Greenhouse techniques to identify field resistance to the brown planthopper, Nilaparvata lugens (Stal) (Homoptera: Delphacidae ), in rice cultivars

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1 CROP PROTECTION (1986) 5 (5), Greenhouse techniques to identify field resistnce to the brown plnthopper, Nilprvt lugens (Stl) (Homopter: Delphcide ), in rice cultivrs R. VELUSAMY *' E. A. HEINRICHS** AND F. G. MEDRANO Deprtment of Entomology, Interntionl Rice Reserch Institute, PO Box 933, Mnil, Philippines ABSTRACT. Two seedbox screening techniques were compred to determine levels ofresistnce ofrice cultivrs to three brown plnthopper, Nilprvt lugens (Stl), biotypes. The commonly used stndrd seedbox screening test (SSST) where eight second-instr nymphs were infested on 7-dy-old see.dlings ws compred with modifiction (MSST) where four second-instr nymphs were infested on ech of20-dy-old plnts. In the SSST, the initil infesttion killed the susceptible cultivrs wheres in the MSST the F 1 progeny killed the susceptible cultivrs. The cultivrs ASD 11, Wgwg, Utri Rjpn, IR46 nd Kencn were susceptible in the SSST but modertely resistnt or resistnt in the MSST, indicting n increse in the level of resistnce with plnt ge. Feeding rte nd popultion growth of N. lugens were the fctors involved in the resistnce of 30-dy-old plnts. The MSST provides rpid method for identifying field-resistnt cultivrs in the greenhouse nd cn be used to identify field-resistnt lines in rice-breeding progrmme for stble resistnce to N. lugens. Introduction Rice cultivrs with resistnce to the brown plnthopper hve been developed nd relesed for commercil cultivtion in mny countries of South-Est Asi. IR26, the first cultivr resistnt to N. lugens, ws relesed in 1973 nd ws extensively grown in Indonesi, Philippines, nd Vietnm. After two or three yers of such cultivtion, IR26, which hs mjor resistnce gene (Bphl) becme susceptible becuse of selection for virulent popultion, nmed biotype 2 (Pthk nd Heinrichs, l 982). In 1976 two further cultivrs, IR36 nd IR42, with the recessive g;.;; "ph2 for N. lugens resistnce, were relesed nd these currently occupy much of the rice-growing res in the Philippines, Indonesi nd Vietnm nd re plnted in severl other countries, where IR36 lone occupies bout 11 million hectres (IRRI, 1983). N. lugens popultions tht re 'virulent' to both IR36 nd IR42 hve been reported from Mindno in the southern Philippines (IRRI, l 983b) nd North Sumtr in Indonesi (Sogw, Kilin Djtnik nd Bhgiwti, 1984). The resistnce ofir36, however, hs been more durble thn tht of IR42. In ttempts to rer these field popultions of N. lugens continuously from Mindno on IR36 in the greenhouse, the popultions grdully declined on this cultivr, wheres high popultions were mintined on IR42 (E. A. Heinrichs, unpublished work). IR46, which hs the Bphl gene, is susceptible to biotype 2 of N. lugens s 7-dy-old seedling but is resistnt to the sme biotype s n older plnt in the greenhouse nd in the field (Pnd nd Heinrichs, 1983). In interntionl tests co-ordinted by the Interntionl Rice Reserch Institute (IRRI), IR46 hs shown field resistnce in Asi despite the fct tht it is susceptible to the locl biotype in the seedling stge in severl countries (IRRI, l 982)., IR36, IR46 nd severl other cultivrs which hve been evluted in the breeding progrmme for N. lugens resistnce t IRRI, hve either mjor genes which re not expressed in young seedlings, or both mjor nd minor genes. The minor genes would provide polygenic resistnce which my be more effective thn mjor-gene resistnce in combting the genetic plsticity of pests (Simons, 1972). According to Tkse ( 1962), mjor-gene resistnce is qulittive, wheres polygenic resistnce, which he equted with field resistnce, ws quntittive. Vn der Plnk ( 1968) * Present ddress: Tmil Ndu Rice Reserch Institute, Aduthuri, Tmil Ndu, Indi. * * Present ddress: Deprtment of Entomology, 402 Life Sciences Bldg, Louisin Stte University, Bton Rouge, LA , USA /86/05/ $ Butterworth & Co (Publishers) Ltd

