Currcnt Plant Science and Biotechnology in Agriculture

VOLUME 18

Scirnrifi Adv&ory Bmrd: P.S. l3=nziger, Unlvsrsiry of Nebwka-Uncoln, Utuoh N e b m h , USA K. Burton. Agmcetus Cow.. Middleton. Wisconsin, USA F. Cannon. Blotechnktl Iru.. CiunbHd~e. M(us(~churrru. USA A. GnlStOh Yde Unlwrsiy, New Haven. C m e c t i c y ~ , USA I. L y r l l ~ Snow. Rutgers University, New BCWVW~C~, N m Jerry. USA C.P. Meredith. Unfvsrsly of Culifumiu at Davis, CoN/ornk USA N,C. Nitlaen. Pnrdut Unlwrsiry, West 4uye11c. lndkw, USA I. Sprcnt, University of O d e , Dundrr, UK D.P.S. Venna, The Ohio State University, Colmbur. Ohlo, USA

Ainu and Scop The hook d c s is intended for readers mnging fmm Pdvancd students to senior research ac iedsa and corpomtc d i m t o n interested In acquiring in-depth, slucqf-the-m knowledge .bout m a r c h findingr ud hhniqued related to plant reiencc and bioechnulogy. Wlulc Ihc rubject matter will relam more partlculady 10 a&iculnual applications, timely loplcs in bwic science and biotechnology wlll also k caplorad. Same volumes will report progress in rapidly advancing d i lc ipf ik through pmc&nm of symposia and worluhopr whi6 orken wlll dcuil fundamental information of an Wurlnp nature rku will be refwcnccd rtpPledly.

Durability of Disease Resistance

TH. JACOBS

and J.E. PARLEVI-IET Apnculrural Un~bcnl~y. Wagen~ngcn, The Ntlhcrland-i

KLUWER ACADEMIC PUBLISHERS W R D R E C M I BOPlPN I LONDON

pa ~ ~ b r o h r n o r - y a m ' . 0- M c D o n a l d 2 . L.J. Reddy3, S.I-4. Nigorn3 and D.H. Srnith2.

1) SADCC/ICRISAT Groundnut Project, C h i t e d z e A g r i cu l t u ra l Resea rch Stot;on, p - 0 . Box 1096, L i l ongwe . M L A W I . Legurr-cs Proyrclrn, ICRtSAT C e n t e r . pcr toncheru P - 0 . . Andhro P r u d c s h 502 324, lP-JDt.4 ' ~ ~ p r o v r d o m coefsrrncr poprr C P n r 700 b y ICRI~AT

The groundnut (A roch i s hypogoeo C.) - g r o u r r d n u t rus t ( P c r c c i r r ~ u u r a c h r d i s Speg.) pathosystem o p p e o r s t o h o v e coevolved in Peru, S o u t h A m e r i c a . where t l r c h o s t i s known to h o v e been cu t t i vo ted f o r o l rnos t 4800 yeurs . The g rounc l r l c~ t s p r . e c x d e d t o the rest o f t h e world o f i o r the SpmnisFi ond P o d \ ~ g u t r s e r o l o r i i ~ n t i o r i o f S u u t h A r n e - r ica. P r io r to 1969 t h e pathogen w h i c h wcis Ioryefy coniined to South Americca, but it got f i r m l y established in all t h e g r o u n d r i i r t growing cc>t,ntr;+rs in a short s p o n o f time (o ' r e - e n c o u n t e r * p h e n o m e r l u n ) . The p o t h u a e n is hishly t r o s t - s p c c ~ f i c and r s known by i ts u r e d i r l i ~ l s t a y e . R u s t is an e e c o n a m i ~ c t l l ~ i r r rpo r ton t d i s e a s e on g r o u n d - nut, o f t w r x c a u s i n g more t h o r x 50'%0 y i t r r l d lasses i r i rnc2s.t igr.OLJndfrut growing a r c o r . R u s t res is ton t genotypes hove been id=-nt i f ied. The resistcxvvce is of a q ~ ~ o n t i t a r i v c n a t u r e und its i r -her i to r ice d o e s not seem t o be s a r r ? y . a l e . Rust rec,;stc~nce ir-i mas* r J e - no types is s t a b l e over o wide r c z n g e o f ~eogr .oph; r Ic,cot;orls e x c e ~ t ,n o few l o c a - tions, indicotir-IQ poss ib l e vuricst ion in t h e pothogen, wb-ich n e e d s c o r r i i r m o t i o n . Rust resistor-ce in ~ r o u n d r - ~ r t f i ts neither typ ica l roce-sk>r>ctfic n o r r ace -oo r> -spec i f i c p a t - te rns and o p p c a r s t o be o r > i n te r r r i cd ra te type fo ; l i ng i r r t h e cur- t inuurr- of t )>ese hue e x t r e m e types.

