Euphytica73 : 1 27-135,1994 .

127© 1994 Kluwer Academic Publishers . Printed in the Netherlands .

Current and future strategies in breeding lentil for resistance to biotic andabiotic stresses

W. Erskine', M. Tufail2 , A. Russell', M. C. Tyagi4 , M. M. Rahman 5 and M. C. Saxenal1 ICARDA, P O. Box 5466, Aleppo, Syria; 2Pulses Research Institute, Ayub, Faisalabad, Pakistan ; 3Departmentof Scientific and Industrial Research, Private Bag, Christchurch, New Zealand; 4Genetics Department, IndianAgricultural Research Institute (JARI), New Delhi 110 012, India, and 5Regional Agricultural Research Station,Ishurdi 6620, Pabna, Bangladesh

Key words : ascochyta, cold, drought, lentil, Lens, resistance, rust, tolerance, wilt

Abstract

Lentil production is limited by lack of moisture and unfavorable temperatures throughout its distribution . Waterlog-ging and salinity are only locally important . Progress has been made in breeding for tolerance to drought throughselection for an appropriate phenology and increased water use efficiency and in breeding for winter hardinessthrough selection for cold tolerance .

The diseases rust, vascular wilt, and Ascochyta blight, caused by Uromyces viciae fabae, Fusarium oxysporumf. sp . lentis, and Ascochyta fabae f . sp . lentis, respectively, are the key fungal pathogens of lentil . Cultivars withresistance to rust and Ascochyta blight have been released in several countries and resistant sources to vascularwilt are being exploited . Sources of resistance to several other fungal and viral diseases of regional importance areknown. In contrast, although the pea leaf weevil (Sitona spp.) and the parasitic weed broomrape (Orobanche spp .),and to a lesser extent the cyst nematode (Heterodera ciceri), are significant yield reducers of lentil, no sources ofresistance to these biotic stresses have been found . Directions for future research in lentil on both biotic and abioticstresses are discussed .

Introduction

In West Asia and North Africa, lentil (Lens culinarisMedikus) is winter-sown at elevations below about 850meters and is usually spring-sown at higher elevations,representing two contrasting agro-ecological regions .Diagrams of the climate at sites representing these twoagro-ecological regions are given as Figure 1 (Mueller,1982). In these areas the crop is grown in drier and cold-er environments than other food legumes . These agro-ecological regions are characterized by wet winters,springs with rapidly rising temperatures and hot, drysummers. The major limiting factors to crop growth arelow moisture availability and high temperature stressin spring, and, at high elevations, cold temperatures inwinter. Consequently, selection for tolerance to theseabiotic stresses is of prime concern to lentil breedersin both agro-ecological regions .

By contrast, in South Asia where 50% of lentil issown (Anonymous, 1990) biotic stresses, particularlydiseases such as rust (Uromyces viciae fabae), vascularwilt(Fusarium oxysporum f. sp . lentis), and Ascochytablight (Ascochyta fabae f. sp . lentis) are major limitingfactors addressable by breeding . Moisture and temper-ature stresses are also important in limiting yield . Aclimato-diagram for a representative lentil producingsite in South Asia is also given in Figure 1 . The rel-ative importance of various biotic and abiotic stressesglobally on lentil is indicated in Saxena (1993) andJohansen et al . (1994) .

This paper addresses current and future strategiesof lentil breeding for resistance to abiotic stresses suchas the extremes of temperature, moisture stress and tobiotic stresses focusing primarily on fungal pathogens .

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Aleppo (390m) 399mmMean temperature (C)

Rainfall total (mm)50

Sep

Temp-f- Rainfall

Oct NovlDec Jan Feb Mar Apr1 May Jun Jul, Aug

SW

S

Hw H

Kanpur (127m) 823mmMean temperature (C)so

100

80

60

40

20

0

0Sep Oct, No Dec Jan Feb Mar Apr May Jun Jul Aug

S

1H

Fig. 1 . Climato-diagrams showing monthly mean temperatures (°C) and rainfall totals (mm) in Aleppo, Ankara and Kanpur, which arerepresentative of the three major agro-climatic regions of lentil production in the developing world . The elevation above sea level (m) andlong-term mean rainfall totals (mm) are given . Typical dates for sowing (S) and harvest (H) are shown ; in Ankara dates for winter-sowing (Sw)and harvest (Hw) are also shown .

