PertanikaJ. Trop. Agric. Sci. 20(1): 1-11(1997) ISSN: 0126-6128© Universiti Putra Malaysia Press

Susceptibility of Capsicum Species and Cultivars to Ralstonia solanacearum:Anatomical Differences and Bacterial Multiplication in Resistant and

Susceptible Cultivars

M.A. RAHMAN and HIRYATI ABDULLAH

Department of Plant ProtectionFaculty of Agriculture

University Putra Malaysia43400 Serdang, Selangor, Malaysia

Keywords: susceptibility, Capsicum spp., Ralstonia solanacearum, anatomy, bacterial multiplication

ABSTRAKPenilian rumah hijau terhadap 42 aksesi/kultivar Capsicum spp. hepada penyakit layu bakteria yang disebabkanoleh Ralstonia solanacearum, memperlihatkan bahawa kultivar tempatan Malaysia "Kulai" menunjukkanparas keresistanan yang tinggi. Tiga aksesi, iaitu CO 1969, CO 1970 dan CO 1553, menunjukkan keresistanansederhana sementara aksesi yang lain adalah rentan kepada sangat rentan. Kajian anatomi terhadap kultivarresistan dan rentan menunjukkan perbezaan bermakna bagi ciri-ciri anatomi yang tertentu. Bilangan uratvaskular per akar lebih banyak, korteks lebih tebal dan kepanjangan unsur xilem lebih pendek dalam akar kultivarresistan berbanding kultivar rentan. Bilangan purata berkas vaskular per pokok dan bilangan unsur xilem perberkas vaskular juga lebih banyak dan kepanjangan diameter unsur xilem lebih pendek di bahagian batang tengahkultivar resistan. Ciri-ciri anatomi pokok resistan ini mungkin menjadi faktor penyumbang yang menghad ataumelambatkan proses penjangkitan, pergerakan dan penggandaan patogen. Penggandaan R. solanacearum padabahagian batang tengah, kolar dan akar bagi kedua-dua kultivar yang diinokulatkan menerusi batangmenunjukkan bahawa dalam kultivar rentan populasi bakteria tidak berbeza pada setiap bahagian yang dikaji,tetapi terdapat perbezaan yang bermakna antara kultivar pada bahagian penyempalan yang sama. Penemuan inimenunjukkan bahawa mekanisma resistan kepada penyakit adalah, sebahagian besarnya, bergantung kepadaketidakbolehan patogen untuk membahagi di dalam perumah yang resistan,

ABSTRACT

Greenhouse evaluation of 42 accessions/cultivars of Capsicum spp. to bacterial wilt caused by Ralstoniasolanacearum revealed that the Malaysian local cultivar 'Kulai' was highly resistant. Three accessions, namelyCO 1969, CO 1970 and CO 1553, were moderately resistant; all other accessions were susceptible to highlysusceptible. Anatomical study of resistant and susceptible cultivars indicated significant differences in certainanatomical characters. The number of vascular strands per root toas fewer, the cortex was thicker, and the lengthofxylem elements was shorter in the roots of resistant cultivars than the susceptible cultivar. The average numberof vascular bundles per plant and the number of xylem elements per vascular bundle were fewer and the lengthand diameter of xylem elements were shorter in the mid-stem of resistant cultivars. These anatomical charactersof resistant plants may contribute to limit or slow down the infection process, movement and multiplication ofthe pathogen. Multiplication ofR. solanacearum at mid-stem, collar and tap root in stem-inoculated susceptibleand resistant chilli cultivars revealed that in susceptible cultivars the bacterial population did not differ at allsites tested, but it differed significantly between cultivars at similar sampling sites. These findings show that themechanism of resistance to the disease is mainly due to the reduced rate of multiplication of the pathogen withinthe resistant host.

M. A. RAHMAN AND HIRYATI ABDULLAH

INTRODUCTIONBacterial wilt of capsicums and chillis (Capsicumspp.) caused by Ralstonia solanacearum (previ-ously called Burkholderia solanacearum) is one ofthe major constraints for chilli/capsicum pro-duction in the humid tropics of Malaysia(Abdullah and Kamaruzaman 1992; Syed andLoke 1995). The bacterium is a soil-inhabitantand rhizosphere survivor and enters the rootsfrom artificial or natural openings (Kelman andSequeira 1965). So far, limited work has beendone on bacterial wilt-resistance of chilli pepper(Peter et al 1984; Jyothi et al 1993; Perera et ai1993 ).