The host

The c u l t i v a t e d grour idr . rut (Ar.ochis hy- pogoto) is b e l i e v e d t o have o r i g i n o t c d s o r n c w h e r e c~iony t h e 6-c~stcrn s lopes of t h e A n d e s in s o r ~ t h e r n Bolivia and r ro r t t ~wes te r n Argentina (Krc lpov ickas 1 9 6 9 ) . T h e spec iss is s u b d i v i d e d into subspec ies and bo to r - i co l cul t ivmrs with CJ spec i f ic geogrcspklic d i s t r i b ~ r t i o n in S o u t h Ar r le r ico . Six g e r ~ e cen te r s o f A. hypegaca have been i d e r l t i f i c d in r e l a -

tion t o the d i s t r i b u t i o n of subspec ies and bcr ton ica l cui t ivnrs. T h e r e aerie cer-tcrs i r -c lude t h e G r ~ a r o r - i r e g i o n ( r iver bosir-s o f Pciragcroy o r i d upper Parar-lcr!, Goios a r i d M ; r x c s r G e r o g s r t - gion (r iver brrsims of Tacor i t ins ond Soo Frczr,c;sco in socrth eastc-rn Brazil), Ron - d o r - r i a crrsd n o r r i ~ > e o s t e r n h4rrto G r o s s o r e g ion (wcsterr, Brmri!), B o l i w i c ~ n r e g i o n (eastizrra s lopes of the Ar-des), P e r u v i o n region (upper A m o z c ~ n u r ~ d west coast ) o n d riut+hecastttrrr Brazi t . T h e Soliviarr r e g i o r i i5 b e l i e v e l d t o be the p r i r ~ r a r y r e cen te r of g r u u r > d n u t clnd the,

Th. J a c o b s and i E. Porlevlint (cdr.) , n u r a b ; l i r y of d ~ s e o s e r e s ; s t c s r ~ c e . 1 4 7 - 1 5 s . 0 1993 Kluwrr Acodom~c Publ ishers PrintcJ in The Ne?et,>r-rlcrnds.

P. Subrahmanyam, D. McDonald, L.J. Rmddy, S.N. Nigom and D.H. Smith

other re~ions to be tire secondary gene centers (Gregory et 01, 1980).

Distribution

Groundnut is known to have been cultivated in Peru sirice ca, 2000 0.C (Hornmons 1973). Frorn its centers of origin, groundnut could hove spread to the rest of the world orily oher the Spanish and Portuguese colonilotion of South America. There is no credible evidence for any pre-Columbian spread of grouridnut from South Ame- rico (Hammons 1973). Groundnut is now cultivated throughout the tropical, sub-tropic01 and warm temperate re- gions of the world, lying opproximatcly between latitudes 40' N and 40" S. Groundnut is firmly established as orie of the most imporiont legumirious crops i r i these regions. In 1989, over 20 milliu~i ho worldwide were planted with groundnut ond from thot 22.6 million tonnes of dried pods were harvested, with an average yield of 1124 kg ha.'. Asia was the largest producer (15.4 million t), followed by Africa (4,6 million t), North and Ceri- tral Arnerico (2.0 milliori t), ond South America (0.5 million 1). Among i~ id i - vidual countries, India wos the largest producer (8.0 nill lion t) of grouridnut, followed by ttie People's Republic of China (5.4 million t), arid the USA (1.83 million 1). Approximately 90% of the world production comes from de- veloping countries (FA0 1989).

