. Russell, was called toexamine the fields of Racine andKenosha counties. Russell concludedthat the malady was the black rot diseasecaused by the bacterium Bacilluscampestris recently described on rutabagaby Professor Pammel in neighboringIowa. Since Russell's publication in 1898(7), there has been a growing recognitionof the seriousness of black rot(Xanthomonas campestris) on crucifercrops throughout the world.

A Worldwide ProblemBlack rot must be considered the most

important worldwide disease of crucifers,attacking all cultivated brassicas,radishes, and numerous cruciferousweeds. As with many bacterial diseases,the pathogen thrives in warm, humidclimates and is most serious in tropical,subtropical, and humid continentalregions. Black rot is known to exist in thecool coastal climates of northern Europeand North America but is seldom aproblem there, rarely progressing to thepoint where the whole plant is destroyed.

After gaining entry through the waterpores at the leaf margins or throughwounds, AX campestris colonizes thevascular system. When temperaturesexceed 25 C, the bacteria move rapidlythrough the vascular system, producingcopious quantities of an extracellularpolysaccharide known as xanthan (5).(An interesting aside is that xanthan gumproduced by X. campestris is animportant commercial product. Xanthanis used in the food industry as an additivein salad dressings, cake mixes, and manyother products. Large quantities are alsoused in the oil-drilling industry; special

properties of the polysaccharide decreasethe viscosity of the drilling mud, thereby

0191-2917/80,08073607/$03.00/0©1980 American Phytopathological Society

A Continuinglubricating the drilling bit.) In thevascular system of crucifers, bacteria andxanthan plug the xylem vessels, restrictingwater flow and resulting in the character-istic V-shaped chlorotic lesions origi-nating from the margins of crucifer leaves(11) (Fig. 1).

As bacteria move throughout theplant, the vascular tissues darken-thereason for the name black rot. The typicalearly symptoms are yellowing and dryingof affected tissues. Invasion of the fleshypetioles and head leaves by Xanthomonasis rapidly followed by a soft-rottingbacterium (Erwinia carotovora orPseudomonas marginalis), which isprimarily responsible for the rottingphases of the disease. Under idealconditions, black rot symptoms appear10-14 days after entry of the bacterium. Iftemperatures are suboptimal, however,the organism may persist in the vascularsystem without producing symptoms.Invasion of the flowering stalks may leadto infection of the seed (2).

When Seed Plants Are InfectedSeed of vegetable crucifers is produced

primarily in temperate coastal regions ofthe world where cool, mild winters areideal for the vernalization needed toinduce flowering in the biennial cropforms such as cabbage, Brussels sprouts,cauliflower, turnip, rutabaga, Chinesecabbage, and oriental radishes (Fig. 2).Crops for seed production are denselysown in beds in midsummer andtransplanted to production fields afterabout a month. Plants grow slowlythroughout the winter, bolting to flowerthe following spring. Seed ripens throughthe summer and is harvested about thetime the new seed crop is transplanted.Seed plants may be infected any timeduring the growing season, but thepathogen is rarely detected becauseconditions for expression of disease aresuboptimal during most of the seedproduction cycle.

The seed crop is most vulnerable toinfection during the late summer in theseedbeds before transplanting and duringlate flowering and seed maturation. Atthese times, temperatures are mostfavorable for bacterial growth, andbacteria on leaves, on plant debris, or insoil can be spread by heavy rain oroverhead irrigation to other plants in theseedbed or seedfield.

Plants infected in the seedbed areunlikely to be detected for severalreasons. As seedlings approach trans-

planting size, seasonal temperatures are

736 Plant Disease/Vol. 64 No. 8

Threat to World Crucifersdecreasing below the optimum forsymptom expression. A second disease,downy mildew (Peronospora parasitica),is common in seedbeds and frequentlyinjures the leaves, causing senescence andmasking early symptoms of black rot.When transplants are pulled, senescent orinjured leaves often fall from the plantunnoticed. Black rot infection is difficultto detect during flowering and seedmaturation because of the massive plantgrowth and heavy senescence of lowerleaves. Furthermore, systemic invasion inflower stalks may not result in typicalexternal symptoms, although internalvein darkening of the seed stalks andpremature leaf drop may occur.

