Euphytica 89: 319-324, 1996 .

319© 1996 Kluwer Academic Publishers. Printed in the Netherlands.

Initial screening of chili and sweet pepper germplasm for resistance to chilithrips, Scirtothrips dorsalis Hood

N.K. Krishna Kumar', M. Aradya2 , A.A. Deshpande3 , N. Anand3, * & PR. Ramachandar4'Division of Entomology & Nematology, Indian Institute of Horticulture Research, Hessaraghatta Lake Post,

Bangalore 560 089, India ; 2 Department of Botany, University of Hawaii at Manoa, Honolulu, Hawaii 96822

U.S.A . ; 3Division of Vegetable Crops, Indian Institute of Horticulture Research, Hessaraghatta Lake Post,

Bangalore 560 089, India ; 4Section of Economics & Statistics, Indian Institute of Horticulture Research,Hessaraghatta Lake Post, Bangalore 560 089, India ; * present address : Pioneer Overseas Corp. (India Branch),283, 8th Main, I Block R . T. Nagar, Bangalore- 560 032, India

Received 6 January 1995; accepted 26 October 1995

Key words: Capsicum annuum, chili pepper, resistance, Scirtothrips dorsalis, sweet pepper, thrips

Summary

A preliminary evaluation for resistance to chili thrips, Scirtothrips dorsalis Hood of 41 and 194 pepper (Capsicumannuum L.) germplasms during 1987 and 1988, respectively, indicated chili accessions may be a promising sourceof resistance . In contrast, all sweet pepper accessions tested were highly susceptible . A number of chili accessionsproduced a moderate yield, in spite of a high thrips infestation indicating tolerance to S. dorsalis . Rating for thripsdamage was more reliable and efficient than estimating thrips numbers in screening pepper accessions for resistanceto thrips. Highly significant, positive correlation between ratings at the seedling stage and final rating for thripsdamage indicated the feasibility of screening pepper accessions at the seedling stage . Comparing the similaritiesin rating among accessions resulted in 40 distinct groups. The variance-covariance matrix of the data from these40 groups was subjected to principal component analysis . This accounted for 56 and 18 per cent of the variationacross the two principal axes, respectively . Projection of chili and sweet pepper accessions along these two axesrevealed three distinct clusters . About 80 per cent of chili accessions formed the first cluster, 58 per cent of thesweet pepper accessions formed the second cluster and a third cluster exhibited intermediate ratings for thripsdamage. The significance of these findings in relation to geographical divergence and resistance to thrips amongpepper accessions is discussed .

Introduction

Chili thrips, Scirtothrips dorsalis Hood is a seriouspest of Capsicum annuum L., both of chili (AnanthaKrishnan, 1973 ; Amin, 1979) and sweet pepper types(Jagan Mohan et al ., 1980) . Adults and nymphs ofS. dorsalis pierce plant cells and suck the sap fromtender leaves, growing shoots and developing fruitsand flowers. Affected leaves curl upwards and exhibitcharacteristic `leaf curl' symptoms . Further, feedingscars as a result of thrips damage can seriously affectthe market quality of sweet peppers . In addition, S .dorsalis is a vector of tomato spotted wilt tospovirus(Amin et al., 1981), a viral disease of peppers (Cho

et al ., 1989) . Use of insecticides for the control ofmajor pests of chili peppers has not always been suc-cessful (Rao & Ahmed, 1986) . With a view to reduc-ing pesticide use in vegetable production, host plantresistance to insect pests merits attention and resistantcultivars could be a cornerstone to an integrated pestmanagement programme for peppers . Resistance towestern flower thrips, Frankliniella occidentalis (Per-gande) in five commercial pepper cultivars was shownto be based on mechanisms of tolerance rather thanto antibiosis or antixenosis (Ferry & Chalk, 1991) .Preliminary evaluation of chili pepper accessions forresistance to S. dorsalis is reported from India (David &Natarajan, 1983 ; Sanap & Nawale, 1985 and Tewari et

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al ., 1985) . However, sweet pepper accessions are yet tobe evaluated under tropical and subtropical conditionsfor resistance to S . dorsalis . With few reports of resis-tance in peppers to insects and mites, detailed investi-gations on screening protocols, genetics of insect resis-tance and transfer of resistance to new pepper varietiesneeds further investigation (Poulos 1994) . In a plantbreeding programme it is important to classify largenumbers of indigenous and exotic genotypes to identi-fy suitable parent material for selection and/or furtherbreeding . Principal component analysis (PCA) sug-gested by Lawley and Maxwell (1963) may be a usefultool for the analysis of resistance to S. dorsalis . Theobjectives of this research were a) to evaluate availablechili and sweet pepper accessions for resistance to S .dorsalis b) to standardize sampling methods and c) toanalyze the variability among pepper accessions forresistance to S. dorsalis .