The pathogen

Occurrence and spread

The first record of groundnut rust dotes bock to 1827 or 1828 in a collection

made in Surinam (Hennen et nl. 1987). Prior to 1969, the d' ~seose was largely confined to South ond Centrol Arnerico, with occasional outbreoks occurring in tlic southern USA. Rust was olso recorded from Mauritius i r i 1914 (Stockdale 191 4) and from the People's Republic of Chino in 1934 (Toi 1937). A record of the occurrence of groundnut rust fronr the USSR (Ja- crewski 1910) wos reportcd to hr: erroneous (Tranzschtrl 1939). In the early 1970s groundnut rust sprcnd rapidly to, and become firmly estn- blished in, rnony countries i r i Asio, Australasia and Oconnia ond Africa (Subrohmanyorn and McDonuld 1983). How do we exploin this suddsrt wide spreod of the disease? How were the fungo1 propagules corricd to all these courrtries? Did the pothogen spread to oll those countries from South America or, from the People's Republic of Chino? We have no sotis- factory answers to these questiolis. Although in the past it was thought thot groundrlut rust might hove spreud through exctionae of germplosm, spread of the disease to olmost all groundnut growing countries ou~side South Arnerica in such o short time is difficult to exploin on this basis. Thero is no reliable evidence of groundnut rust being intcrnolly seedborne (Subro- hmonyam orid ~ c D o n u l d 1982; Moyee 1987).

Groundnut rust is known almost exclu- sively by its urediniol stage, It is not known whether the fungus can pro- duce pycnia and oecia or if any 01fer. nate host is involved in the life ~ycle- Urediniospores arc the main rneons ol

dissemination and spreod of ground. rtut rust. There are a few records of tlrc occurrence of teliospores on the cultivotcd groundnut nnd on wild Aro- clris species (Henncn et nl. 1987) in South Arnerico. There is no outhen- ticated report of ttie orcurrerice of teliospores of grouridnut rust frorn other countries.

The pnthogen is highly host specific. There is no rnrord of thc ocrurrence of oily host of groundnut rust outside the genus Arochis (Sa~hrnhman~nm nnd McD~ritrld 1982). Bccorrsc there is no knowledge o l spsrrnogonicr, oncin, and hosts fliuf busidio\purcs will irlfcct. the life cycle of groundriut rust is un- known and the tuxoriortii~ po,itiun of the fungus is obscure and orily ten- tative (Herrncn et 01. 1987).

A case o i "re-encounter" plieriorii~:no~i

Groundnut rust is thuuylit fo hove originated in South America olorig with the do~ritrsticotiori of the groundnut in prehistoric tirnes (Leppik 197 1 ). The restricted distributiori of thc pnthogen in Souflr Arnerico urrlil reco~itl~, and I ~ S

host restriction to members of tlie Qenus ArucIi~s, strongly support this hypotliesis (Subrotrmonyorii ct al. 1989). The groundnut rust wos gee. Qlaphicnlly separated from its hart during sevr?rol centuries in Asro, Afriro, A~stralosio, orrd Occnnin, before being reurjited i r i the eorly 1970s. All Qroundriut cultivors grown by ttre for- fwrs outside South America are sus- ceptible to rust. A period of several centuries of seporotion of the host from the pathogen might hove wor- ranfed re-ordering of host plant prio- 'ities to selection pressure in the new environments. The genes governing