2 suggested the term 'horizontl resistnce' to denote resistnce governed by polygenes. Russell ( 1978) used the terms, 'dult resistnce', 'mture plnt resistnce' nd 'field resistnce' for the phenomenon where the cumultive effect of minor genes for resistnce is incresingly expressed s plnts grow older. In this pper, we re using Russell's definition of field resistnce, where the level of resistnce increses with plnt ge regrdless of whether mesurement oflevels of resistnce re mde under lbortory, or greenhouse or field conditions. Becuse of the severity of the N. lugens biotype selection problems in rice, the breeding strtegy t IRRI is to develop field-resistnt vrieties which hve horizontl resistnce to ll biotypes nd to incorporte field resistnce into cultivrs hving mjor genes for resistnce. In the ltter cse, the field resistnce, lthough providing lower level of resistnce thn the mjor gene, would continue to provide resistnce fter selection of biotype virulent to the mjor gene hs occurred. A mjor constrint to the breeding of field-resistnt cultivrs hs been the lck of n efficient technique to evlute breeding lines for field resistnce. We hve identified severl rice cultivrs which re resistnt to N. lugens both in the vegettive nd reproductive stge in the field nd s 30-dy-old plnts in greenhouse studies, but which re susceptible in the seedbox screening of 7-10-dy-old seedlings. However, occsionlly the field-resistnt cultivrs re rted s modertely resistnt (MR) in the seedbox screening, when plnt dmge is rted on the dy tht the susceptible check plnts re killed. When rtings re tken few dys lter, the MR cultivrs re rted s susceptible. The stndrd seedbox screening test ws thus modified to provide for n efficient method of identifying field-resistnt cultivrs in the greenhouse. This study ws conducted to compre the performnce of cultivrs in the stndrd nd modified seedbox screening tests nd to determine the effect of fieldresistnt cultivrs on the feeding ctivity nd popultion growth of N.!ugens. Mterils nd methods Screening tests were conducted in the greenhouse t IRRI t n mbient temperture rnging from 20 to 35 C. Three N. lugens popultions served s test insects. Biotype 1 ws rered on cultivr Tichung Ntive 1 (TNl) from Tiwn, which hs no gene for N. lugens resistnce nd is highly susceptible; biotype 2 ws rered on Mudgo, n Indin cultivr which hs the Bphl gene for resistnce; nd biotype 3 ws rered on ASD7, n Indin cultivr with the bph2 gene for N.!ugens resistnce. The popultions were rered on the respective cultivrs for bout 10 yers (100 genertions) prior to the tests. Seprte tests were conducted for ech biotype in split-plot design, with the screening method s the min plot, nd the cultivrs s subplots. R. VELUSAMY, E. A. HEINRICHS AND F. G. MEDRANO 329 Seedbox screening test 1 Fifteen cultivrs were evluted for resistnce to biotype 2 nd 3 using the stndrd seedbox screening test nd the modified seedbox screening test. IR26 nd ASD7 served s the susceptible checks for biotype 2 nd 3, respectively, nd IR56 s the resistnt check for both biotypes. Stndrd seedbox screening test (SSST). Seed of ech of the cultivrs ws sown in 35 cm row in wooden seed boxes mesuring 60 x 40 x 10 cm. Seven dys fter sowing (DS), seedlings were thinned to 20 per row nd infested with eight second-instr N. lugens nymphs per seedling. Plnts were rted for dmge 7 dys fter infesttion (DI) when the susceptible check ws killed, nd gin 5 dys lter, using 0-9 scle on which 0-3 is clssified s resistnt, 4-6 s modertely resistnt nd 7-9 s susceptible. Modified seedbox screening test (MSST). Seed ws sown nd seedlings thinned s described for the SSST. Twenty DS the plnts were infe_sted with four secondinstr N. lugens per plnt nd dmge rtings were recorded t 25 DI when the susceptible check ws killed nd gin 5 dys lter. Tretments in both the SSST nd MSST in test 1 were replicted five times. In the SSST the initil infesttion killed the susceptible plnts wheres in the MSST the progeny of the initilly infested insects killed the susceptible plnts. Seedbox screening test 2 Six cultivrs were evluted in the SSST nd MSST ginst N.!ugens biotypes 1, 2, nd 3. Cultivrs consisted of ASD7, IR46, Kencn nd Utri Rjpn. Rections of the ltter three cultivrs rnge from modertely resistnt to susceptible in the seedbox screening of 7-dy-old seedlings in the SSST but were reported to hve high levels of field resistnce to biotype 2 (Pnd nd Heinrichs, 1983). Cv. Sinn Sivppu from Sri Lnk, which hs two unnmed genes for N. lugens resistnce, is highly resistnt to the three biotypes nd served s the resistnt check. TN 1 served s the susceptible check. The technique used in the SSST ws the sme s tht described in test 1 except tht rtings were tken t 2-dy intervls from 6 to 16 DI. The MSST differed in tht 10-dy-old seedlings were infested with three nymphs per seedling nd plnt dmge rtings were tken t 2-dy intervls from 26 to 36 DI. Seprte experiments were conducted for ech biotype. The experimentl design consisted of splitplot with the screening method s the min plot nd cultivrs s the subplots. Popultion growth studies The cultivrs evluted in Test 2 were further studied to determine whether they ffected the popultion

