S. L. J. TpSci. 73(1). 29 - 38, 2008. Printed in Sri Lanka "Evaluation of Tea (Camellia sinensis L.) Germplasm for Host- plant Resistance to Shot-hole borer, Xyleborus fomicatus Eichh. (Coleoptera: Scolytidae) R S Walgama1, M T K Gunasekare2, M M Jayathilake1, J D Kottawaarachchi2 and D D Liyanage1 ('Entomology Division, Tea Research Institute of Sri Lanka, Talawakelle. Sri Lanka 2Plant Breeding Division, Tea Research Institute of Sri Lanka, Talawakelle, Sri Lanka) \ ABSTRACT Shot-hole borer (SHB), Xyleborus fomicatus Eichh. is a major pest in tea plantations in many south asian countries including Sri Lanka and India. Presence of high degree of resistance to this pest is an important consideration in developing new tea cultivars for tea growing regions where the pest is a major threat for crop productivity. However, high degree of resistance is not found in cultivated tea. Evaluation of germplasm helps identification of sources of resistance and promotes effective and greater utilization of the germplasm in breeding and crop improvement. The present study investigate the extent of variations in host plant resistance to SHB in tea germplasm, in order to search for high levels of resistance enabling identification of candidate parents for breeding SHB resistant tea cultivars. A representative sample of germplasm collection consisting of 74 accessions covering cultivated and unadapted tea genotypes conserved in the ex situ field gene bank was evaluated in the field under natural infestation to study the profile of tea genotypes for SHB infestation. More than 81 % of the accessions showed high level of susceptibility to SHB. Only 3% of the accessions were found to have higher levels of resistance than TRI 2023, which is currently considered as the highly resistant tea cultivar among the adapted gene pool. Accessions, DG 7, DG 39 and DG 66 were shown to have higher resistant levels than all the other accessions evaluated. The present study provides information on new and additional sources of SHB resistant genotypes present in the existing germplasm collection which could be used in the breeding program to incorporate high levels of SHB resistance. INTRODUCTION Tea (Camellia sinensis L.) is an important plantation crop in Sri Lanka, which contributed 1.2% of the share of agriculture to the GDP (Anon, 2005). It is imperative to increase the productivity of tea plantations to enhance the contribution by the tea sector to the national economy. Productivity and sustainability of tea plantations depend on many factors and among them availability of improved tea cultivars possessing 29 desirable traits is the main factor. Furthermore, genetic resistance against insect pests has become a decisive factor determining the productive value o f a cultivar. Shot Hole Borer (SHB) is a major pest in tea, particularly in plantations situated in the elevation range o f about 200 - 1400m amsl (Walgama and Pallemulla, 2005). Control o f the damage due to shot-hole borer results in a yield increase o f about 25% over the cycle on average and the percent increase is the highest in the second year o f the pruning cycle (Cranham, 1963; Cranham et ai, 1966; Judenko et al., 1962). Around 40% of yield increase could be achieved by controlling severe SHB infestations. Host-plant resistance has been viewed as the most environmentally friendly and economically feasible method o f control of many pests (Waage, 1996), including SHB (Danthanarayana, 2003). Cultivars possessing genetic resistance to pests are promising components in an integrated pest management system and have the potential to reduce pesticide use and production cost while increasing on-farm yields. Tea breeders face formidable challenges in developing high yielding cultivars incorporated wi th genetic resistance to pests and diseases owing to inaccessibility for sources wi th adequate levels o f resistance to pests and diseases. In order to search for sources o f resistance to be