rust resistonce rn~ght have been lost or their frequency greotly reduced in the ohsence of contint~al selection pressure by the pnthcrgan leading to a genetic erosion phenomenon (Von der Plank 1963). Ho,~ever, th~s proposition seen>$ to be less likely in sclf-polli- r ~ t e J urid cloriol crops (Buddenhogen and De Ponti 1YSdj. It is possible thot the early explorers might hove intro- duced groundnut from South Arnerico including the rust-resistant gerrotypes, irrespective of their botunirol bpe, pod type and seed colol,r. However., since most of the r~st~reststant genotypes nre prrrn~tlve lnndraces w ~ t l i low yield potent101 and unacceptable pod ond secd choracterlstics, they might have urider<jonc conscious selection by forrtiers to rr.noc:t thei; sperafic needs purtirulurly for high yield, pod type, and seed colour. This could hove fur. fher eroded rust res~stonce of the host popuiotlons outside South Arnerico.

Rust is now uri ecorion~icolly important disease i r i rriost groundnut-growing oreos of the world, losses being por- tirulorly severe if the crop is olso at, tacked by l e d spots (Cercosporo ara. ch,d~colo Hori orrd Phoeoisoriopsb personoto (Berk. ond Cur*.) v. A n ) . In the Coribbenn and Central Arrierico, cornrnerciol production of groundnuf lios beuri olnlost eliminated by the combined attack of rust and leof spots (Hammonr 1977). Losses frorn rust measured ot two locotions in Texas, USA, were 50 ond 7046 (Harrison 1973). Felix and Ricaud (1 977) re- ported losses from rust amounting to 70% i r i Mauritius. In India, Subroh- manyam et 01. ( 1 980 ond 1984) re.

ported significant losses in groundnut yields from combined anock of rust ond leaf spots, while rust alone was responsible for 52% reduction in pod ~ie ld, Ghuge et al. (1981), olso from Indio, estimated that rust caused a 49% loss of pod yield and lowered the 100.reed weight to the extent of 19%. Zhou et al. (1980) recorded a loss of 20% for spring groundnut and 17% for autumn groundnut in the People's Republic of China. The early establish. merit of the disease is known to od- vance harvesting by about o morith, resulting in poor pod filling (O'Brierr 1977). In odditiorr, haulm (boy) yields ore lowered drastically.

Resistance to groundnut rust

Although rust car1 be controlled effec- tively Ity certoin furrgicides (Smith and Littrell 1980), this is not economically feosible for the vast mojority of small- holder formers in developing courr, tries. Hence, genetic control is deci- dedly the best solution (Gibboris 1980). Identification of sourccs of genetic resistonce is therefore highly imporiant. Recognition of this hos stimulated research in many countries to exploit host plorrt resistance to rust.

Screening methods

Effective field screening methods hove been developed for use in oreos where notural disease pressure is high or where such pressure con be orti- ficiolly induced. Genotypes to be screened are sown in a diseose nur- sery comprising infector rows of highly susceptible cultivars orranged systcma- tically throughout the nursery. To en-

honce diseose development, plants in infector rows are inoculated with $us- pensions of rust spores. This is most successful if done in ttie evening, fol- lowing irrigation. Potted spreader plants heavily infested with rust are olso placed systematically throughout the field to provide further sourccs of inoculurn. As required by climatic con- ditions, fields may be irrigated fre- quently using overheud sprinklers until harvest. Some 10 days before hurvcst each genotype is scored for the de- velopment of rust using o 9-point scole (Subrahmarryorn et 01. 1982).

Screening of germplosm for resis. tonce to rust can olso be done on a limited scale in the glasshouse using poned plonts, or in the loborotory using detached leaves (Subrohmun- yam et al. 1982, 19830, and t983b). A glosshouse or loborotory screeriirrg method could be useful in oreos where rust epidemics do not occur regularly or where other folior diseases or insect pests interfere with field screening. However, these techniques hove lirr~ito. tions in identifying moderate levels of resistonce ond require further verificn tion in the field.