Materials and methods

Germplasm . Forty one pepper accessions represent-ing 24 chili pepper and 17 sweet pepper were evaluatedfor the incidence of S. dorsalis during the summer of1987. Another 156 chili and 38 sweet pepper acces-sions were evaluated for the incidence of S. dorsalisduring the summer of 1988 . Accession origins are pre-sented in Table 1 . Seedlings were grown in raised seedbeds in a field nursery . Nursery pests were controlledusing 0.05% monocrotophos [or 1 .25 ml of NuvacronR(Ciba-Geigy of India Ltd .,) in 1000 ml of water] 15days after germination. This prophylactic measure wasused to initially standardize the thrips population in allaccessions . Seedlings were transplanted 45 days aftergermination in rows of 10 plants per accession . Plantswere spaced 60 cm between rows and 45 cm betweenplants. Wettable sulfur @ 2g/l was sprayed to controlbroad mite, Hemitarsonemus latus (Banks) when nec-essary. Chili and sweet pepper accessions were evalu-ated in adjacent but separate blocks .

Sampling . Five plants were selected at random fromeach of the accessions and visually rated for thripsinfestation based on damage symptoms . The ratingwas done at 8-10 day intervals. Chili peppers wererated for 8 weeks and sweet peppers for 7 weeks . Thescale was as follows:

Score Symptoms

01

2

3

4

5

No symptomsTerminal 3-4 leaves showing tiny eruptionsin interveinal area of leafTerminal 3-4 leaves showing upward curlingalong leaf marginSevere scarring of terminal and a few basalleavesStunted plants, leaves severely curledand leaf area greatly reducedPlants with no leaves and only stemremaining

The number of thrips was estimated by sampling threeapical leaves from different plants in each accession .The buds were collected in glass vials containing apinch of acid fuchsin in 50% ETOH, and were fil-tered with Whatman #2 filter paper and the thrips werecounted under a stereo binocular microscope (Olym-pus). Acid fuchsin was added to facilitate the iden-tification of thrips larvae on white filter paper back-ground. Mature green sweet pepper fruits were har-vested at fortnightly intervals and number and weightwas recorded. Chili pepper fruits were allowed to ripenon the plant before harvest . Fruits were harvested twiceat 21 days interval and oven dried at 40 ° C (till two con-secutive weights were the same) to record dry weight .

Data analysis . Pearson's correlation was used to cor-relate between a) rating thrips damage on different daysafter transplanting (DAT), b) number of thrips recordedon different DAT, c) yield and rating in sweet pepperaccessions, d) yield and rating in chili pepper acces-sions. Accessions evaluated in 1988 were comparedfor similarities in thrips damage observed during thelast 4 weeks, because this damage was highly corre-lated with the final rating . Based on thrips damagesimilarities the total number of accessions (194) wereeffectively reduced to 40 distinct types . The resultingvariance-covariance matrix (40 x 4) was subjected toPCA. All computations were done using NTSYS-PC(Exeter Software, New York) . The first two principalcomponents were plotted to visualize the clusters .