utilized in resistant-breeding program, evaluation o f available tea genetic resources forbiotic stress resistance is an essential component (Gunasekare, 2007). Despite the potential use o f unadapted sources for breeding purposes, the germplasm of tea in Sri Lanka has not been sufficiently evaluated or utilized effectively in resistant breeding programs (Gunasekera and Kumara, 2005). As the pool o f resistant germplasm for tea breeding is small, additional genotypes must be identified with the increased levels of resistant to the SHB than the existing cultivars. The objective of this study therefore, was to identify tea germplasm (both cultivated and unadapted), having higher levels of resistance to SHB than what is currently available in the cultivated gene pool. MATERIALS AND METHODS Seventy-four germplasm accessions conserved in the ex situ gene bank at Uva Extension Centre o f Tea Research Institute at Passara (Agro Ecological Region - IU3, Lat: 6° 5 6 N , Long: 81°07E, Elevation 1120m amsl) were evaluated to identify sources of resistance to SHB. Details of the accessions evaluated are given in Table 1. Selection of regional gene bank site for this study was based on the pest pressure at the site (elevation 1120m amsl) to ensure effective field screening to assess the sensitivity o f the accessions to SHB attack. These accessions were planted as a germplasm backup collection and have been therefore nearly 60 years demonstrating their adaptability to the environment in the particular site. 30 Twenty randomly selected plants per accession from a plot containing 30 plants per each accession were evaluated for pest attack just before the plants were pruned. All the normal cultural practices were followed with the exception that no chemical treatment was used to control SHB in the gene bank block. From each plant a 30cm long stem piece, that originated from the previous prune cut was removed (Vitarana, 2003). Those stems were then split open longitudinally and number of galleries was recorded. The infestation levels, in terms of the average number of galleries per accession, were computed as (total number of galleries/ number of stems in the sample) and sensitivity to SHB was evaluated for all accessions using the following descriptor state (scale): 1 Highly resistant where average gallery numbers are in the range of 0 -1 .0 2 Resistant where average gallery numbers are in the range of 1.1 - 2.0 3 Moderately susceptible where average gallery numbers are in the range of 2.1- 3.0 4 Susceptible where average gallery numbers are in the range of 3.1 - 4.0 5 Highly susceptible where average gallery numbers are > 4.0 RESULTS AND DISCUSSION Screening of the germplasm collection conserved at the Uva Extension Centre of Tea Research Institute showed that the shot-hole borer infestation levels varied in a wide range. The lowest infestation level was 0.9 and the highest infestation level recorded was 6.5 galleries per stem (Figure 1). The high values of infestation indicate the existence of extreme pest pressure at the gene bank site and extreme vulnerability of some tea accessions to SHB infestation. The distribution of SHB sensitivity of the genotypes was continuous and skewed towards high severity levels (Figure 1). Of the 74 accessions evaluated, 81 % of the accessions showed infestation levels of more than 3 (Figure 2), which is considered as a high infestation level. Among the tea cultivars recommended, TRI 20213 and TRI 2025 have been confirmed as tolerant and susceptible respectively, to SHB with several years of observations and through screening procedures (Thirugnanasundaran and Calnaido, 1968, 1969). Compared to the highly resistant, TRI 2023, which is currently considered as ''resistant" to SHB, three accessions namely DG 7, DG 39 and DG 66 that belong to estates selection category showed higher levels of resistance to SHB and infestation was recorded as less than 2.0 (Figure 1). All three accessions were the selections made from old seedling tea population on the same tea estate (Balangoda Estate) in the agro-ecological region WM3. In the past, when mass selection from seedling populations were adopted, one of the criteria for selecting promising seedling bushes were the resistance to major pests and diseases (Visser and Kehl, 1958). Furthermore, SHB incidents in that location where the particular accessions have been originally selected were reported to be high (Walgama and Pallemulla, 2005). 