Sources of resistance

A number of sources of resistarrce to grourrdnut rust hove been identified (Mozzoni and Hinoiosa 1961 ; McVey 1965; Bromfield and Cevorio 1970: Cook 1972; Brornfield 1974; Morocs and Sovy Filho 1983). At lCRlSAT Center, Indio, on intensive research program wos staded in 1977 to con- firm earlier reported sources of resis- tance and to search for additional sources of resistonce for use in the breeding program to develop high

yrerurrig ayronumrcally superror rusr. resistorit rultivors. A world collection of over 12,000 ~ermplosm lines hor been screnncrl in thc fiold for rust resistnncn doring tho period 1Y77 to 1990, ond 124 rust-rasistnnt nrrd 29 rust orjd 10tu loof spot-resistorit gem,- plosrn I;ries were identified (Sutrr~)i- moriyurli et ul. 1989). Alttivugh many sources of resisfurice tu rust hove beer1 identified frarn the avoiluble gernr. plosrn cullertioris, riot r r iu~l i is knvwri obout their yerietir bose. Ttiere is o need to study the genetic bose of this moterial nnd diversify it fc~rthor or, diverse nenotypcs with rust resistonre ond superior ogronornic trcrits ore required for utilizotiori iri breeding progrorns.

Co-evolution of groundnut - groundrru! rust polhosystcr?i

It is gerietully occeptcd tho? the centers of origin and durriesticutiorl of culti. voted plants ore the best locutions i r i which to find genuine sources of rssis- tonce to common pests ond potliuysrls (Leppik 1970: Dinoor ond Estied 1984). During tlie coevolution of host and porosite, both porticiporrts dcvclop complimentary genetic systems if they hove long bccn ossocrated in their centers of origin (Leppik 1970; Brow. ning 1974; Hnrlon 1976; Ariikster and Wahl 1979; Segol et 01. 1980). Ttia evolution of new or more virulent races of ttie po~trogeri rnoy be coun. ter-bolonced by the development of higher levels of resistonre irr i ts host (Flor 1956). It is interesting to note hat the ~roupirrg of the ovoiloble rust-rasistont A. hypoyoeu genotypes based on botonicol type ir~dicotcs thot about 89% of them belong to vor.

rasrtgrora, I via ro vor. nypogoea, on0 1% to var. vulgaris. A study of all the avoilobla rust-resistant gerioh/pes re- veals thot over 90% of t!wm originated irr South Arnericn or hod South Arne- ricrrn conrindions ond ubout 83% oriyir~oted in Peru. The origin of other yeriotypes is not clearly documented. Avoiloble gcrrnplusm records iridicote thut ttie origin of resistclrvt types could be trored to the Taropoto region of Peru. The osrurriptiun tlrc~t groundnut - yroundnut rust puttiosystem coevolved i r i Peru is further suppor'ad by the foct that Peru i s o recandory gene center with predorninonce of prirnittve tos- tig;oto types. A mojority of rust,resis. ton! grot~ndnuts ore primitive fost~grata fypes (Rur~roriolho Roo 1987; Subroh- rnuriyurn el ol. 1989). Such types hove beer1 uridur ~ul~ivcrtion sirice obout 2000 B.C us evidenced by o number of orchoeologicol findirrys (Hommons 19731. Why ore the sources of rust resistonce prcdorniriontly of Peruvign or i~ in? Why does the rust resistonce riot occur in the primary center or in other scrondnry centers? Is it because ttie iust;g;cio types prcdomrnontly occur in the Peruvlon region? If so, whot are the levels of resisfonce ovoil- ohle in hirsuta types wtiirh huve coho- bitotcd olong w~th the fost~gtoto types in the Peruviori region? A better un. derstonding of fodors associated with these questtons is impor:ont for gerretic erlhoncamcnt ond rust resistorice bree- diriy progrorns.