Various attempts to control bacterial wilthave been reported (He 1990; Hartman et al.1993) but the success was limited. Breeding forresistant varieties, therefore, remains the bestcontrol strategy.

Little is known about the mechanism ofresistance of chilli to R solanacearum. In cultivarsof alfalfa which have been cultivated for a longtime in the USA and Canada, the mechanism ofresistance to Clavibacter michiganensis subsp.insidiosum appears to be stable due to morpho-logical and/or physio-chemical barriers to thepathogen (Cho et al 1973).

Vascular anatomy of plants is known to playan important role in susceptibility to vascularpathogens (Elgersma 1970; McNabb et al 1970;Cho et al 1973). Restricted upward or downwardmovement of Erwinia amylovora in resistant appleshoots appears to be due to immobilization ofthe pathogen as well as anatomical features suchas reduced vessel diameter and vessel branching(Tomasik and Goodman 1986). Therefore, ana-tomical characters may be important for anunderstanding of the mechanism of resistanceduring host-pathogen interactions.

Grimault and Prior (1993) detectedPseudomonas solanacearum in wild resistant tomato,but the infection was found only in selectedprogeny, indicating that resistant plants may be-come symptomless carriers. The information onthe growth and multiplication of the pathogen inresistant and susceptible hosts is important inunderstanding the mechanism of infection.

Literature on anatomical differences andbacterial multiplication in chilli plants resistantand susceptible to bacterial wilt is lacking. Thepresent study was, therefore, undertaken to evalu-ate the susceptibility of some chilli accessions/cultivars to K solanacearum; whether anatomical

differences between resistant and susceptiblecultivars of Capsicum spp. are related to bacterialwilt-resistance; and to investigate the multiplica-tion of the pathogen in chilli with differentlevels of susceptibility.

MATERIALS AND METHODSPropagation of Test Plants

Seeds were germinated in the greenhouse intrays with sterilized soil mixture (soil : sand :compost) in the ratio of 3 : 2 : 1 for a period of10 days. They were then transferred to indi-vidual pots containing approximately 1.5 kg ofsterilized soil mixture

Evaluation of the Susceptibility of 42 Accessions/cultivars of Chilli to R. solanacearumSeedlings of 42 accessions and cultivars (Table1) of Capsicum spp., 40 from AVRDC, Taiwan;and one each from Taiwan and Malaysia weretested. Plants were inoculated when they wereapproximately 35 days old. The K solanacearumstrain used in this study was isolated from wiltedchilli (cv. Chili Merah), C. annuum L. plantscollected from UPM research field. On isolationon triphenyl tetrazolium chloride (TZC) agarmedium (Kelman 1954), the pathogen yieldedgreyish white, fluidal colonies with light pinkcentres. After purification, the isolate was char-acterized as race 1 and biovar III according topathogenicity and host range (Buddenhagen etal 1962), and biochemical test (Hayward 1964).The isolate was kept as stock culture inlyophilized form and in 5 ml sterilized tap waterat 15°C. Before each preparation of inoculum,the bacteria was streaked on TZC from the stockculture. The inoculum was prepared by cultur-ing the virulent, wild type and fluidal colony ofR. solanacearum on casamino acid-peptone-glu-cose (CPG) agar medium incubated at 30°C for48 h. A micropipette tip carrying 20 ul of thebacterial suspension, containing approximately2.0 x 108cfu/ml was inserted at the axil of 3rdfully expanded leaf from the top of the plant.The initiation of wilt symptoms after inoculationand the number of wilted plants for each acces-sion were recorded at 7-day intervals-up to 63days after inoculation. The reactions of species/cultivars were graded on a 0-5 rating scale basedon percentage of wilting obtained at 63 days.The rating scale was as follows: 0 = HR (highlyresistant, plants did not show any symptoms ofwilt); 1= R (resistant, 1- 20% plants wilted); 2 =

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 20 NO. 1, 1997

SUSCEPTIBILITY OF CAPSICUM SPECIES AND CULTIVARS TO RALSTONIA SOIANACEARUM

MR (moderately resistant, 21-40% plants wilted);3 = MS (moderately susceptible, 41-60% plantswilted); 4 = S (susceptible, 61-80% plants wilted);and 5 = HS (highly susceptible, more than 80%plants wilted). The incubation period (time takento produce 50% wilt at days after inoculation,DAI) and temperature and relative humidityusing hygro-thermograph were also recorded.Plants were arranged on greenhouse benchesfollowing the complete randomized design withthree replicates. One replication consisted of 25plants.