Abiotic stresses

Temperature

Low temperatureIn the agro-ecological region of lowland Mediterraneanand south Asia, winters are usually mild . Low temper-ature is a factor limiting lentil production, but is lessimportant than low moisture availability. For exam-ple, at Tel Hadya (280 m.a .s .l .) total seasonal rainfallaccounted for 80% of the variance in mean seed yieldand the addition of the number of frost nights to themodel lifted the variance of seed yield accounted for to93%, with each frost night reducing seed yield by anaverage of 15 .5 kg ha t (Erskine & El Ashkar, 1993) .Late frost is known to damage early-sown plants morethan late-sown material (ICARDA, 1990) .

In contrast at higher elevations, where lentil isspring-sown because of the severe winter cold, exper-iments in Turkey have shown that autumn-sown lentilcan yield 50% to 100% more than the traditional springsowing using cultivars with winter hardiness (Sakar etal., 1988) .

Winter survival of lentil often requires toleranceto factors other than cold : e.g ., frost-heaving, water-logging, and diseases such as various root pathogensand Ascochyta blight . Cultural practices also stronglyaffect winter survival, as do frost, and disease and pestattack prior to the onset of winter . In short, a complexof factors is involved in winter survival (Murray et al .,1988), and a myopic preoccupation with cold toleranceis to be avoided . Screening in lentil for winter hardi-ness has been confined to evaluating survival in thefield (Erskine et al ., 1981). No recourse has been madeto using associations with other morphological, phys-iological and/or chemical traits or controlled environ-ment facilities . In other species, controlled freeze testsand measurements of plant moisture offer the breederthe best means of predicting cold tolerance, but finalevaluation must still be done in the field (Murray et al .,1988). The variation of the field screening environmentcaused by large differences in winter cold between sitesand seasons and local differences in snow cover andsoil fertility make progress slow . Use must be madeof many different locations each season . A search fornormally snowless, cold, winter areas for screening iswarranted .

Despite the problems of field screening, severalwinter hardy lentil cultivars have been released inTurkey (Sakar et al ., 1988) and sources of winter har-

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diness registered in the USA (Spaeth and Muehlbauer,1991). A world collection of 3592 lentil accessionswas screened for cold tolerance near Ankara, Turkeyover a severe winter with temperatures going as low as- 26.8 ° C with 47 days of snow cover (Erskine et al .,1981). A total of 238 accessions were found undam-aged by the cold winter with origins mostly from Chile,Greece, Iran, Syria, and Turkey, where natural selec-tion for cold tolerance had occurred . Further confirma-tion of their cold tolerance was found in joint screen-ing in Italy ; and, as a result, a nursery of cold tolerantsources is distributed to cooperators annually in theFood Legume International Testing Program . Pure lineselection and mass selection of landraces has, and willcontinue to have, a role in selecting for winter hardi-ness .

Additional sources of winter hardiness are beingsought at ICARDA within Lens culinaris ssp . oriental-is (see Cubero, 1981), the distribution of which spansareas of severe winter cold . As this species is crossablewith the cultigen, utilization of new genetic variationfor winter hardiness should be simple.

A major effort is now underway at ICARDA torecombine sources of winter hardiness with otherattributes for high elevation areas in simple crosses,and segregating populations with one winter hardy par-ent are being distributed in the Food Legume Interna-tional Testing Program . There are no reports on theinheritance of cold tolerance in lentil, but collabora-tive research between the USA, Turkey, and ICARDAon this topic is in progress . In view of the difficultyin measuring winter hardiness due to environmentalvariation, it may be useful to study linkages betweenmolecular markers (allozymes, RFLPs and RAPDs)and winter hardiness in order to explore the useful-ness of marker-assisted selection . Efforts have begunat ICARDA to recombine various sources to increasethe level of winter hardiness using simple recurrentselection facilitated with a new source of cytoplasmic-genetic male sterility (Muehlbauer, pers . comm .) .

High temperatureHigh temperatures are encountered by lentil in themajor production regions mainly during the reproduc-tive stage of growth, usually accompanied by condi-tions of low moisture availability . Initial efforts to sep-arate the effects of heat and water stresses in the fieldusing supplemental irrigation treatments and heat treat-ments during the reproductive growth stage by cover-ing with a plastic tunnel have been made at ICARDA

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(ICARDA, 1989). The paramount importance of mois-ture stress in a dry season was established and a heattreatment of about 10°C resulted in no change in totalbiomass but a reduced distribution of dry matter intoreproductive growth . Refinement of the technique forproviding heat stress is required .