Components of resrsionce

Rust resistonce is not correlnted with either the frequency or the size of stomnto, urediniospores germinote ond germ tubes enter through stomoto

P. Subrahmonyom, D. McDonold, L.J. Raddy, S,N. Nigom ond D.H. Smith

irrespedive of whether a genotype is immune, resistant, or susceptible to rust. In immune genotypes (found only in wild Arachis species) the fungus dies shortly after entering the substomotol cavity.

The differences between resistance arid susceptibility are associoted with differences in rate and extent of myce. lial development within the cavity and within leof tissues (Subrahmonyom et al. 1980). Rust-resistant genotypes hove increased incubation period, dccrcosed infection frequericy, arid reduced pustule size, spore production and spore viability (Lin 1981; Sokhi and Jhooty 1982; Subrahmonyam et 01. 1983a and 1983b; Lin et al. 1984). The effects of these compo- nents of resistonce to rust appear to be cumulative over the course of the disease epidemic. In general, the din scose on rust-resistant genotypes builds up slowly, arid does little op- parent darnuge to ttie foliage, os shown by the lower rates of disease development (r), and the area under disease progress curve (AUDPC). The panern of inheritance and the number of genes governing these components of resistance is not known.

The extent of rust damage to fo. lioge is dependent on the physiolo- gical age of the plarit. Your~g plorits are most susceptible to rust onack and the susceptibility declines with age (Suhruhmoriyorn and McDonold 1987).

Suryokumori et 01. (1984) reported o strong associotiorr between tlie num- ber of tonniri sacs in leaves and rust resistance. The susceptible genotypes had few tannin sacs, but highly rerise tont or immune wild Arochis species had large numbers of them. Subba Rao (1987) found a correlation bet-

ween the degree of resistance ond the amounts of total phenolic compot~nds in leof tissues. Biosynthesis of pliyto- olcxins (methyl linolenote, dieriic ol- cohols, tricarboxylic proporric acid, noriyl phenol arrd olkyl-bis-phenyl ether) was stimulated by infection in rust-resistant genotypes. Such a stimu. lotory response was also observed in rust-susceptible genotypes, but tho amplitude was smoll ond the response was usually deloycd.

Utilization of resistance

Most rust-resistant germplasm lines are primitive landraces, and have unde. siroble pod ond sccd choroctars strch as dark tcsto colour, ond hcovily rcti- culated pods. At ICRISAT Center, n1or.e than 1500 single, two-way, and ttiree- -woy crosses wcrc made between lines with good ogronornic choroctars and lines resistont to rust. Lorgc F2 popu. lutioris, arid subsequent generot~ons, were grown in the field during thc rainy season and screened for resis- tance using the infector row niethod. Several high-yielding, agronomically superior lines, with high levels of resis- tance to rust and rnoderute levels of resistance to late leaf spot were bred by pedigree ond mass pedigree methods (Reddy et al. 1984). Back. crossing wos also used in a few instan* ces to improve pod, seed, and plant characters. Several of these resistant lines outyielded released susceptible cultivors when tested in multilocation trials ond some ore in advanced stoges of testing in several countries Two high-yielding cultivors ICG(FDRS) 10 ond ICGV 86590 resistont to rust and moderately resistant to late leaf spot hove recently been released for

cultivntion in the por~ir~st.rlnr zone of Indin, where rust orid lotc Inof spots cuusc snvere yield losse, (Roddy et 01. 1997). Another high yieldirig cultivor ICG(FDRS) 4, with rrit~ltiple rssistnnces to rust, lntn leaf spot, stcrr~ ond pod rots cuuscd by Sclerotrum rulf,;; Socc , leof miner, artd with low field irici- dence of bud necrosis disease hos stiown wide odupt~bility by otrtyielding locol ct~ltivars in Iridiu. Swnzilnnd, Molowi, Mynnrnor, orid Tht: Philippines (Niyom ct nl. 1992). A folior disease, resisturlt cultivor, Tifrust 2, ioirltly deve. loped try USDA-ARS (Urjiversity of teorgiu, Tiftorl, U.S.A.), and ICRISAT lros heen releusod as 'Cordi Poyrre' it1 Jornaica.