Table 1 . Accessions and their origin evaluated for resistance to S. dorsalis

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Crop Origin Accession number

Chili Australia

BrazilBulgariaCosta Rica

DenmarkEthiopiaFranceHungary

India

IHR 292-2, IHR 207, IHR 292-3

IHR 688IHR 104IHR 470P-21, IHR 321-B

IHR 893, IHR 919IHR 497, IHR 500, IHR 387IHR 615, IHR 622, P-3X-206, NP-46, Pusa Jwala, IHR 429-9, IHR 328-9, K-2, IHR 990, KCS-1, BR-Red, Sel-1, IHR309-3-18, IHR 576, G-4, X-235,IHR 721, CA 235, Pant C1, Musalwadi Selection, CA-206, IHR310-3-1, IHR 45-9-18-2, Pant C1-11-1, KC-15-16-58-18, Cr 197-80-5/1-8-80, IHR 226-8-1-5,IHR 226-3-1-3, IHR 226 6 7-5, IHR 226-8-1-5, IHR 238, IHR 240A-1-2, IHR 243-1-1-5, IHR243-3-3-1-1, IHR 263-23-2-1, IHR 265-11-16-12, IHR 266-1-3-9, IHR 266-23-6-9, IHR268-4-7-5, IHR 274, IHR 276-15-16-4,IHR 276, IHR 277, IHR 277-5-9, IHR 278, IHR280-5/1-8-21, IHR 289, IHR 290-7, IHR 296-4, IHR 303-3, IHR 307-15, IHR 315-1,IHR 315-1-75,IHR 323-29, IHR 334, IHR 337, IHR 339, IHR 340, IHR 343,IHR 344, IHR 352, IHR 352-6-1-10, IHR353-7-1-15, IHR 352-7-1-18, IHR 352-7-2-8, IHR 352-7-1-22, IHR 354, IHR 352-7-1-47, IHR352-7-2-1, IHR 356-2, IHR 357, IHR 358, IHR 358-3, IHR 360, IHR 359, IHR 395-1, IHR 397-9,IHR 400-15, IHR 405, IHR 406, IHR 407, IHR 410, IHR 408-4, IHR 413-13, IHR 417-19, IHR 417,IHR 419-12, IHR 420, IHR 421, IHR 422-2, IHR429-19, IHR 429, IHR 434-1, IHR 436-3, IHR 437,IHR 438-15, IHR 438-4, IHR 440-8, IHR 440-8(OP), IHR 464, IHR 470, IHR 507-27-1, IHR511-18-8-10-3, IHR 511-13-28-2-1-1, IHR 514-578, IHR 581, IHR 598, IHR 606, IHR 611-11-1-1,IHR 716-1, IHR 721-1, IHR 723-3, IHR 848, IHR 990, IHR 310-3-16, AR-2, AR-5, AR-7, P1, P3,

IranIHR 185, Arka Lohit, IHR 328-9IHR 865, IHR 874

Japan IHR 113A-11, IHR 399, IHR 400, IHR 400-1, IHR 987

Korea (S) IHR 897, IHR 901, IHR 959, P-16

Malaysia IHR 859, IHR 860, IHR 860-2-11, IHR 860-7-2Mexico

Sri Lanka

IHR 82A-3

IHR 338, IHR 471-5, IHR 471-11-4-3, IHR 472-12Taiwan IHR 930, P-20Turkey IHR 869, IHR 871

Yugoslavia IHR 850, IHR 851, IHR 854, IHR 855, IHR856, IHR 858

USA IHR 726, IHR 43A-1-4-3, IHR 525-1-4-1-12, P-12USSR IHR 908-1Zambia IHR 921

Sweet pepper Australia IHR 680, IHR 681

Brazil IHR 526, IHR 536, IHR 993, IHR 994, IHR994-1, IHR 995, IHR 996, IHR 998France IHR 499IndiaItaly

HC 202, IHR 67-3, Bharat F1, KT 1, Arka Gaurav, Arka Mohini

IHR 675, IHR 213-1-1, IHR 943, IHR 968-1, IHR 1022, IHR 1023, IHR 1024Mexico IHR 563, IHR 564Peru IHR 140-1-1Rumania IHR 900, IHR 911USA IHR 88-1, IHR 478-1, IHR 131, Sel-16, IHR131-2-1-3, IHR 484-1-1, IHR 485-7, IHR 487, IHR 946,

USSRIHR 981-4, California Wonder, IHR 60-1-1IHR 841, IHR 846, IHR 846-1/4

Yugoslavia IHR 890-4, IHR 174

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a : Resistance based on rating for thrips damage .b : Tolerance based on yield .