3 330 Screening for N. lugens field resistnce in rice growth nd feeding rte of the three biotypes. Three plnt ges were tested-30, 45 nd 60 DS. In order tht plnts of the three ges could' be provided simultneously, seeds were sown t 15-dy intervls, in cly pots. At 7 DS, seedlings were trnsplnted into soil in cly pots (dimeter 16 cm) with three seedlings per pot. Ech cultivr ws replicted six times. When the plnts reched the desired ges they were enclosed with Mylr film cge (90cm heightx locm dimeter). The plnts in ech pot were infested with five pirs (mles nd femles) of 3-dy-old N. lugens dults. Firstgenertion progeny ws counted t 30 DI. Feeding rte studies Plnts were grown in the sme mnner nd test cultivrs, tretments nd experimentl design were the sme s in the popultion growth test. The honeydew excreted ws collected in feeding chmber, s described in Pgui, Pthk nd Heinrichs ( 1980). The feeding chmber consisted of n inverted plstic cup plced over bromocresol green-treted filter pper resting on plstic Petri dish. The plnt ws inserted through the hole in the centre of the chmber which surrounded the stem portion t the bse of the plnt. Five 3-dy-old dult femles which hd been strved for 5 hours were plced in the chmber through hole which ws then plugged with cotton to prevent their escpe. The insects were llowed to feed overnight, fter which the filter ppers were exmined. Spots produced by the rection of the honeydew with bromocresol green were trced on filter pper (Pthk nd Heinrichs, 1982b). The re of the spots ws determined by trcing over the spots with trcing pper nd counting the number of squres occupied by the spots over millimetre-squred grph pper. Feeding rte ws expressed s the re of the spots produced in feeding period of 18 h. Results Seedbox screening test 1 IR56 nd IR60 were resistnt to both biotype 2 nd biotype 3 in the SSST nd MSST (Tble 1). ASD 11, Utri Rjpn nd Wgwg hd rtings of6 t 7 DI nd 9 t 12 DI in the SSST for biotype 2 but hd rtings of 1 in the MSST. All other cultivrs hd rtings of 9 in both screening methods. Results using biotype 3 s the test insect were the sme s for biotype 2 except tht IR36, which hs both seedling nd dult-plnt resistnce to biotype 2 (bph2 gene), ws susceptible to biotype 3 t the seedling stge in the SSST but resistnt in the MSST. Seedbox screening test 2 In compring the dmge rtings of the six cul ti vrs in the SSST nd MSST, when infested with N. lugens biotype 1, 2, or 3, TNl ws highly susceptible nd Sinn Sivppu highly resistnt, to the three biotypes with both screening methods (Figure 1 ). The rections of the other cultivrs vried, depending on the biotypes nd the seedbox test employed. In the SSST, with biotype 1, Kencn nd Utri Rjpn hd rtings of 6 3 nd 6 0, respectively, when TN 1 ws rted 9 t 12 DI, while IR46 nd ASD7 hd rtings of 1 5 nd 2 O, respectively. By 16 DI, Kencn nd Utri Rjpn hd susceptible rtings of 9 nd 7, respectively, while IR46 ws modertely resistnt. In the MSST Kencn, Utri Rjpn, IR46, nd ASD7 were resistnt when TNl ws killed t 30 DI. At 36 DI, Kencn nd Utri Rjpn were modertely resistnt with rtings of 4 4 while IR46 nd ASD7 continued to be resistnt. Results using biotype 2 (Figure 1) were generlly similr to those with biotype 1. However, biotype 2 ws virulent to cultivrs with the Bphl gene in the TABLE 1. Resistnce of trditionl nd modern rice cultivrs to N. lugens biotype 2 nd 3 in the stndrd (SSST) nd modified seedbox screening test (MSST). IRRI greenhouse, 1984 Plnt dmge rting* Biotype 2 Biotype 3 Cultivr Origin SSST MSST SSST ASD 11 Indi Tt l O b Binto Philippines T C029 Indi T C032 Indi T GEB24 Indi T Int n Philippines T Pet Indonesi T Rmind Philippines T Wgwg Philippines T l O Utri Rjpn Indonesi T l O b Vzhipoo Smb Indi T IR36 Philippines M* l 8 b l O b IR60 Philippines M l 6 b l O b l 4 b IR56 (R check for biotype 2 nd 3) M l 4 b l 0 b 1 4 b IR26 (S check for biotype 2) M ASD7 (S check for biotype 3) T - - MSST l O b l O l 0 l O b l O b 1 0 b * l-3=resistnt; 9=susceptible. In column, mens followed by common letter re not significntly different t the 5% level by Duncn's multiple rnge test. t Trditionl cultivr. * Modern high-yielding cultivr.