Early generation brccding moteriuls resistant to rust have been widely dis- tributed to hreeders orid scinntlsts in notional und intcrnationul proglorns to enable ttieni to ccirry out furttier sr:lac. tion In sriu under locol ogrociinlatic conditions. This hns resulted in the successful developrncnt ond r a l t . ~ ~ ~ ~ of cultivors such as Girriur 1 nnd ALR 1 in lndio (Subrahmanyurn ct al. 1990).

Genetics of resisturicr:

Rust rcsistcrnce in grourid~l~rt is re. ported to be governed by duplicate recessive genes (Bruriifield ond Boilcv 1972; Knauh 1987), Dependirig 011

the resistont parents used in the study, Kishorc (1981) observed both digerii~ ond trigenic inharito~icc. Nigurrt et 01. (1 980) observed continr~ous segrego- tion for rust resisturica within t t~c ~dvonced generations of highly resis- tent progenior, refuting the volidity of the two gene model. While, there is no clear ogreerrrent on the nurriber of Banes involved, all the workers have

reported thot res~stunce wt!triri r ~ ~ l t i . vuted spnries behoves as o recess~ve truit. Qu(.rtltitotive genet;< onolysis of porerrts, F,, F2, BC1 and BCs Qenera- tioris of rust resistont x susceptible crosses using gsnerotiorr rneon ana- lysis at ICRI5AT Center indicated thot rust-resistartre i,: pr,cdominontly con- tro1lc.d by odditivs, odditive x odditive, orid ndditive x dorniriunce gene effects (Rodcly ct ol. 19871. These ohservo- tiuris on the importonce of both od- ditivc. trnd nonoddi!ive gene odion in fl ier rust inherctonce ;r confirmed by other wurLcrs (Anderson ut 01. 1990; Vatmurt et ol 1991 ). Wild Arachis spp. niuy ticrve mechonisrns of rust resisturire thrlt differ from thore in the r~~ltivoted groundnut. In sorne diploid wild Aroch.5 sop., rtrst resistonce ap- pcors to be puriiolIy dominont (Singh et ol. 19841, uriiike in the cultivated grourr~j~irrt, tnd;cntiriy thot dtfferent gancJ n7ov be ~tivolved. Cornbinution of tljerc reris!anres n10y result in inore stuble resistonce in the cultivated

Stobility of host resistonce is on impor- tnnt obje~five of breeding programs. Sonre of tlie rust-rasistarit genotypes identified,'ossernbled ot ICRISAT Cen- ter ure be~ng tested in diCferent locotiuns i ~ r the International Ground- n t~ t Rust Diseases Nursery (IGRDN). Ttie results obtoiried indicate thot rust resistance of most gerjotypes is stable over o wide rnnge of geographic loco- tions (e.g,, USA, Niger, Molawi, and India). However, the results from the People's Republic of China ond Toi- won were dilferent from those of other countries. The genotype NC AC

P. Subrohmanyom, Do MeDonold, L.J. Reddy, S.N. Nigom and D.H. Srnth Rust rrsi>tonre in groundnu!

17090, is highly resistant to rust at ICRISAT Center, only moderotely resis- tant in the People's Republic of Chino, ond susceptible in Taiwon. In controst, the genotype PI 2981 15, is only mo- derately resistant at ICRISAT Center, but is highly resistont in the People's Republic of China ond in Taiwan. Although this indicates the possibility of variation in the pothogen, there is no authenticotgd repori on the occur- rence of roces.