Results and discussion

A number of promising chili pepper accessions pos-sessing field resistance to S. dorsalis were identifiedbased on lower rating for thrips damage and numberof thrips recorded (Table 2) . Accessions IHR 930 andIHR 328-9 from Taiwan and India, respectively, hadthe least number of thrips and lowest rating amongpepper accessions in 1987 . However, accessions X-235, G4 and Pant- Ct (all from India) with a relativelyhigher rating than IHR 930 and IHR 328-9, recordedhigher yield indicating innate tolerance to S. dorsalis.

During 1988, IHR 930, IHR 328-9, IHR 343, IHR472-12, IHR 497, IHR 507-27-1, IHR 723-1 and IHR723-3 recorded the smallest rating for thrips damageand no yield indicating absence of tolerance . Further-more, accessions IHR 437, IHR 606, IHR 615, IHR850 and IHR 859 had the highest yield among 156 chiliaccessions in 1988, in spite of a relatively higher ratingfor thrips damage, suggesting tolerance to S. dorsalis .Horber (1980) defined tolerance as "all plant responsesresulting in the ability to withstand infestation and tosupport insect populations that would severely damagesusceptible plants". Thus, our results indicated that rat-ing (a measure of plant damage from thrips infestation)and yield despite thrips infestation (tolerance) are twodistinct traits in chili peppers and are not significantlycorrelated .

In contrast, we were unable to locate any sweetpepper accession with appreciable resistance or tol-erance to S. dorsalis . However, in a few sweet pep-per accessions small sized fruits were observed, thatwere extensively damaged with feeding scars cover-ing almost the entire fruit surface. Unlike chili peppers

Figures followed by * indicate a significant correlation at the0 .05% probability level .Figures followed by ** indicate a significant correlation atthe 0.01 % probability level .

which are dried, powdered and used as condiments,sweet pepper is a premium vegetable in the tropicaland subtropical Asia and even a marginal reduction inthe quality of fruits will significantly reduce the mar-ketability of the fruits.

Figure 1 shows rating for thrips damage in chiliand sweet pepper accessions . There was considerablevariability within pepper germplasm for tolerance toS. dorsalis, but sweet pepper accessions were moresusceptible than chili peppers. These trends were con-sistent both during 1987 and 1988 . The results are inconformity with the speculations of Singh & Cheema(1989).

Chili peppers are the progenitors of sweet peppers .Each of the domesticated peppers may have passedthrough an evolutionary bottleneck associated withdomestication . It is likely that intense selection underdomestication for fruit size and absence of pungency,especially in sweet peppers, has narrowed the variabili-ty at the cost of various other traits such as resistance totropical pests and diseases. This may have contributedto the mechanism of tolerance to thrips in chili pep-pers. Thus, sweet peppers selected for centuries to suittemperate agriculture are not likely to possess requisiteresistance to tropical pests such as S. dorsalis.

Table 3 shows coefficient of correlation betweenweekly rating for thrips damage and final rating inboth chili and sweet pepper accessions . Highly sig-nificant correlations between ratings recorded in thefirst few weeks of transplanting and final ratings forthrips damage in both chili and sweet pepper accessionswere observed . The ability to effectively screen pepper

Table 2 . Chili pepper accessions showing field resistance toS. dorsalis

Table 3 . Coefficient of correlation between weekly rating forthrips damage and final rating

1987 1988 1987 1988Resistant' Tolerantb Resistant' Tolerantb DAT Chili Sweet DAT Chili Sweet

IHR 930 X-235 IHR 930 IHR 437 pepper pepper pepper

IHR 328-9 G-4 IHR 328-9 IHR 606 47 0.47* 0 .33 21 0.01 0.44**Pant Cl IHR 343 IHR 615 56 0.47* 0 .58* 30 0.15 0.67**

IHR 472-12 IHR 850 63 0.77** 0.47* 42 0.30** 0.64**IHR 497 IHR 859 70 0.68** 0 .68** 50 0.22** 0.78**IHR 507-27-1 81 0.85** 0 .76** 58 0.24** 0.82**IHR 723-1 90 0 .81** 0 .87** 65 0.23** 0.86**IHR 723-3 73 0.31**

80 0.34**

Rating5

4

3

2

0 I

I

J

1

2

3

4

6

6

8

WeeksChill

87 + S. Pepper 87

*- Chili 88S.Em =0.12-0 .19

0.09-0.12

0.05-0.16

accessions at the nursery stage for resistance to S. dor-salis, will reduce further field testing . However, thismay not locate accessions having inherent toleranceto S. dorsalis. Further, correlation values between rat-ings were more pronounced as the scores approachedthe final rating . Therefore, initial screening of a largegermplasm for resistance to thrips can be accomplishedat the nursery stage but promising accessions must beevaluated for tolerance by recording yield parameters .