Seed Production RegionsOf the regions in the world where

crucifer seeds are most commonlyproduced (Fig. 2), the Pacific NorthwestCoast of North America, including thePuget Sound region of Washington, theWillamette Valley of Oregon, and valleysof central California, is considered ideal.In these areas, low summer rainfall(normally less than 2 cm per month)combined with abundant subsoil moistureminimizes the potential for spread of X.campestris in the seed crop. Seed grownin the Pacific Northwest has thereputation of being free from X.campestris, which accounts for thisregion producing much of the world'svegetable brassica seed.

Such ideal conditions for disease-freeseed production do not exist in manyother regions where crucifer seed isgrown. The potential for crop damage byX. campestris is low in northern Europe,Japan, New Zealand, parts of Australia,and Brazil, but relatively frequentsummer rains favor spread in the cropand cooler temperatures result inundetected movement of the organism inseed plants. In tropical and subtropicalregions where production of numerousleafy forms of Brassica campestris andthe oriental greens pe-tsai, pakchoi, andchoi sum are grown the year round andwhere the cole crops are grown at higherelevations, X. campestris is constantlypresent. The pathogen cycles from cropto crop or from crop to weeds and back tocrop. Seed production in the humidtropics is commonly associated with highlevels of seedborne X. campestris.

Hot Water Seed SoakRecognizing that seedborne infection

played an important role in the spread ofX. campestris, J. C. Walker at Wisconsin

(12) and E. E. Clayton at Cornell (1) inthe 1920s determined that soaking freshlyharvested, well-developed seed in waterat 50 C for 25-30 minutes virtuallyeliminated seedborne X. campestriswithout adversely affecting seed vigor.They also noted that seed stored forseveral years, weak seed, or seed ofcertain crops, such as cauliflower, wasdamaged by hot water treatment, andshorter treatment periods were advisedfor the more delicate seeds (1).

By using only seed produced in thePacific Northwest and treated with hotwater, crucifer growers have, by andlarge, been able to avoid seriouswidespread epidemics of black rot overthe years. Occasionally, however, blackrot has been a problem in cabbage orcauliflower crops grown from disease-free seed. In some instances, disease couldbe attributed to carry-over of bacteria inresidues from previously infected crops,to spread from crucifer weeds, or toinfected transplants from contaminatedseedbeds.

Risks of Shipping TransplantsLarge numbers of cabbages and

cauliflowers are grown in seedbeds inTexas, Georgia, Alabama, Florida, andTennessee and shipped north as trans-plants for early spring planting. Theregion around Tifton, GA, is particularlysuited for growing and shipping trans-planted vegetables and has long beenknown for producing high-quality

transplants of tomatoes, peppers, onions,cauliflowers, and cabbages. Each yearmillions of transplants are shipped northto Wisconsin, New York, and the NewEngland states. Because plants are alsoshipped south to Florida for wintercabbage production, the transplantoperations around Tifton can be pivotalwith respect to wide distribution ofdiseases.

The state of Georgia has set upinspection and certification procedures tominimize the possibility of diseasedplants being shipped out. To obtain statecertification of their plants, growers musthave their seed assayed and plant bedsinspected for freedom from pests anddiseases, including black rot. Inspectioncan be useful in detecting pockets ofserious black rot infection (Fig. 3), butthe systemic nature of seedborne X.campestris plus cool temperatures in thespring often delay symptoms, makingdetection virtually impossible.