Possible occurrence of races

Hennen et at. (1987) speculated that the fungus completes its sexuol life cycle in South America and genetic diversity of the pothogen is predicted to hove occumuloted there. In foe, the susceptibility of several wild Arachis species to yrotrndnut rust in their notu- ral habilots in Soutlr America slr.ongly supports this hypothesis. All these Ara. chis species are found to be inrrnune or highly resistarrt to groundnut rust it1

India (Subrahmotryom et al. 1983c), and elsewhere (Bromfield and Cevario 1970; Hommons 1977). The results obtained from the People's Republic of Cliino and Taiwon also indicote the possible occurrence of roces of groun. dnut rust. There is a need for syste- matic identification of races arrd deter. mination of their geographical distri- bution. Recently a set of preliminary differential hosts has been proposed (P. Subrohmonyam, unpublished).

How durable i s the rust resistance in groundnut?

The genetic composition of rust resis- tance in groundnut i s not conclusively

estoblished. Thore are indications tho rust-resistance is controlled by a few major genes, o situation typical o race-specific resistonce (Van der Planl 1963). However, quantitative ~enetic anulysis of porents nnd pronenie! indicated thot rust resistance is predo minantly controlled by odditive, ad. ditive x additive, and additive x doini. nonce gene effects with duplicate epis. tasis (Reddy el 01. 1987). These fin. dings suggest thot rust resistonce doer not fit o typic01 roce specific ponern. I1 is probably an internredinte type ex. hibiting smaller roce-specific effects as indicated in other pothosystems (Pnrle. vliet 1981). The muiority of rust-resis- tont grouridnut genotypes typically show fully developed rust lesions, which are small and orily sparsely sporuloting. However, in cerioin geno- types, olthough lesions are initially fully developed, they show some necrosis around the lesions ot Inter stuycs of development resembling a hypersen. sitive reoction. These observations suggest two types of resistonce, thc loner one possibly being of o race- specific nature. When resislonce is conferred by the action of o single gene, preventing the development of the polhogen during or immcdiutely oher penetrotion, there is strong sclec. tiori pressure on the pothogen popu- lotion. However, the resistance that is governed by the additive effects of several genes, retarding several or- peds of pathogen development, exerts o weaker selection pressure. This kind of resistance may not be subiect to sudden "breakdown" and is more likely to be durable. Durability of disease resistonce can best be identified by growing o resis- tont-cultivar on a large scale for 0

long period of time in on environment

fovouriny the diseuse (Johnson 1984). Thc stability of groundnut rust resis. tarice fnund in differerlt ~oogiuphicnl locutions is urtly on inJictltion of durn. bility, ond there is no stiict cousul rc- lotion between stability or,ld durability. The moiority of rust-rcsistoirt gonotypes identified in recent yeors ur.e not ogio- nomicolly accsptcthle oitd can not nteet tlie fnrrners' and trodn's Je. morrds for yiald and quality. However, these gerrotypes trovn served as sources of resistonre in rust it.$istonce breeding pioOroms iri many cotrntrias ond scvcrol ugron~rri icnl l~ superior rust-resistant cultivnrs tluvc beerr dc. vsloped. When recently reloosad rust. resistant yroundiiut cultivors ore grown by formers on lorger orcos we mu? obtoiri rclioble infoirnotion os to the durobility of rcsistonce. The role of these cultivors in stobilicing the pntho. gcn evolution (Leonnrd uitd Czoct1or 1980) needs to be exomincd.

Most of the world's groundnuts ore Qrown in Jcvelopirlg couiitrics ( F A 0 1989) under subaisterlcc ngiiculttrre i r i diverse oyro.eiivironrnents. Agronomic practices such os iritercropping, iriixed cropping ond use of cultivnr mixtures mny olso ir~fluenre the durobility of discose resisturrce.

There ore mony gaps in o u r know- ledge of the gioundnut . giouirdnut rust pathosystcrn. Ttrc occurrence of roces of tho pothogen, the gene-for- gene relotioristrip, and host-pothogen specificity need to be cleorly esto- blished to provide o better understo~r- ding of the durobility of resistonce.

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