4 R. VELUSAMY, E. A. HEINRICHS AND F. G. MEDRANO Biotype I SSST MSST...i K i 1 TNl Biotype I 7 5 IR46 3 AS07 Sinn Sivoppu Sivoppu 9 SSST TNl MSST TN1 Biotype 2 Blotype 2 o7 c: :;: c... Q) g5 E c 0 Utri Rojopon b IR46 3 cd c AS07 c AS07 ~Sinn Sivppu Blotype 3 SSST ~,c AS07 hnl Ulri Rojopon b be ~ncn MSST _...: ~ ~otype3 _- TN1.I ~ AS07 Utri Rjpon Jib blr <1~ dSinno Sivppu ',.~ c 'l i c 1 sinn Sivoppu Dys fter infesttion FIGURE I. Dmge rtings of selected cultivrs to N. lugens biotypes 1, 2, nd 3 in the stndrd seedbox screening test (SSST) (left) nd modified seedbox screening test (MSST) (right) t given dys fter infesttion (DI). For ech biotype, screening method nd DI points with common letter re not significntly different t the 5% level by Duncn's multiple rnge test. SSST nd thus IR46 hd rting of 8 t 16 DI. In the MSST there ws distinct seprtion in dmge rtings between TN 1 nd the other cultivrs. IR46, Utri Rjpn, nd Kencn were modertely resistnt. ASD7 ws susceptible in both the SSST nd MSST in the biotype 3 test. Utri R jpn, Kencn, nd IR46 hd rtings of 6 3, 6 0 nd 4 0, respectively in the SSST wheres they hd rtings of bout 5 in the MSST. Popultion growth studies Becuse N. lugens popultions generlly differed little on plnts ged 30, 45 or 60 DS, only the 30 DS dt re

5 332 Screening for N. lugens field resistnce in rice TABLE 2. Popultion growth nd re of honeydew excreted by N. lugens biotypes infesting 30-dy-old rice plnts.* IRR! greenhouse, 1984 Popultion growth (no.) Are of honeydew (mm2) Cultivr Biotype 1 Biotype 2 Biotype 3 Biotype 1 Biotype 2 Biotype 3 Sinn Sivppu 1 d 4 d 7 d 71 b 29 c 34 c ASD7 95 c 137 c b 545 b 891 IR be 382 b 69 c 51 b 273 b 48 c Kencn 274 be 122 c 370 b 172 b 205 c 95 c Utri Rjpn 262 be 470 b 359 b 208 b 512 b 302 b TN! * Men of six replictions. Mens within column, followed by common letter re not significntly different t the 5% level by Duncn's multiple rnge test. presented (Tble 2). Plnts in the MSST were t similr ge to those in the popultion growth test (30 DS) nd results of the two tests re comprble. Sinn Sivppu ws highly resistnt to ll three biotypes, with numbers of progeny rnging from one to seven, wheres TNl ws highly susceptible with bout 1200 progeny of ll three biotypes. Progeny on ASD7 ws highest ( 1222) with biotype 3, being 10 times tht of biotype 1 nd 2, nd not differing significntly from tht on TNl (1233). Progeny number on IR46 ws highest with biotype 2 (382) but ws still only onequrter tht on TNl. The number of N. lugens produced on Kencn ws significntly less thn tht on TNl with the three biotypes while biotype 1 nd biotype 3 progeny were less on Utri Rjpn thn on TNl. Feeding rte studies Results of the feeding test were similr to tht of the popultion growth test. The lowest honeydew excretion ws on the resistnt check, Sinn Sivppu, with the highest on the susceptible check, TN 1 (Tble 2). Biotype 3 fed the most on ASD7, the feeding being equl to tht on TNl. Biotype 2 fed most on IR46 but only one-qurter s much s tht on TNl. Feeding ws significntly less on Kencn nd Utri Rjpn thn tht on TNl with ll three