Water

Water deficit - droughtIn lowland Mediterranean environments lentil is usu-ally sown in the zone with between 300 to 400 mmannual precipitation in the months of December andJanuary. The early stages of vegetative growth arerestricted by low radiation and temperature, but thisis the time of increasing rainfall and low evaporativedemand. From March until maturity in May, the cropexperiences increasingly strong sunshine, a rapid risein maximum temperature (see Figure 1), a fall in rain-fall, and high evaporative demand. Drought stress iscommon during this period, which coincides with thephase of reproductive development in the crop, andas a consequence yields are frequently low. The keyimportance of moisture in a lowland Mediterraneanenvironment is illustrated by the fact that total season-al rainfall accounted for 80% of the variance in meanseed yield at a single site over several seasons (Erskine& El Ashkar, 1993) . Although lentil is grown in drierenvironments than other food legumes in West Asiaand North Africa, the drought strategy of the species istypically one of drought avoidance with forced senes-cence and crop maturity induced by conditions of hightemperature and/or drought stress .

The breeding program at ICARDA uses simulta-neous yield tests at sites spread along a rainfall cline .Selection at these sites has resulted in increases in wateruse efficiency.

Other approaches to screening for drought toler-ance have been tried on lentil at ICARDA with varyingdegrees of success. Testing performance across sitescontrasting in rainfall presupposes that sites differ inmoisture availability and not in soil or other factors.To avoid problems, moisture availability may be con-trolled at a single site . At a dry site water may be addedin varying amounts by a line-source sprinkler system,or at a wet site a rain-out shelter may be used to excludemoisture.

Initially at ICARDA, the approach to droughtscreening was through the exclusion of moisture at awet site using plastic sheets on the soil in October and

November, until a particular rainfall total was passed(e.g., 50, 100 mm), prior to sowing . An additionaltreatment of supplementary irrigation was applied, cre-ating different levels of moisture supply at a single site(ICARDA, 1983 and 1984) . More recently we haveused a line-source sprinkler system at a dry site andfound that in a dry season (180 mm), 49% of the vari-ation in seed yield among lines was accounted for byvariation in flowering time . Drought escape was clearlythe key response to drought and, for severely drought-prone areas, selection for early flowering is required(Silim et al ., 1993). But, while the line-source sprin-kler system is useful in identifying differences betweenpotential parents in response to moisture, it cannot beused for single plant selection .

As another screening method, late sowing to screenfor drought tolerance was tried assuming that the cropwould experience severe drought and heat stress as itmatured late under conditions of low moisture avail-ability and high temperature. However, vegetative andphenological development was abnormal even in wetseasons, because late sown plants were strongly affect-ed by both temperature and photoperiod . Selectionunder these conditions was clearly for many charac-ters other than drought tolerance, and late sowing as amethod of drought screening was abandoned .

In view of the slow progress in other crops inresponse to direct selection for yield under drought,analytic breeding has been suggested, whereby selec-tion is practiced on another trait, which is stronglyassociated with yield under drought and has a higherheritability than yield itself (Richards, 1987) . Althoughit is facile to expect any single character to determineyield under drought stress, in view of the variationof stresses and the complexity of yield development,many such characters have been proposed for othercrops (Srivastava et al ., 1987). In lentil, we have exam-ined traits associated with yield under rainfed condi-tions and found that early vigor was strongly correlatedwith biomass and seed yield (Silim et al ., 1992). Theassociation of isozyme and RAPD markers with yieldunder drought conditions and the feasibility of marker-assisted selection for drought is also worth exploring .

Selection for deep rooting has been advocated toincrease food legume productivity under moisture lim-iting conditions (Buddenhagen & Richards, 1988) . Inthe lowland mediterranean environment the lentil isfavored in rotation with wheat over many other legumesbecause the crop does not utilize all the moisture storedin the soil profile, leaving some residual stored mois-ture for the succeeding wheat crop (H . Harris, pers .

comm.) . While an increase in lentil rooting depth mayresult in increased lentil yield, it might, however, be atthe expense of wheat productivity.