Some Hazardous PracticesA number of practices associated with

transplant bed production of crucifers areparticularly hazardous with respect toblack rot. Some growers traditionallygrow rows of radishes, turnips, ormustard among the beds of cauliflower orcabbage seedlings as "indicators" ofwhether the soil nutrient balance issatisfactory. When the quick-growingindicator rows begin to show deficiencies,side dressings of fertilizer are applied tothe seedlings. But these indicator rows

Fig. 1. Characteristic symptoms of black rot on cabbage.

Plant Disease/August 1980 737

Fig. 2. Regions of major production of vegetable crucifer seed.

the early crop, outbreaks of X. campestriswere sporadic and limited. In Florida andTexas, however, black rot was a seriousthreat to production every year, withoutbreaks often tied to heavy rainfall andhigh temperature. Then, during the late1960s and early 1970s, the frequency andseverity of black rot epidemics increased.Particularly serious and widespreadoutbreaks occurred during 1972 and 1973in the northeastern and midwesternUnited States.

A number of reasons are associatedwith the resurgence of black rot. Duringthe 1960s, the advantage of F, hybridbrassicas, particularly cabbage, Brusselssprouts, and broccoli, became apparentto crucifer growers throughout the world.Several Japanese seed firms produced F1hybrids of superior uniformity andmarket quality that were in demand bygrowers and processors. Because many ofthese hybrids were produced in regions ofJapan where X. campestris was endemic, aproportion of the seed lots entering theUnited States from Japan was infestedwith the organism. Much of this relativelyexpensive and frequently delicate seedwas not treated with hot water, andinfested seeds were commonly sown inseedbeds along with disease-free seedsproduced in the Pacific Northwest. Insouthern seedbeds, seedborne infectionfrequently went unnoticed and wasoccasionally intensified by heavy rains,overhead irrigation, and clipping.

In the 1973 epidemic, approximately70% of several million early transplantsoriginating from a single seedbed weresystemically infected, but disease was notdetected at transplanting time. InWisconsin and other midwestern states,the infected transplants were planted inApril on the earliest tillable fields.Portions of several of these fields wereselected for early sowing of the local ormain crop of transplants. Infectionremained undetected during the coolspring, and not until mid-June did thefirst signs of black rot appear in thetransplants. Then it was too late toprevent the spread of X. campestris fromthe early transplanted crop to theadjacent main crop seedbeds. Theepidemic was widespread. Cabbagegrowers, processors, transplant operators,and seedsmen all suffered!

Blackleg, a Fellow TravelerThe epidemiology of blackleg, caused

by the fungus Phoma lingam, isremarkably similar to that of black rotexcept that Phoma is more active thanXanthomonas on crucifer tissues at lowertemperatures. P. lingam is also seedborneand spreads most rapidly in seedbedsthrough water-splashed pycnidiospores.

Phoma is frequently found on seedcrops produced in cool, humid regionsand causes spreading lesions on the leavesand stem cankers. Lesions have lightbrown centers with darkened margins

Fig. 3. Black rot Infestation in a transplant bed. The Infection level rose from 0.5% Initiallyto 65% after 3 weeks.

occasionally carry seedborne X.campestris!

Excessive fertility, warm temperatures,or a too-low seeding rate frequentlyresults in oversized succulent transplants.In such instances, growers commonly"clip" seedbeds (Fig. 4). Rotary mowersor specially designed flails pass over theplants, removing excess foliage andtrimming the plants to fit into shippingcrates. If pockets of infection are in theseedbed, clipping virtually ensuresuniform and widespread introduction ofX. campestris among the transplants; thepathogen is not detected until thetransplants are growing vigorously in theproduction fields, several weeks later.

Another hazardous practice is soakingcrates or bundles of transplants in tubs ofwater before transplanting. Such soakingspreads X. campestris inoculum fromplant to plant through leaf scars andwounded roots.