31 Table 1. Accession identity, origin and its agro-ecology, and type o f material o f the tea accessions evaluated for SHB sensitivity Accession Origin/ Parentage/ Agro-ecological Type o f the Accession location o f selection region TRI 18 St Coombs W U 2 Unadapted TRI 23 St Coombs W U 2 Unadapted TRI 25 St Coombs W U 2 Unadapted TRI 740 St Coombs W U 2 Unadapted TRI 1076 St Coombs W U 2 Unadapted TRI 2012 St Coombs W U 2 Unadapted TRI 2016 St Coombs W U 2 Cultivated TRI 2020 A S M 4/10 O.P. W U 2 Cultivated TRI 2023 A S M 4/10 O.P. W U 2 Cultivated TRI2024 A S M 4/10 O.P. W U 2 Cultivated TRI 2025 A S M 4/10 O.P. W U 2 Cultivated TRI 2026 A S M 4/10 O.P. W U 2 Cultivated TRI 2027 A S M 4/10 O.P. W U 2 Cultivated TRI 2043 Indo-China Introduction WU2 Cultivated TRI 2086 St Coombs W U 2 Unadapted A M A 5/60 Ampittikanda I U 3 Unadapted A M A H 3/12 Ampittikanda I U 3 Unadapted B 3 5 Gartmor W U 1 Unadapted C 21 Chapelton W U I Unadapted CH 13 Craighead W M 2 Advanced breeding line C Y 9 Thangakelle W U 1 Cultivated D Dayagama W U 2 Unadapted DG 3 Balangoda W M 3 Advanced breeding line DG 7 Balangoda W M 3 Advanced breeding line D G 3 2 Balangoda W M 3 Unadapted D G 3 9 Balangoda W M 3 Advanced breeding line DG 52 Balangoda W M 3 Unadapted DG 66 Balangoda W M 3 Unadapted DW 3 Downside I U 3 Unadapted DW 5 Downside I U 3 Unadapted DW 12 Downside I U 3 Unadapted DW 16 Downside I U 3 Unadapted D W 19 Downside I U 3 Unadapted DW 26 Downside I U 3 Unadapted DW 29 Downside I U 3 Unadapted DW 32 Downside I U 3 Unadapted HELD 163 Kirkoswald W U 1 Unadapted G M T 9 Gonamottawa I U 3 Unadapted contd. to page 33 32 contd. from page 32 Accession Origin/ Parentage/ Agro-ecological Type of the Accession location of selection region K145 Kirkoswald WU 1 Cultivated K 150 Kirkoswald WU 1 Unadapted KEN 15/2 Kenil worth WM 1 Unadapted KEN 16/3 Kenilworth WM 1 Cultivated KEW 14/1 Kew WU 1 Unadapted MG 3/31 Cannavarella IU3 Unadapted MT 18 Balangoda WM 3 Cultivated MT 20 Balangoda WM 3 Unadapted MT 35 Balangoda WM 3 Unadapted MTBG Balangoda WM 3 Advanced breeding line NK3B1 Cannavarella IU3 Unadapted NL 3/1 Neluwa IU3 Unadapted NL 8/3 Neluwa IU3 Unadapted OK 4 Ouvakelle WU 1 Unadapted PD 14 Gonakelle IU3 Unadapted PGG2 Poonagala IU3 Unadapted PLLG 1 Poonagala IU3 Unadapted PLLG2 Poonagala 1U3 Unadapted PUH 1 Poonagala IU3 Unadapted QT 1/3 Queenstown IU3 Unadapted QT 4/4 Queenstown IU3 Unadapted QT 7/1 Queenstown IU3 Unadapted SS/P Passara IU3 Unadapted T5/2 Thotalagala IU3 Unadapted T5/3 Thotalagala IU3 Unadapted TK 42 Thalankanda WU 1 Unadapted UH 3/4 Uva Highlands IU3 Unadapted UH 9/3 Uva Highlands IU3 Advanced breeding line UR 12 Uda Radella WU 2 Unadapted VK9 Ouvakelle WU 1 Unadapted W7W1/1 Welimada IU3 Unadapted W/W3/10 Welimada IU3 Unadapted W7W5/1 Welimada IU3 Unadapted W/W6/1 Welimada IU3 Unadapted W/W7/1 Welimada IU3 Unadapted Y2/3 Yapame WU 1 Unadapted O.P. : Open Pollinated IU : Intermediate zone - Up country WU : Wet zone -Up country WM : Wet zone -Mid country 33 7.0 6.0 5.0 4.0 3.0 2.0 O O 5 1.0 0.0 II i8»3Mgai]l6;i§BB;il3Mii;gl!iBgiIEBlg|8jB[aggSMM|aiE3M!^ O o ! 3 Accessions F igure 1. Infestat ion levels o f the accesions o f the germplasm in M i d country dry zone as expressed in terms o f the average number o f gal leries/ 30 cm stem o f tea =0-1.0 •1.1-2.0 •2.1-3.0 -#3.1-4.0 • > 4 . 0 >4 . ( ) (39%) 2 . 1 - 3 . 