In another approach to screening for drought toler-ance the differential growth response of genotypes toosmotic stress in a solution containing different levelsof polyethylene glycol (PEG) as osmoticum is beinginvestigated and comparisons with field reaction todrought being made (N . Haddad and F.J . Muehlbauer,pers. comm.) . The results are not yet available . Inves-tigations are also underway at ICARDA and the Uni-versity of Saskatoon, Canada of the value of 13C dis-crimination in selection for drought tolerance in lentil .The naturally occurring isotope of carbon 13C is dis-criminated against during the fixation of carbon in C3plants. A correlation between carbon isotope discrimi-nation and grain yield has been found under conditionsof non-limited water and selection for carbon isotopediscrimination advocated to select for increased yieldin wheat (Condon et al ., 1987) .

Wild lentils, particularly L. culinaris ssp . oriental-is, are often found in habitats receiving low averagerainfall . Indeed, selections from crosses of the cultigenwith L. culinaris ssp. orientalis have been distributed aselite lines in the Lentil International Trials . Wild lentilsrepresent an inadequately explored potential source ofdrought resistance .

Finally, breeders should be clear whether their aimis to stabilize yields in current production areas andto take advantage of good rains through increasedresponse to moisture availability or to spread lentilinto areas currently marginal for production by breed-ing for drought avoidance and true drought resistance .These aims require different approaches .

Water excess - irrigation and waterloggingLentil is sensitive to water-logging and anaerobic con-ditions causing the low response of the crop to irri-gation. In a study of genetic variation in responseto irrigation in lentil, large differences were foundamong genotypes in irrigation response (Hamdi, 1987) .An anatomical study, using fluorescent microscopy ofgenotypes contrasting in their response, revealed thatirrigation-responsive genotypes had large aerenchymaor air-spaces in their roots whereas unresponsive geno-types had no such spaces when grown under anaerobicconditions . The value of this anatomical trait in screen-ing for irrigation response requires investigation .

Salinity

Salinity is not considered a major problem in lentil pro-duction, consequently there has been limited researchon screening for resistance to salinity in the crop . How-ever, a rapid and reliable hydroponic culture techniquehas been developed for salinity tolerance and comparedwith field results (Jana and Slinkard, 1979) . It has beenshown that lentil responds to toxicity of specific ions(e .g ., S02- , Cl - ) rather than total salt concentrationor osmotic potential .

Biotic stresses

Fungal diseases

RustRust, caused by Uromyces viciae fabae, is the mostimportant foliar disease of lentil. Complete crop lossis possible from an early infestation with the fungus(Khare & Agrawal, 1978 ; Sepdlveda, 1989) . Epi-phytotics of rust are common in Bangladesh, Chile,Ecuador, Ethiopia, India, Morocco, Nepal, and Pak-istan ; the disease is widespread but unimportant eco-nomically elsewhere around the Mediterranean basin .

Field screening has been undertaken at several loca-tions where infection occurs annually, for example,Ishurdi in Bangladesh, Pantnagar in India, and Akaki,Ethiopia . A method to artificially inoculate rust hasbeen described (Kramm & Tay, 1984) . Genetic differ-ences among genotypes and sources of resistance havebeen reported by many authors (Nene et al ., 1975 ;Khare et al ., 1979; Gurha, 1983 ; Reddy & Khare,1984; Shukla, 1984 ; Amin, 1985; Mishra et al ., 1985 ;ICARDA, 1988; Singh & Sandhu, 1988). Resistanceto rust has been recently found to be monogenical-ly inherited with resistance dominant to susceptibility(Sinha & Yadav, 1989 ; Singh & Singh, 1990) .

ICARDA is breeding for rust resistance throughjoint screening with national programs in Ethiopia,Morocco, and Pakistan. As a result, national pro-grams have released rust resistant lines received fromICARDA in Chile, Ecuador, Ethiopia, and Morocco .An international nursery of rust resistant sources waslaunched in 1990. The nursery will clarify the host-pathogen relationship in different regions and assist inidentifying variation in the fungus . Linkage of the rustresistance gene to potential markers is being sought at

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Washington State University and ICARDA is screen-ing wild lentils for additional sources of resistance .