Differences in Anatomical Characters of Resistant andSusceptible Chilli CultivarsTwo cultivars of chilli viz. highly resistant cv.Kulai and highly susceptible cv. Long Chilli 455(LC 455) were selected from the previous study.One hundred plants of each cultivar were raisedin trays and carefully removed from the soilwhen they were 30 days old. Two 5-mm sectionswere taken 1.5 cm below the ground for rootsamples and two stem sections of the same lengthwere taken from the upper part of the firstinternode of the plants. All the samples werefixed and preserved in formalin-acetic acid-alco-hol solution. One pair of root and stem seg-ments was processed for microtomy and anothersample was used for measuring the length of thexylem elements.

Samples for microtomy were dehydratedthrough an ethanol series and embedded inparaffin blocks. Sections 7-8 jim thick were pre-pared using a rotary microtome and stainedwith either safranin-fast green or 0.5% toluidineblue O with 1% borax, and observed under thelight microscope. The thickness of the cortex,diameter of xylem vessels and number of vesselelements were measured or counted.

For measurement of the length of vascularelements, tissues were macerated in 10% chro-mic acid and 10% nitric acid in 1 : 1 ratio for 16h. The length of 10 complete vascular elementswas taken randomly for each specimen. Meansof the above characters were subjected to statis-tical analyses using the SAS package.

Population Dynamics ofR. solanacearum in Resist-ant and Susceptible Chilli CultivarsFifty plants each of resistant cv. Kulai and sus-ceptible cv. LC 455 were propagated as above,then transferred into pots containing sterilizedsoil mixture. Thirty-five day old plants were stem-

inoculated as above with a bacterial suspensioncontaining approximately 2. 2 x 108cfu/ml.

Bacterial population was assessed by ran-domly harvesting five plants for each cultivar at4-day intervals. Samples from the tap root, mid-stem and collar region were cut and maceratedwith phosphate buffer in a stomacher laboratoryblender for 2 min. The tissues were incubatedfor 20 min at 15°C, after which serial dilutionswere made and 0.1 ml of the bacterial suspen-sion was plated on tetrazolium chloride (TZC)agar medium (Kelman 1954) in five replica-tions. Plates were incubated at 30°C for 48 h.The bacterial population was expressed in logunit per g fresh matter (FM). Sampling of thesusceptible plants was carried out for 16 dayswhen plants showed complete wilting. Assess-ment of bacterial population in the resistantcultivar was assessed 32 days after inoculation.The experimental design was a completerandomized design of 5 replications with 20plants per replicate. All bacterial populationdata were analysed statistically using the SASpackage.

RESULTS

The reaction of the chilli accessions and cultivarsis shown in Table 1. Analyses of variance of thepercentage of wilted chilli pepper accessionsrevealed significant differences among the ac-cessions (p= 0.01). The cultivar "Kulai" showeda high level of resistance with complete absenceof wilting caused by R. solanacearum under thepresent test conditions. Three accessions,CO1969, CO1970 and CO1553, were moderatelyresistant with wilt incidence of 21-40%, whereasall other accessions were moderately to highlysusceptible.

The moderately resistant accessions andsome of the moderately susceptible accessionsdid not show 50% wilting even after 35 daysafter inoculation (DAI), whereas most of thesusceptible accessions were more than 80% wiltedwithin 10 DAI. The moderately resistant acces-sions had longer incubation periods (8 - 10 days)than the susceptible accessions (3 - 9 days).

Mean difference of thickness of root cortexin resistant and susceptible varieties was signifi-cantly different (Table 2), but not in the case ofstems. There were significantly fewer vascularbundles in the stem and roots of plants of theresistant cultivar than the plants from suscepti-ble cultivar.