No significant association between rating for thripsdamage and number of thrips was observed in chiliand sweet pepper accessions. The results indicate thatplant factors that influence preference to S . dorsalis inan accession and characteristics that determine planttolerance to thrips infestation may be independent ofeach other. Similarly, Fery & Schalk (1991) observedthat tolerance and not antixenosis was a major factor ofresistance to F. occidentalis in sweet pepper cultivars .

Rainfall is a major mortality factor for thrips andmay significantly alter thrips counts (Lewis, 1973) .Thus, screening pepper accessions for resistance tothrips based on insect counts may not reflect the innateantixenosis response of an accession . On the contrary,rating for thrips damage is a cumulative index, easier,rapid and less likely to be altered by fluctuations inweather parameters.

Correlation between rating and yield: Significantnegative correlation between rating for thrips damageon different DAT and yield was observed only whendata on chili and sweet pepper accessions were pooled .None of our sweet pepper accessions recorded any mar-

Figure 1 . Rating for thrips damage in chili and sweet peppers .

-s- S. Pepper 880.0-141-0-09

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ketable yield. Sweet peppers are very highly suscep-tible to thrips as compared to chili peppers (Singh &Cheema, 1989), pooling yield data from chili and sweetpeppers may lead to erroneous conclusions . When dataon rating for thrips damage and yield from chili acces-sions were analyzed separately, correlations were notsignificant . Accessions vary in their ability to yieldboth under controlled and protected conditions . It isthis inherent ability to yield under biotic stress (dam-age due to thrips) that could influence tolerance . Thus,the association between yield and rating across pepperaccessions is unlikely to be linear.

Relationship between chili and sweet peppers :Forty distinct clusters were observed when pooled dataon rating for thrips damage were subjected to clusteranalysis. When the variance-covariance matrix fromthese clusters was subjected to PCA, the first two prin-cipal axes accounted for 56 and 18 per cent of the totalvariation, respectively . The projection of these acces-sions along these two axes, revealed distinct demarca-tion between chili pepper (Cluster I) and sweet pepper(Cluster III) . However, a third cluster (cluster II) over-lapping from chili and sweet pepper accessions wasvisualized that consisted of 20 per cent of the chiliaccessions and 58 per cent of the sweet pepper acces-sions. Cluster II may represent an intermediate group,consisting of accessions originating as a result of intro-gression between these two groups. The clusters thatinclude chili or sweet pepper indicate genetic diver-gence between these groups for thrips damage . Thelevel of divergence to this important trait further indi-

324

cates differential selection regimes in areas where theseaccessions were collected .

India is considered to be a secondary center ofdiversity for peppers . Hot peppers have been selectedfor centuries for different traits in the tropics, and haveevolved into different plant types to fit area- specificcropping systems and utilization patterns. In contrast,sweet peppers were selected for centuries in the milderclimates of temperate countries . Thus, it is unlikelythat sweet pepper accessions have a high level of resis-tance to tropical pests such as S. dorsalis . Our findingssupport the observations of Yoon et al . (1989) whoreported that lack of suitable sweet pepper cultivarslimits successful cultivation in the tropics .

Acknowledgement

The authors are indebted to Dr. T. R. Subramanian,Director, IIHR for providing the necessary facilitiesin conducting the experiment. We thank Dr. S. Gane-shan, Division of Plant Genetic Resources., IIHR foruseful suggestions and critical review of the manu-script. Authors sincerely thank Mr . M. B. Muniratna-iah for field assistance and innumerable help duringthe course of the experiment .

References

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Amin, P.W., D.V.R. Reddy, A.W. Ghanekar & M .S . Reddy, 1981 .Transmission of tomato spotted wilt virus, causal agent of budnecrosis of peanut, by Scirtothrips dorsalis and Frankliniellaschultzei . Plant Dis . 65 : 663-665.

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