Ascochyta blightAscochyta blight is caused by Ascochyta fabae f. sp .lentis (syn . Ascochyta lentis) (Gossen et al ., 1986) .It is economically important in Argentina, Canada,Ethiopia, India, New Zealand, Pakistan, and USSR .Foliar infection has caused yield losses of up to 40%(Gossen & Morrall, 1984), but economic losses frominfected seed are much higher in Canada where suchseed can only be sold as livestock feed.

Methods of artificial inoculation are known andgenotypic differences in foliar reaction followingscreening for resistance have been reported fromArgentina (Mitidieri, 1974), Canada (Slinkard et al.,1983), India (Khatri & Singh, 1975 ; Gurha, 1983),Pakistan (NARC, 1986), and Syria (ICARDA, 1984) .Clearly, resistance to Ascochyta blight is relativelycommon. Genetic differences in seed damage are alsoknown (Morrall, pers. comm .) . Resistance to blighthas been found in wild Lens (Bayaa et al ., 1991) .

Breeding for resistance to Ascochyta blight is beingconducted in a joint ICARDA/Pakistan breeding pro-gram with screening undertaken in Islamabad, a "hotspot" for the disease. As a result the cultivar "Manser-ha 89" has been released with multiple resistance toAscochyta blight and rust in Pakistan . Studies of theinheritance of resistance to the disease are being con-ducted and it is important that the major genes involvedare located on the lentil genome to allow the possibilityof using marker-assisted selection .

A nursery of Ascochyta resistant sources has beendistributed in the International Food Legume TestingProgram since 1988 . Information on the differentialreaction of a common set of lines to indigenous iso-lates in different countries suggests the presence ofvariability within the pathogen (Erskine, pers . comm.) .Knowledge of the variation in Ascochyta species affect-ing other food legumes suggests that variation in thelentil pathogen should be carefully monitored .

Wilt

The most serious disease of lentil is vascular wiltcaused by Fusarium oxysporum f. sp. lentis, which pro-duces major economic losses in parts of South Ameri-ca, the Mediterranean basin, and South Asia . A screen-ing method for wilt has been developed (Bayaa & Ersk-ine, 1990) and genetic variation in reaction reported

(Khare, 1981 ; Bayaa & Erskine, 1990) . Recently fiveindependently segregating genes for resistance havebeen found (Kamboj et al ., 1990). At ICARDA, breed-ing for resistance to vascular wilt and a study of thelinkage of resistance to isozyme and RFLP markershave been started, the latter in collaboration with Wash-ington State University. Resistance to Fusarium wilt(race 1) in pea is controlled by a locus closely linked toEst-s, coding a seed esterase (Hunt & Barnes, 1982) . Inview of the extent of conserved linkage groups betweenpea and lentil, this possible linkage is worth investigat-ing in lentil . A recent survey of the wilt reaction of220 wild lentil accessions revealed resistance in thesubspecies orientalis, nigricans, and ervoides (Bayaaet al., 1991) .

Other fungal pathogens

Several other fungal pathogens cause economic losseson occasion but their importance has not warranted spe-cial attention in lentil breeding (Khare, 1981) . Geneticvariation in disease reaction has been reported follow-ing field infestation for downy mildew (Peronosporalentis) (ICARDA, 1982), anthracnose (Colletotrichumtruncatum) (NARC, 1987), collar rot (Sclerotium rolf-sii) (Khare et al ., 1979), powdery mildew (Erisyphepolygoni) (Mishra, 1973), and Botrytis blight (Botrytiscinerea) (NARC, 1987) .

Viruses

Pea seedborne mosaic virus (PSbMV) is of economicimportance in the USA, causing stunting, malforma-tion of leaves and stems, and reproductive abortion .Immunity has been identified and is controlled by asingle recessive gene (Haddad et al ., 1978). Toleranceto pea enation mosaic virus transmitted by aphids hasbeen identified in the USA (Aydin et al., 1987). Sub-terranean clover red leaf virus (SCRLV) is endemicto cropping areas in New Zealand and lentil cultivarsrequire a high level of resistance to it . Seven PI linesfrom the USDA collection have adequate resistance,with one line PI 212610 also resistant to PSbMV andthe blue-green aphid (Acyrthosiphon kondoi) (Jermyn,1980) .