PERTANIKAJ. TROP. AGRIC. SCL VOL. 20 NO. 1, 1997

M. A. RAHMAN AND HIRYATI ABDULLAH

TABLE 1Reaction of 42 chilli accession s/cultivars to Ralstonia solanacearum

Accessionno.

FI hybridCO 1436COO 801COO 751CO 2800CO 2489

COO 800COO 222-2COO 788CO 3865

COO 829COO 832COO 767

COO 722CO 1966CO 3982-ACOO 707COO 721CO 1964CO 1970CO 1969Local

CO 1175

COO 268

COO 758CO 1826COO 679COO 835CO 1664

CO 4394CO 1553

COO 776-ACOO 776CO 4166

CO 1379CO 4171

Name of accession

Capsicum annuumLong chilli 455P3/78Nose GayChilli TepinePI 271322Chile Duke Num-135 RJEP 204/76PI 163201Little TomatoW-C 2068HabaneroFordhookRed Cherry HotThick Wall WorldBeaterVP 46API 165588PI 439274PimentoVP 34PI 165510PI 166368PI 65691Kulai

Capsicum spp.Crillo de Marelos334Paul Smith's ASerano 1534CAP 291/96PI 138560Num 598Orias KossarvuP 493/80

Capsicum chacoensePI 260435P498/78

Capsicum fruiescensOse UtoroOse Utoro136 BGH 412Pimenta WildP8/85146 BGH 823Pimenta

Source

TaiwanMexicoUSAUSAMexicoMexico

USAIndiaUSAMexico

USAUSAUSA

IndiaIndiaUSAUSAIndiaIndiaIndiaIndiaMalaysia

France

USA

GuatemalaIranGuatemalaHungaryDenmark

BoliviaEngland

NigeriaNigeriaBrazil

MexicoBrazil

% ofwilt*

1009696969292

92888884

*848080

7668686452444032

0.0

96

96

9288887248

9636

1009292

6060

Reaction**

HSHSHSHSHSHS

HSHSHSHS

HSS

s

ssss

MSMSMRMRHR

HS

HS

HSHSHSS

MS

HSMR

HsHSHS

MSMS

Incubationperiod(day)

4577

5

6555

767

676779

1010

-

4

3

45577

68

567

77

50%wilt

DAI***

67

101076

8777

101010

81299

11

-

-

7

5

789

11-

9-

67

10

119

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 20 NO. i, 1997

SUSCEPTIBILITY OF CAPSICUM SPECIES AND CULTIVARS TO RALSTONIA SOLANACEARUM

Table 1 (contd.)

Accessionno.

CO 4675CO 4681CO 4685CO 4690CO 4682CO 4678

Name of accession

Capsicum chinensePI 441598PI 441604PI 441608PI 441613PI 441605PI 441601

Source

BrazilBrazilBrazilBrazilBrazilBrazil

LSD = 14.47

% o fwilt*

10010096928880

Reaction**

HSHSHSHSHSS

Incubationperiod(day)

556756

50%wilt

DAI***

79778

10

*Means derived from angular transformed data.**0 = HR (highly resistant, plants did not show any symptoms of wilt); 1 = R (resistant, 1-20% plants wilted) ;2= MR (moderately resistant, 21-40% plants wilted); 3 = MS (moderately susceptible, 41-60% plants wilted); 4= S (susceptible, 61-80% plants wilted); and 5 = HS (highly susceptible, more than 80% plants wilted).***DAI= Days after inoculation

The numbers of vessel elements in the stemwere fewer and the lengths of vessel elements inroots and stems of resistant plants were shorterthan in susceptible cultivars with the differencesstatistically significant. Xylem elements in rootsand stems of cv. LC 455 were 83.03 and61.00 |j,m longer respectively than those of cv."Kulai". The mean diameter of xylem elementsin roots did not differ statistically but was signifi-cantly different for the stem.

Both cultivars tested were infected by thepathogen, which then multiplied regardless oftheir susceptibility. Bacterial population densitydiffered significantly at similar sites of detectioni.e. mid-stem, collar and tap root at differenttimes for susceptible cv. LC 455 and resistantcv. "Kulai" cultivars (Table 3, 4, 5). However,bacterial population density among the mid-stem, collar and tap root regions did not differsignificantly in the susceptible cultivar (Table6). Some susceptible plants showed symptomsof wilt after 5 days' inoculation, with completewilting by 16 days. Accordingly, the test plantswere no longer suitable for furtherinvestigation.

