the Symposium on Wheats for More Tropical Environments - cimmyt

92work on helminthosporium, supportedby a special project research grant fromUNDP, has resulted in <strong>the</strong> identificationofadvanced lines with varying degreesof resistance.Resistance to head scab, caused byFusarium spp., is especially needed in<strong>the</strong> more humid tropical environments,where conditions are ideal for <strong>the</strong>development of <strong>the</strong> disease. CIMMYThas recently intensified its efforts todevelop high-yielding lines with scabresistance. By <strong>the</strong> end of 1983, lineshad been identified that possessed someresistance.O<strong>the</strong>r disease problems confrontingwheat in tropical climates are <strong>the</strong> soilbornediseases, such as foot rot andseedling blight, caused by Sclerotiumrolfstt. Damping-off of seedlings oftenoccurs when temperatures are high and<strong>the</strong> soil saturated with moisture; it isstill not known whe<strong>the</strong>r it will bepossible to identify resistance to thisparticul~disease. Possibly morepromising would be <strong>the</strong> development ofa chemical seed-treatment technique fordisease control. Ano<strong>the</strong>r soil-borneproblem is fusarium, which causesseedling blight. root rot and <strong>the</strong> spikeinfection, scab.Bacterial stripe, caused byXanthomonas translucens, is ano<strong>the</strong>rthreat to wheat cultivars under tropicalenvironments. A severe infection of thisbacteria kills most, if not all, of <strong>the</strong>foliage of <strong>the</strong> wheat crop, causing adrastic reduction in yield. Bacterialstripe is widely distributed, but it onlybecomes important where susceptiblevarieties are grown and <strong>the</strong> growingseason is predominantly wet.Insect resistanceFor tropical wheat varieties, thischaracteristic is also important, sinceinsects such as <strong>the</strong> rice stem borer,corn earworm, corn semilooper,armyworm and aphids can be seriouspests.Desirable gralD qualityGrain color and appearance andacceptable milling and baking qualitiesare important qualities. The wheatproduced in <strong>the</strong> tropical areas will bedestined primarily for local millers;hence grain type and quality will playan important role in farmer decision toadopt a particular variety.Sprouting resistanceThis is an obvious requirement forwheat varieties for tropical areas, as ithinders grain from sprouting in <strong>the</strong>spikes before harvest, which wouldlower <strong>the</strong> quality characteristics of <strong>the</strong>variety. Because of humid productionconditions in <strong>the</strong> tropics, especiallywhen rain is frequent dUring grainripening. sprouting can be a seriousproblem.Breeding andScreening MethodologyCIMMYT's principal breeding approachis <strong>the</strong> development of a base of widelyadapted, high-yielding and diseaseresistantwheat germplasm thatpossesses attributes necessary toachieve higher productivity undervarying conditions. New lines must bedeveloped qUickly if <strong>the</strong>y are to be ofvalue to <strong>the</strong> less-developed countries.Toward this objective, CIMMYT growstwo generations in Mexico each year,resulting in 8,000 to 10,000 simple andtop crosses which combine differenttraits needed to develop lines SUitablefor those areas of <strong>the</strong> world includedunder <strong>the</strong>ir operational mandate(Table I). The germplasm used in <strong>the</strong>crosses is obtained from allover <strong>the</strong>world and is chosen because of usefulgenes. Before crosses are made,candidates for parents are studiedcarefully for <strong>the</strong>ir agronomic, pathologicand quality characteristics. Decisionsare based on experience and rapidassessment of available data, ra<strong>the</strong>rthan on a genetical analysis of eachparent. The number of lines produceddiffers from year to year, but 40 to 50%