biotypes. Discussion The levels of resistnce expressed by the cultiv.rs tested were dependent on plnt ge. The MSST provided better seprtion oflevels of resistnce thn the SSST, where young plnts were ttcked by the hoppers. However, when dmge rtings in the SSST were tken t the time when TNl ws first killed (t 7 DI), cvs ASD 11, Wgwg nd Utri Rjpn were rted s modertely resistnt, with rtings of 5 8, but were rted s 9 t 12 DI in the biotype 2 test. Thus, if dily grdings for plnt dmge re tken, some of the field-resistnt cultivrs my be detected in the SSST but they will often be rted s susceptible. The mechnism involved in the field resistnce of IR46, Utri Rjpn nd Kencn ws low feeding rte of N. lugens nd low reproductive rte. However, tolernce, lthough not mesured, my hve lso been involved, s ws reported by Pnd nd Heinrichs (1983) for biotype 2 on Utri Rjpn nd Kencn. The MSST is rpid method of identifying fieldresistnt cultivrs in the greenhouse. The cultivrs identified s hving field resistnce in the MSST - IR46, Kencn, nd Utri Rjpn-were previously shown to hve field resistnce to biotype 2 under field testing (Pnd nd Heinrichs, 1983). These cultivrs were field resistnt to ll three biotypes in this study nd hve been reported to hve field resistnce in Kore, Philippines, Solomon Islnds, Thilnd nd. Indi (IRR!, l 982,b); this indictes tht the type of resistnce which they express is probbly generl or horizontl. Three of the trditionl cultivrs, which formerly were widely grown by frmers, were identified s hving high levels of field resistnce in the MSST. Wgwg, Philippine cultivr, is still grown on smll scle becuse the Filipinos prefer its grin qulity to tht of the modern high-yielding cultivrs. Utri Rjpn nd Kencn re trditionl cultivrs from Indonesi, where Kencn is still grown on smll scle. Kencn hs been reported to be resistnt under field conditions, s n older plnt in Indonesi, lthough it is susceptible t the 7-10-dy-old seedling stge in the greenhouse (Mochid, Whyu nd Suryni, 1979). Utri Rjpn, in ddition to hving field resistnce to biotype 2, s reported by Pnd nd Heinrichs ( 1983), lso hs field resistnce to biotypes 1 nd 2 s shown in this study nd is resistnt to rice tungro nd grssy stunt virus diseses (Pnd, Heinrichs nd Hibino, 1984). Rice breeders re interested in utilizing fieldresistnt cultivrs s donors in the breeding pror grmme. Cultivrs such s Utri Rjpn, which hve. field resistnce to N. lugens nd the whitebcked plnthopper, Sogtell furc1fer (N. Pnd nd E. A. Heinrichs, unpublished work) nd resistnce to rice viruses, re of interest in the development of highyielding cultivrs with stble resistnce to insects nd diseses. The lck of n efficient mens of evluting lrge numbers of breeding lines hs hmpered the use of such cultivrs s donors. However, with the use of the MSST, such breeding lines cn be evluted nd the MSST is now being used for this purpose t IRRI where field-resistnt cultivrs IR46, Utri Rjpn nd Triveni hve been used s prents. The difference between susceptible nd resistnt plnts is sufficiently distinct to llow the use of the MSST for genetic nlyses.

