the Symposium on Wheats for More Tropical Environments - cimmyt

141Identification ofSources of ResistanceSources of resistance to B. sorokinianain species o<strong>the</strong>r than Triticumaestivum are of special interest in <strong>the</strong>breeding program. The use of aliengene pools for disease resistance haslong been known (8). Breeding forspecific resistance againstPhytophthora infestans in potatostarted as early as 1949. when a genepool for resistance was discovered inSolanum demtssum (16). In recentyears. <strong>the</strong>re have been several reportson <strong>the</strong> use of gene pools for resistanceto rusts. septoria and mildew in wheatfrom some closely related species (2.17).CIMMYT has been making some effortto incorporate alien resistance genesalready in <strong>the</strong> wheat background (6).Recently. <strong>the</strong>y have obtained somesegregating lines resistant toB. sorokiniana from wheatcombinations with Aegtlops elongatumand Elymus giganteus (M. Kazi.personal communication).Never<strong>the</strong>less. information on <strong>the</strong>sources of resistance to B. sorokinianain species o<strong>the</strong>r than T. aestivum isst111 very scant. and attempts should bemade to identify sources of resistance inas many alien species as possible. Suchinformation would help for geneticallycombining different sources ofresistance. Also. relatively littleinformation is available about <strong>the</strong>underlying genetic mechanism ofresistance. Adlakha et al. (1) reportedthat resistance in wheat toB. sorokiniana was conditioned by oneor two dominant factors; similarconclusions were reached by Srivastavaet al. (15). Fur<strong>the</strong>r work may benecessary to understand more about<strong>the</strong> inheritance of resistance.Transfer of Resistanceto Advanced Lines or VarietiesUntil recently. very little emphasis hasbeen given to breeding for resistance toB. sorokiniana. During <strong>the</strong> past tenyears. much improvement has beenmade in wheat production. especially in<strong>the</strong> Latin American region. through <strong>the</strong>introduction of Mexican varieties withhigh yield potential and improveddisease resistance. mainly to <strong>the</strong> rusts.Some of <strong>the</strong> varieties have failed tocreate an impact in some areas due to<strong>the</strong>ir susceptibility to B. sorokinianaand <strong>the</strong> local complexes of diseases.There are only a few examples of fieldresistance to B. sorokiniana withineXisting commercially grown varieties.Among <strong>the</strong>m are <strong>the</strong> Mexican varietiesAlondra and Cocoraque. which haveshown reasonably good performanceagainst spot blotch in farmers' fields.Alondra. a semidwarf wheat with a bigspike. has become an importantprogenitor in numerous crosses withBrazilian wheats. Among <strong>the</strong> Brazilianvarieties. BH1146. CNT1 and PAT7219have <strong>the</strong> best resistance. Thus. due to<strong>the</strong> fact that a majority of commercialvarieties are susceptible or highlysusceptible to B. sorokiniana. breedingfor resistance to B. sorokiniana hasrapidly gained importance in recentyears.Once <strong>the</strong> sources of resistance inT. aestivum or alien species areconfirmed. <strong>the</strong> problem of incorporatingsuch resistance in agronomicallydesirable varieties arises. By and large.<strong>the</strong> pedigree selection method isfollowed by most breeders andpathologists. After passing throughseveral field and greenhouse screeningtests. resistant lines are tested for yield.included in <strong>the</strong> crossing block, andselections for resistance and agronomic