In the resistant chilli cv. "Kulai", the ftsolanacearum population differed significantly(p=0.01) between mid-stem and tap root regions(Table 6). The highest population obtained fromall sites tested was c. 107cfu/g FM.

The bacterial population increased initially,remained stable for a few days and then ap-

peared to decline in the resistant cv. "Kulai"(Table 7) during the period of investigation.The population differed significantly among thesites, i.e. tap root, collar and mid-stem regions,and at various sampling times. The resistantplants did not produce any symptom of wiltduring the whole study period.

In the susceptible cultivars, bacterial popula-tion did not differ significantly at 12-16 samplingdates among mid-stem, collar and tap root. Thepopulation obtained was c. 109 cfu/g FM from8-16 days after inoculation (Table 6 & 7).

DISCUSSION

Forty-two accessions/cultivars of Capsicum spe-cies were evaluated for resistance to ftsolanacearum under greenhouse conditions. Re-sults revealed that cv. "Kulai" showed a highlevel of resistance to the pathogen without anyvisible signs of wilting. Three accessions (CO1969,CO1970 and CO1553) were moderately resistantwhereas all the other accessions were moder-ately to highly susceptible.

For tomato, expression of resistance underhigh temperature may be lacking (Krausz andThurston 1975) or may break down (Mew andHo 1977). Resistance in potato was also found tobe temperature sensitive and strain specific(French and De Lindo 1982). Difference inresistance may also be due to the occurrence ofdifferent races. Buddenhagen and Kelman (1964)reported that three races can occur in

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 20 NO. 1, 1997

TABLE 2Anatomical characters of susceptible chilli cv. LC 455 and resistant cv. Kulai to Ralstonia solanacearum

SflKA

??Oy>2pG

oO

O

1997

Characters

RootThickness of cortex (p.m)Number of vascular bundlesLength of xylem element (|im)Diameter of xylem element (jam)

StemThickness of cortex (|J.m)Number of vascular bundlesLength of xylem element (fim)Diameter of xylem element (|im)

Long chilli

Range

61.34.0

149.022.0

125.04.0

208.120.0

Xylem elements per vascular bundle 13.0

- 127.25.0

- 460.0- 55.0

- 396.05.0

- 588.1- 63.2- 23.0

455

Mean

88.2 ±4.54 ±

303.8 ±28.5 ±

222.0 ±4.5 ±

438.5 ±32.8 ±17.1 ±

Variety

±S.E.

1.650.07

11.90.61

9.90.07

12.020.830.94

Kulai

Range

80.0 - 187.54.0 - 5.0

103.6 - 334.517.0 - 50.0

165.0 - 372.04.0 - 5.0

138.0 - 334.518.7 - 60.011.0 - 17.0

Mean ± S.E.

117.9 ± 3.874.18 ± 0.06

220.5 ± 7.0828.0 ± 0.55

238.0 ± 5.744.18 ± 0.06

377.5 ±17.328.5 ± 0,4414.0 ± 0.58

Significance

**

ns

ns*

******

CV(%)

8.484.56

11.496.52

10.754.618.786.56

15.86

S.E. • standard error, ns= not significant; * and **= significant at (p = 0.05) and (p = 0.01) respectively; CV = coefficient of variation

SUSCEPTIBILITY OF CAPSICUM SPECIES AND CULTIVARS TO RAI^IONIA SOIJiNACEARUM

TABLE 3Bacterial population at the mid-stem region of chilli

for the susceptible cv. LC 455 and resistantcv. Kulai after stem inoculation

Days afterinoculation

48

1216

Bacterial population(log cfu/g FM)

LC455

8.889.289.299.22

± 0.03± 0.2± 0.06± 0.1

± S.E.

Kulai

6.097.698.298.01

±±±±

0.050.20.30.02

Signifi-cance

**

******

** = significant at p = 0.01 (t-test); S.E. = standarderror; FM = fresh matter

TABLE 4Bacterial population at the collar region of chilli

for the susceptible cv. LC 455 and resistantKulai after stem inoculation

Days afterinoculation

48

1216

Bacterial population(log cfu/g FM)

LC 455

7.729.139.439.19

± 0.03± 0.05± 0.04± 0.1

± S.E.