0 ( 1 5 % ) 3.1-4.0 (42%) Figure 2. Percentage o f accessions in d i f ferent in festat ion categor ies Thus, it cou ld be assumed that those accessions wh ich were adapted to the said locat ion show S H B resistance. A m o n g those three accessions, highest level o f resistance were f ound in D G 39 and D G 66 for w h i c h o n l y less than 1.0 ga l le ry percentage was recorded (F igure 1). O n l y few resistant tea cu l t i vars to S H B have been reported in the past (V i ta rana, 2003 ) , but that was not as h i g h as the resistant levels captured i n the present study. Results o f the present s tudy also revealed that a h igh level o f resistance is f ound o n l y in a l im i t ed number o f accessions and that is accounted for 3 % o f the tota l accessions evaluated. Even in r ice, resistance to insects was reported to present o n l y in 0.01 to 2 % o f r ice germplasm accessions (Heinr iche, 1986). Successful ident i f icat ion o f candidate parents w i t h resistant traits depends p r imar i l y on the avai lab i l i ty o f adequate levels o f resistance and the present s tudy reveals that those t w o par t icu lar accessions posses h igher levels o f resistance. A s the poo l o f resistant germp lasm for b reed ing for S H B resistance in tea is sma l l , o n l y T R I 2023 have been used recur rent ly in the breed ing p rog ram to incorporate S H B resistance to the new progenies. There fo re , h i g h levels o f resistance captured f r o m the present local germp lasm co l lec t ion are h i g h l y usefu l as a source for future breed ing p rog ram a i m i n g at incorpora t ing S H B resistance to p roven cu l t ivars . Th is is the first study conducted on eva luat ion o f a w i d e range o f ge rmp lasm on compara t ive basis f o r S H B at a site where h igh S H B pressure exists. A l t hough there are ample evidence that introgression breeding cou ld p lay a v i ta l ro le in t ransferr ing resistant genes f r om w i l d relatives or related species (Ocampo et ai, 2000 ; 35 0-1.0 1.1-2.0 2.1-3.0 3.1-4.0 >4.0 Damage range (in terms of gallery numbers) Figure 3. Frequency d is t r ibu t ion o f tea accessions based on S H B sensi t iv i ty (So l i d arrow indicates the posi t ion o f resistant cu l t ivar [ T R I 2023] wh i l e the hatched ar row indicates the pos i t ion o f susceptible cu l t i var [ T R I 2025 ] Saxena and Kumar , 2003; Clement , 2002; Russell , 1981), problems exist when attempts were made to cross such candidate parents, ma in l y due to incompat ib i l i t i es (Mar ta et al., 2004; Cohen et al., 1984) and l inkage drag o f undesirable or agronomica l ly in fer ior traits w i th the target gene (Russel l , 1981). As a general rule, the best sources o f resistance are l i ke ly to be those in loca l ly adapted germplasm because they can easi ly and qu ick l y be exp lo i ted in a b reed ing p rogram (Russel l , 1981). Hence, it is economica l l y feasible and techn ica l l y -sound i f the sources o f resistance for the trait cou l d be found among the cu l t i va ted gene pool or advanced breeding l ines. F r o m a b reed ing perspect ive, it may be easier to t ransfer resistant genes f r om cu l t iva ted or ex is t ing o ld var iet ies than f r om the w i l d re lat ives or re lated species/genera o w i n g to the issues discussed above. As two o f the accessions, D G 7 and D G 39, do not be long to w i l d types or unadapted germplasm l ines, those cou ld be readi ly used in the breeding p rogram w i thou t negative impacts on l inkage drag o f undesirable traits into the resu l t ing progeny. Thus, t ransfer o f S H B resistance captured in the present study in to h igh y i e l d i n g cu l t i va rs or to a more adapted genetic background w i t h other desirable trai ts w i l l be considered in the tea-breeding p rog ram. REFERENCES A n o n 2005 A n n u a l Report o f the Central Bank o f Sri Lanka, p 22, Centra l Bank o f Sri Lanka, Sri Lanka. 36 Clement S L 2002 Insect resistance in the wi ld relatives o f food legumes and wheat. 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