Insects

In West Asia the major insect pest of lentil is pea leafweevil (Sitona crinitus) . Adult weevils eat the leafletsof seedlings and the larvae consume the root nodules of

adult plants . Despite finding differences among diversegenotypes in seedling damage by the weevil, no differ-ences in nodule damage have been found (ICARDA,1981, 1984) . Host-plant resistance to Sitona does nothave major scope as a control alternative . Through jointresearch with USA universities, the gene for deltatoxinproduction from Bacillus thuringiensis has been intro-duced into rhizobial strains as a potential method ofSitona control . This may be seen as a model for theeventual introduction of the same gene into the hostplant.

The second most important group of insect pests arethe aphids Aphis craccivora and Acyrthosiphon pisum,which are of particular importance in dry seasons .Genetic differences in susceptibility to A. craccivorahave been seen in Egypt following natural infestation(Hamdi, pers . comm .) and to Acyrthosiphon kondoi inNew Zealand (Jermyn, 1980) .

In the Indian sub-continent the key insect pest is thepod-borer (Etiella zinkenella) . Resistance to pod-borerhas been found (Chhabra & Kooner, 1980; Sachan,1991), but no attempt has been made to breed for resis-tance .

Among seed weevils on lentil Bruchus lentis is themost widespread and injurious . Genetic differences insusceptibility have been found (Chopra & Pajni, 1987),but breeding for resistance has not been initiated .

Parasitic weeds

Broomrape (Orobanche spp.) is an important parasiteof lentil in the Mediterranean basin . Host plant resis-tance to Orobanche has been exploited in other cropsto produce resistant cultivars as a method of control .In lentil, a total of 1774 germplasm accessions havebeen screened in the field in infected soil and a rangein reactions recorded (Erskine & Witcombe, 1984) .However, the reaction of the most resistant acces-sions was examined in petri dishes in the laboratory(Sauerborn et al ., 1987) and there were no signifi-cant differences among accessions in the number ofinfections of Orobanche per unit length of root . Thelow incidence of Orobanche infection on the roots of"resistant" accessions in the field was probably due topoor root growth . Despite further extensive screeningin petri dishes, resistance to Orobanche has not beenfound in the cultigen . Screening has continued withinwild Lens, but resistance remains elusive .

Nematodes

Among the nematode species that affect lentil, screen-ing for resistance has been undertaken for cystnematode (Heterodera ciceri) as part of an ICAR-DA/Institute of Nematology, Bari project . A total of175 germplasm accessions of the cultigen and a sin-gle accession of Lens orientalis were screened in Bari,Italy and 75 lines were screened at ICARDA in pottrials with infested soil (ICARDA, 1985) . Differencesin susceptibility were observed among the lentils, butno resistance found . Although nematodes are not cur-rently major limiting factors of lentil production, it isimportant to monitor the reaction of promising linesto cyst nematode in West Asia, in order to avoid therelease of highly susceptible new cultivars .

Lodging

It is increasingly uneconomic for farmers in West Asiaand North Africa to harvest the crop by hand due to therising cost of labor and research has identified optimumsystems for mechanical harvest of the crop (Snobar etal ., 1985; Erskine et al ., 1991). Genotypes with a highdegree of standing ability are desirable in these sys-tems because lodging results in yield losses if it occursearly in the development of the crop (Penaloza & Mera,1988) or if a lodged crop is machine harvested (Ersk-ine et al ., 1991). Additionally, in wet seasons, lodgedcrops are more prone to infection from such fungi asAscochyta, Botrytis, and Sclerotinia . Lentil genotypesvary in standing ability (Mera, 1987) ; genotypes withthick stems generally have good standing ability (Ersk-ine & Goodrich, 1988) . Increasing standing ability is anaim of breeding programs in the Mediterranean region,the USA, and in New Zealand .

Concluding remarks

Sources of resistance to the key stresses - cold, wilt,rust, Ascochyta blight, and of drought avoidance arenow known in lentil and breeding is underway to intro-duce these resistances into adapted lines . It is impor-tant that pathogen variation is monitored in the future.Sources of resistance to several other fungal and viraldiseases of regional importance are known . In con-trast, although the pea leaf weevil (Sitona spp .) and theparasitic weed broomrape (Orobanche spp.), and to alesser extent the cyst nematode (Heterodera ciceri),are significant yield reducers of lentil, no sources of

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resistance to these biotic stresses have yet been found .The real challenge is now to create appropriate com-binations of the resistances to different stresses withinindividual cultivars, as stresses often appear togetherin the field .

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