The practice of locating seedbeds inproximity to production fields greatlyincreases the chances of black rotspreading from the production fields tothe local crop (Fig. 5). Normally, well-drained light soils ensure uniformseedling emergence, provided moisture is

738 Plant DiseaseNol. 64 No. 8

adequate. Such light soils are frequentlythose that can be worked earliest in theseason and, in addition to being selectedfor transplant seedbeds, are occasionallychosen for the earliest planting of"shipped-in" transplants.

Strandberg (10) has shown that X.campestris can persist in the soil incrucifer residue at least as long as theresidue is not completely decomposed.This may vary from a few months inwarm, wet regions to 2 years in coolclimates. For this reason, strict rotationof seedbeds should be practiced, avoidingground cropped to crucifers during theprevious two seasons.

Resurgence In the SeventiesFor many years, black rot was

considered a disease of relatively minorimportance to crucifer growers in themajor northern production areas of theUnited States and in the cool coastalvalleys of California. Because growersadhered to the recommended practices ofplanting seed produced in the PacificNorthwest, soaking seed in hot water,rotating seedbeds, and producing andprocuring inspected certified plants for

and contain the characteristic pycnidia ofPhoma (Fig. 6). The name blacklegderives from cankers on the lower stemand at the ground level that frequentlygirdle the stem, causing wilting and deathof the plant (13). Phoma causes a darkdry rot on root crops and cabbage instorage.

Although blackleg can spread rapidlyunder optimal conditions, spread isseldom as explosive as that of black rot.Numerous strains of P. lingam exist,some having more restricted host rangesand more limited virulence than others.The perfect stage of P. lingam,Leptosphaeria maculans, permits geneticrecombination among isolates. InAustralia, France, and England, airborneascospores produced on crop residues area major source of inoculum in epidemicsof blackleg on oilseed rape.

NCR-100 Takes ActionBecause of the growing threat of black

rot and blackleg to crucifer production inthe United States, a committee wasformed of researchers representing 18states in which crucifer seeds, transplants,or crops are grown. The committee,sponsored by the Cooperative StateResearch Service and designated NCR-100, met first in 1975 and annuallythereafter to plan cooperative research oncontrol of seedborne diseases of crucifers.A component of NCR-100 that hascontributed significantly to its success isthe inclusion, on an unofficial basis, ofrepresentatives of U.S. and internationalseed industries, transplant growers,crucifer growers, shippers, processors,and members of state and federalregulatory agencies.

Each year, the 30-40 attendees havemet in a different state representing aparticular phase in the chain of cruciferproduction. At each location, problemsassociated with crucifer production wereobserved and research initiated toaddress specific needs. The committeemet first in Washington State inproduction areas of the Pacific Northwest,then at Tifton, GA, where transplantoperations are extensive, and next inWisconsin, where sauerkraut and marketcabbages are important. Winter produc-tion in central Florida and production inthe Salinas Valley of California wereviewed in subsequent meetings.

NCR-100's activities are resulting in agrowing list of reports and publicationsrepresenting collaborative researchamong members (3,4,8). Research hasfocused primarily on identifying gaps inunderstanding the epidemiology of blackrot and blackleg and in developingimproved procedures to detect andcontrol seedborne infection. Thedevelopment and implementation ofcrucifer seed lot assays were among thefirst activities of the committee (6,8).Through assays of seed lots produced orpurchased for sale in the United States, a

Fig. 4. Tractor-driven mower "clipping" tomato transplants in a seedbed. Such a practicebefore crucifer plants are lifted Is particularly hazardous In spreading such bacterialdiseases as Xanthomonas campeatris.

Fig. 5. Of this transplanted early crop of cabbage (left) growing adjacent to a largeseedbed, 70% were systemically Infected with X. campestrls. Subsequent heavy rainsspread Infection Into the seedbed.

Fig. 6. Leaf lesion of Phoma lingam, the cause of blackleg, showing characteristicpycnidia.