Kulai

5.157.568.008.17

±±±±

0,0,0.

0,

,2.814

Signifi-cance

**

******

** = significant at p = 0.01 (t-test); S.E. = standarderror; FM - fresh matter

TABLE 5Bacterial population at the tap root region of chilli

for the susceptible cv. LC 455 and resistant cv.Kulai after stem inoculation

Days afterinoculation

48

1216

Bacterial population(log cfu/g FM)

LC 455

8.709.149.309.24

± 0.1± 0.05± 0.4± 0.1

± S.E.

Kulai

4.767.217.628.01

±±±±

0.70.20.30.3

Signifi-cance

**

******

** - significant at p = 0.01 (t-test); S.E.error; FM = fresh matter

standard

Pseudomonas solanacearum. In the present study,under greenhouse conditions the air tempera-ture and relative humidity were 20-33°C and 55-90%, respectively, and the bacterial strain usedmay be different. These factors, together withthe influence of other environmental factors,

TABLE 6Mean bacterial population in chilli susceptible cv.

LC 455 and resistant cv. Kulai after steminoculation upon 16 days of sampling

Sites of Bacterial population*detection

LC455

(log cfu/g FM)

Kulai

Mid-stemCollarTap root

9.15 a*9.11 a8.90 a

7.54 a**7.23 ab6.76 b

* and ** any two means in a column followed by acommon letter are not significantly different atp • 0.05 and p = 0.01 respectively, as determined byDuncan's multiple range test;*** Mean bacterial population was obtained from 20observations of 4, 8, 12 and 16 days sampling afterinoculation of the plants

such as soil moisture and temperature, mayinfluence resistance. However, the results of thepresent study are in agreement with those ob-tained by Hanson et al (1996), where the fieldreaction of tomato lines to different strains of R.solanacearum differed at different locations inSouth East Asia. They found that in Malaysiaand Taiwan most tomato lines were resistant,but in the Philippines and Indonesia they weresusceptible. This indicates that it is necessary toevaluate the germplasm in local conditionsagainst a particular pathogen.

The longer incubation period of the resist-ant and moderately resistant accessions com-pared to the susceptible ones indicated thatresistance could either delay the initial infec-tion of the disease or slow down the rate ofwilting. The incubation period for R.solanacearum in chilli appeared to be generallysimilar to that of tomato (Atabug and San Juan1981); in 10% of the wilted plants assessed forthe incubation period of the pathogen theresistant accessions had longer incubation peri-ods compared to those of the susceptible acces-sions. The present study, based on the assess-ment of 50% wilted chilli plants, producedsimilar results, i.e. the resistant accessions hadlonger incubation periods and took a longertime to produce disease symptoms than thesusceptible accessions.

The present study indicated that fewer ves-sels and a thicker cortex in roots, and a shorterlength and smaller diameter of the xylem ele-

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 20 NO. 1, 1997

M. A. RAHMAN AND HIRYATI ABDULLAH

TABLE 7Development of bacterial population in resistant cultivar Kulai and susceptible LC 455

after stem inoculation for 32-day period of investigation

Sites of Cultivarsdetection

Mean bacterical population at day after inoculation (log cfu/g FM)

12 16 20 24 28 32

Mid-stem

Collar

Tap root

KulaiLC455

KulaiLC 455

KulaiLC 455

6.898.88

6.107.73

5.268.70

ab

bc

cc

7.709.27

7.269.13

7.219.14

aa

ab

bb

8.289.29

8.009.43

7 629.30

aa

ba

ca

8.179,21

8.089.19

7.419.24

aa

ab

ba

7.70 a 4.26 a 6.61 a 6.16 a

7.68 a 7.02 b 6.62 b 5.59 b

6.15 b 5.41 c 5.11 c 4.66 c

Any two means in a row with the same letter are not significantly different at p = 0.01, as determined by theDuncan's multiple range test

merits in stems of resistant plants, may restrictthe growth and movement of the pathogen.Since R. solanacearum is a vascular pathogen,these characters possibly hinder occurrence ofinfection and reduce bacterial movement post-entry. Plugging of vessels by the growth oftyloses (Beckman 1966; Wallis and Truter 1978),accumulation of gums (Elgersma 1967) andocclusion of vessels by vesicles (Shi et al 1992)have been suggested as mechanisms for localiz-ing or slowing the vertical distribution of vascu-lar pathogen. Since large vessels will not beoccluded as rapidly as small ones, the distribu-tion of bacterial cells will not be hindered andthe disease severity of susceptible cultivar willbe higher.