6 R. VELUSAMY, E. A. HEINRICHS AND F. G. MEDRANO 333 One strtegy in the breeding for cultivrs resistnt to N. lugens t IRRI is to incorporte verticl resistnce controlled by mjor genes with field resistnce controlled by either mjor or minor genes. When verticl resistnce 'breks down', through the selection of virulent biotype, field resistnce cn be expected to continue to provide stble yields, especilly if tolernce is lso involved. IR46 is n exmple of such cultivr. The mjor gene Bphl provides verticl resistnce to biotypes 1 nd 3, wheres other mjor or minor genes provide field resistnce to biotype 2, which is virulent to young seedlings of cv. IR46. Field resistnce would be expected to ct ginst ll biotypes. Although field resistnce generlly gives less effective control of N. lugens thn tht of seedling resistnce, combintions of field resistnce nd the ction of biocontrol gents in n integrted control progrmme hve provided excellent control nd minimized yield losses in field tests (E. A. Heinrichs, unpublished work). The two screening methods hve been tested in Chin, where cv. Utri Rjpn ws rted s susceptible in the SSST, but resistnt in both the MSST nd in field tests (Wu, Lingyou nd Zigung, 1984). The MSST provides method of identifying field resistnce under greenhouse conditions nd is expected to be useful tool in the breeding of highyielding rice cultivrs which hve durble resistnce to N. lugens. References IRRI (l 982). Finl Report of the Seventh Interntionl Rice Brown Plnt hopper Nursery (IRBPHN-1981 ). Los Bnos, Philippines: Interntionl Rice Reserch Institute. IRRI (l 982b). Levels of Resistnce of Rice Vrieties to Biotypes of the Brown Plnthopper, Nilprvt lugens in South nd Southest Asi. Report of the 1979 Interntionl Collbortive Project on Brown Plnthopper Resistnce. IRR! Reserch Pper Series No. 72. Los Bnos, Philippines: Interntionl Rice Reserch Institute. IRRI (l 983). IR36 the World's Most Populr Rice. Los Bnos, Philippines: Interntionl Rice Reserch Institute. 4 pp. IRRI ( l 983b). Interntionl Rice Reserch Institute Annul Report for Los Bnos, Philippines: Interntionl Rice Reserch Institute. 532 pp. MOCHIDA, 0., WAHYU, A. AND SURYAN!, K. T. (1979). Some considertions on screening resistnt cul ti vrs/lines of the rice plnt to the brown plnthopper, Nilprvt lugens (Stl). Proceedings of the Interntionl Rice Reserch Conference, April 16-20, Los Bnos, Philippines: IRR!. PAGUIA, P., PATHAK, M. D. AND HEINRICHS, E. A. (1980). Honeydew excretion mesurement techniques for determining differentil vrieties. Journl of Economic Entomology 73, PANDA N. AND HEINRICHS, E. A. (1983). Levels of tolernce nd ntibiosis in rice vrieties hving moderte resistnce to the brown plnthopper, Nilprvt lugens (St5!) (Hemipter: Delphcide). Environmentl Entomology 12, PANDA, N., HEINRICHS, E. A. AND HIBINO, H. (1984). Resistnce of the rice vriety Utri Rjpn to rgged stunt nd tungro virus. Crop Protection 3, PATHAK, P. K. AND HEINRICHS, E. A. (1982). Selection of biotype popultions 2 nd 3 of Nilprvt lugens by exposure to resistnt rice vrieties. Environmentl Entomology 11, PATHAK, p. K. AND HEINRICHS, E. A. ( l 982b). Bromocresol green indictor for mesuring feeding ctivity of Nilprvt lugens on rice vrieties. Philippine Entomologist 5, RUSSELL, G. E. ( 1978). Plnt Breeding for Pest nd Disese Resistnce. London: Butterworths. 485 pp. SIMONS, M. D. ( 1972). Polygenic resistnce to plnt disese nd its use in breeding resistnt cultivrs. Journl of Environmentl Qulity 1, 232. SOGA WA, K., KILIN DJATNIKA AND BAHAGIAWATI, A.H. (1984). Chrcteriztion of the brown plnthopper popultion on IR42 in North Sumtr, Indonesi. Interntionl Rice Reserch Newsletter 9, 15. TAKASE, B. N. (1962). Tests nd screening for resistnce in breeding potto vrieties resistnt to lte blight. Recent Advnces in Breeding 3, VAN DER PLANK, J. E. ( 1968). Disese Resistnce in Plnts. New York: Acdemic Press. Wu, JUNG-TSUNG, LIANGYOU, ZHANG AND ZIGUANG, QUI ( 1984). Screening technique for evluting the resistnce of rice vrieties to brown plnthopper Nilprvt lugens (Stl). Act Phytophylci Sinic 11, Accepted 15 Februry 1986