Plant Disease/August 1980 739

Guidelines for Minimizing the Threat of Blackleg and Black Rot of Crucifers

1. Guidelines for Crucifer Seed ProductionI. All stock seed being planted in seedbeds prior to transplanting or

direct seeded in seed production fields should be determined to befree of Xanthomonas cumpestris and Phoma lingam.

2. Fields selected for seedbeds should not have had crucifer crops onor near them for at least four years, should be located as far aspossible from existing fields of crucifer crops and should not receivesurface run-off water likely to contain contaminants from fieldswith crucifers.

3. Inbreds used in the production of major seedlots of F, hybridsshould be separated within the seedbed areas from other open-pollinated seedlots, and where possible, large seedbeds should bedivided into smaller separated units,

4. Seedbeds should be kept weed free, especially from mustards, wildradish and other cruciferous weeds. Radish or other leafy crucifersshould not be used in seedbeds as soil nutrient markers unlessdetermined to be free from X. campestris or P. lngam.

5. Seedbeds should be thoroughly inspected by a qualified person forthe presence of any obvious foliar symptoms ofdisase caused by P.lingam and X. campestris. Infected plants should be removed anddestroyed and remaining plants observed frequently throughout thereproductive cycle to see if the eradication has been successful.Removal of diseased plants from the seedbed does not insureeradication of the problem.

6. Plants in the seedbeds should be treated with recommendedpesticides in order to prevent the entry or buildup of any potentialdisease or insect in the seed crop and to prevent the occurrence ofpest damage that would mask disease symptoms. Carefulobservation of local rainfall patterns and coordination of pesticidespray schedules should be followed to provide effective protection.

7. Equipment used in the cultivation and management of the seedbedsshould be isolated from that used on other crucifer crops or if usedin crucifercrops,should be thoroughly cleaned and disinfested withsteam or germicidal sprays before being brought into the seedbed.

8. Upon lifting, transplants should not be sprayed or dipped in waterprior to transplanfing. Plants remaining in the seedbed after thetransplanting operation should be destroyed as soon as possible.Seed production fields should not have had a previous crucifer cropfor at least three years.

9. For issuance of a phytosanitary certificate, a seed crop must beinspected while actively growing and during each growing season.

10. Recommended pesticides should be applied to the productionfields.

IL After the seed is harvested and cleaned, representative seed samplesfrom each production lot, taken according to International SeedTesting Association procedures, should be assayed for P. lingam andX. campestris. The assay must show thatneither pathogen is presentin viable condition in a sample of 10,000 seeds for P. lingam and30,000 seeds for X. campestris. If either pathogen is detected,appropriate eradicant treatment should be applied to the wholeseedlot. Reassay of the seedlot should follow treatment.Retreatments may be necessary until tests show the pathogens areeradicated,

12. Seed producers should be prepared to provide well-documentedreports of seedbed protection, inspections, seed treatments, andseed assays if requested to do so by customers.

I. Guidelines for Plant Bed and Container-Grown Operationand Planting1. Fields selected for seedbeds should not have had crucifer crops or

debris on or near them for at least one growing season beyond thetime taken to fully decompose heavy stem tissue residues. (This timemay vary from less than one year in warm humid climates to morethan two years in cool dry regions.) Seedbeds should be located asfar as possible from existing fields of crucifer crops and should notreceive surface run-off water likely to contain contaminants fromfields with crucifers.Ia. Container-grown operations should follow the guidelines in this

section where applicable and particular attention should begiven to overall sanitation.

2. Only seed assayed and shown to be free of P. lingam and X.campestris should be sown in seedbeds. Unassayed trial samplesshould not be grown in or near transplant beds. Large seedbedsshould be divided into smaller separated units.

3. Seedbeds should be kept weed free, especially from mustards, wildradish and other cruciferous weeds. Radish or other leafy crucifers

740 Plant DiseaseNol. 64 No. 8

should not be used in seedbeds as soil nutrient markers unlessdetermined to be free from X. campestris or P. lingam.