In addition, the small number of vascularbundles more deeply embedded in the corticaltissues of the resistant plants were less likely tobecome infected than numerous vascular bun-dles covered by a thin cortex, as seen in suscep-tible plants. The shorter elements of the resist-ant cultivar also probably hinder the movementof bacterial cells compared with the longer ele-ments of the susceptible cultivar, due to thepresence of more obstructions at the point ofunion of the elements.

Results of the present study confirm theearlier report of Cho et al (1973) who foundthat in resistant alfalfa cultivars there were mor-phological barriers to Corynebacterium insidiosum.They indicated fewer vascular bundles, shortervessel elements, and a thicker cortex in resistantalfalfa cultivars than susceptible cultivars.

Similarly, resistance in sugarcane cultivarsto ratoon stunting disease (RSD) bacterium isattributed to the low number of large, continu-ous vessels, which pass through the nodes, whichare effective in reducing pathogen movement(Teakle et al 1975).

Thus, selection of anatomical characters suchas thick root cortex, low numbers of vascularbundles and short vessel elements would beuseful in breeding programmes designed to con-trol bacterial wilt.

All chilli cultivars in this study were infectedby R. solanacearum regardless of their degree ofsusceptibility to the disease. The resistant cultivardid not show any wilt symptoms throughout theperiod of study, even though the pathogen wasdetected in the plants. This shows that cv. "Kulai"could be latently infected. Difference in diseaseexpression as well as the multiplication of thepathogen resulted from subsequent in planta,plant- R solanacearum. interactions. The resultsof the occurrence of latent infection in resistantchilli corroborates those reported for resistanttomato cultivars (Prior et al 1990) and resistantchilli peppers (Prior et al 1994).

The susceptible cv. LC 455 showed initialsymptoms of wilt within 5 days of inoculation,and high bacterial density was recovered fromall sites. Complete wilting occurred within 16days, suggesting that wilt susceptibility resultedfrom simultaneous rapid bacterial colonizationand increase of bacterial population in vasculartissues.

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 20 NO. 1, 1997

SUSCEPTIBILITY OF CAPSICUM SPECIES AND CULTIVARS TO RAISTONIA SOIANACEARUM

Results revealed that the multiplication and

distribution of the pathogen in resistant cultivars

was slower than in the susceptible cultivars. Lower

multiplication rates of Xanthomonas phaseoli on

resistant bean leaves and P. solanacearum on

resistant tomato plants were reported by Cafati

and Saettler (1980) and Grimault et al. (1993),

respectively. Morphological barriers that restrict

pathogen spread have been reported in other

bacteria-plant systems. For example, the popula-

tion of Erxvinia amylovora in susceptible apple

cultivars was more than 104 times greater than in

resistant cultivars (Tomasik and Goodman 1986).

Besides immobilization of the pathogen, ana-

tomical features such as vessel diameter and

vessel branching in resistant plants appeared to

be involved.

From these observations, it is hypothesized

that the anatomy of resistant cultivars plays a

significant role in restricting the spread of patho-

gen in the host. On the other hand, Grimault

and Prior (1993) reported that conducting ves-

sels of tomato plants resistant to bacterial wilt

may have some tolerance capacity to bacterial

mass because resistant chilli plants also showed

an ability to tolerate the pathogen. Thus, the

resistant cv. "Kulai" behaves as a symptomless

carrier. Some other plant species, such as to-

bacco (Winstead and Kelman 1952), potato

(Ciampi and Sequeira, (1980) and tomato lines

(Prior et al 1994), have also been reported to be

symptomless carriers of P. solanacearum.

In conclusion, this study shows that cv.

"Kulai", which bears locally acceptable fruits and

has a high level of resistance to bacterial wilt,

could be used in developing wilt-resistance

cultivars and is recommended for cultivation in

wilt-prone areas.

ACKNOWLEDGEMENT

We thank Prof. A.C. Hayward for drawing ourattention to the correct name of Ralstoniasolanacearum.

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