4. Seededs should be thoroughly inspected by a qualified person forthe presence of any obvious foliar symptoms of disease caused by P.lingam and X. campestris. If positive identification of P. lingam orX. campestris is made in a seedbed, infected plants should beremoved and destroyed. The remaining plants of the seedlot and inthe bed should be kept under close surveillance. If other plants arefound to be infected, the infested lot or whole seedbed unit should bedestroyed. Equipment used in a seedbed in which infection is foundshould be considered to be contaminated and should bedecontaminated using suitable disinfectants.

5. Plants in seedbeds should be treated with recommended pesticidesin order to prevent the entry or buildup of any potential disease andto prevent the occurrence of pest damage that would mask diseasesymptoms. Careful observation of local rainfall patterns andcoordination of pesticide spray schedules should be followed toprovide effective protection.

6. Equipment used in the cultivation and management of the seedbedsshould be different from that used on other crucifer crops or if used incrucifer crops, it should be thoroughly cleaned and disinfested withsteam or germicidal sprays before being brought into the seedbed.

7. Transplants should not be mechanically "topped," "clipped" or"chopped" to toughen them or to reduce their size in order to fit theplants into shipment crates.

8. Upon lifting, transplants should not be sprayed or dipped in waterprior to transplanting.

9. Only new crates which have never been used for shipping cabbage,broccoli, cauliflower, mustard, turnip greens, collards or radishesshould be used for shipping transplants. Reusable plastic or woodenplant containers brought to the field should be cleaned anddisinfested before returning them to the seedbed area for reuse.

10. Plants remaining in the seedbed after pulling the transplants shouldbe destroyed as soon as possible.

III. Guidelines for the Purchase or Receiptof Crucifer Transplants1. Crucifer growers, shippers, and sauerkraut packers should contract

acreage only from growers agreeing to adhere to the guidelines herein.2. Crucifer growers, shippers or kraut packers should purchase

transplants from plant bed growers who can verify that their plantshave been grown in accordance with the guidelines herein.

3. For transplants moving across state lines, only plants certified bylocal state inspectors should be acceptable for sale or purchaseprovided certification is in compliance with the guidelines herein.

4. Purchasers of transplants may request additional specificinformation relative to "certification" of their particular lots oftransplants: a) seedlot numbers; b) seeding and pulling dates; c)verification that plants have not been topped or clipped; d) seedbedspray schedules; e) seeded inspection schedules. Such additionalinformation may be appropriately obtained by persons other thanthe plant bed operator or state certification officials.

IV. Guidelines for the Production of the Crucifer CropI. Follow the practices for the production of healthy, vigorous crops

according to the recommendations of the region or state withparticular attention being given to the following guidelines.

2. Direct seed or transplant the crop in weed-free and crucifer-residue-free ground which has not had an immediately preceding crop ofcrucifers. Crucifers should not be grown on ground that has had P.lingam or X. campestris infected crucifers for at least three precedingcrops, The land should be so located as not to receive surface waterfrom soil or debris from plants likely to contain X. campestris or P.lingam from adjacent fields or previous crops.

3. Water for irrigation should not be drawn from sources which couldreceive surface run-off containing P. lingam or X. campestris.Overhead sprinkler irrigation should be avoided if possible.

4. Good pest management should be practiced. X. campestris can bereadily spread via insects. Weeds, particularly crucifers (mustard,wild radish, cress, shepherds purse, etc.) should be removed from thecrop and surrounding areas.

5. When working in the crop with cultivation or spray equipment, waituntil the crop is dry from dew or precipitation. Boom sprayers are lesslikely to spread X. campestris as rapidly through a field than areairblast sprayers.

profile of the primary sources ofseedborne infection has been obtained.The development of assays together withimproved fungicidal eradicants for P.lingam has significantly reduced seed-borne inoculum as a threat to thetransplant industry (3). Research onmethods to eradicate X. campestris fromseed is under way (4). When fullydeveloped and cleared for use, thetechniques should provide acceptablealternatives to the hot water soak. Atpresent, however, the hot water soak isthe recommended treatment even thoughapplication to old or weak seed can affectseedling vigor. Research on survival of X.campestris in residues (10) and on weedsin seed production and transplant areashas clarified the need for rotation andsanitation in disease control.

A major contribution from NCR- 100 isa set of guidelines designed to minimizethe threat of black rot and blackleg. Theguidelines cover all aspects of cruciferproduction-seed, transplants, andcrop-and are the result of several yearsof deliberation among those directlyinvolved with each phase of production.The emphasis is on sanitation, particularlyin seed production and transplant bedoperations. The procedures can bereasonably followed and should reducethe probability of continuing widespreadepidemics of black rot and blackleg. Theguidelines were unanimously approved18 March 1980 by the 44 members ofNCR-100 attending the annual meetingin Monterey, CA.

An aspect of production that, inconjunction with the guidelines, wouldhelp minimize disease outbreaks is directsowing of assayed seed free from X.campestris and P. lingam into residue-free soil. Transplants are necessary,however, for early crop production andfor seed production of F, hybrids fromlimited quantities of hand-pollinatedinbred seed.

Resistance, a Long Sought GoalDespite many early attempts to

identify and utilize resistance to X.campestris, it was not until 1973 thatstabilized resistance derived from theJapanese cabbage cultivar Early Fuji wasreleased from the Wisconsin AgriculturalExperiment Station as Badger Inbreds 14to 20 (Fig. 7) (14). These inbreds are nowused worldwide, and a number of hybridcultivars bearing X. campestris resistancehave reached the market during the past 2years.

Reports from India, Australia, Japan,South America, and various regions ofthe United States indicate that the EarlyFuji source has resisted a variety ofindigenous strains of X. campestris. Thedurability of this source of resistanceremains to be seen. At present, X.campestris resistance has been identifiedin cabbages and Indian cauliflowers (9).

The need to find new sources of

Paul H. Williams

Dr. Williams is a professor in theDepartment of Plant Pathology at theUniversity of Wisconsin, Madison. Heobtained a B.S.A. from the University ofBritish Columbia, Vancouver, and aPh.D. from the University of Wisconsinin 1962. His interests are in vegetablediseases, with emphasis on thedevelopment of methods for multipledisease resistance screening and onthe genetics and breeding of crucif-erous crops. He is also interested in thecytology of host-parasite relations andpathogenesis.

Fig. 7. Row of cabbage on the right Is a resistant Badger Inbred derived from the Japanesecultivar Early Fuji.

resistant X. campestris, while incorpo-rating existing forms into other brassicacrops, is pressing. Resistance is particular-ly needed for crops grown in the humidtropics and subtropics. Indeed, the newsources of resistance for breedingprograms must be seeded in such regions,where black rot is endemic.

That partial resistance to X. campestrisexists in many Japanese cultivars and thatstrong resistance was found in the old,open-pollinated Early Fuji cannot beattributed to chance. Japan and Chinarepresent the greatest diversity of crucifercrops in the world, and X. campestris isendemic in these regions. The success ofthe Japanese in hybrid crucifer production

has undoubtedly narrowed the geneticbase on which varietal selection will bemade. The Chinese also are pursuing theproduction of F1 hybrids at thecommunal and provincial levels.

Hybrid breeding strategies, combinedwith eradication of black rot as anendemic disease in the breeding andproducing regions, may have profoundeffects in eliminating important sourcesof variation and resistance to diseasessuch as black rot. Resolution of thedivergent objectives of maintaining highlevels of sanitation in the actualproduction of crucifer crops, togetherwith the long-term need for evolution andselection of the pathogen among as

Plant Disease/August 1980 741