the Symposium on Wheats for More Tropical Environments - cimmyt

31Earliness-Earliness is needed if <strong>the</strong>wheat plant is going to be able to fitinto narrow crop rotations. escapediseases and avoid seasonal environmentalstresses. A broad array ofgenetic variability exists. The mainproblem is to incorporate this characterinto lines which are adapted to tropicalconditions.Tolerance to acid soils and aluminumtoxicity-Considerable research hasbeen conducted for this character. andprogress has been made mainly bycombining Brazilian and Mexicanmaterials.Resistance to leaf rust-Many resistantlines are available. but resistance is notof lasting character. Changes in <strong>the</strong> rustVirulence require <strong>the</strong> continuousattention of pathologists and breedersin order to have resistant lines ready forrelease as current varieties becomesusceptible. As <strong>the</strong> wheat area extendsinto warmer environments. <strong>the</strong>re mustbe a constant awareness of new races ofpathogens.Resistance to helminthosporium­Helminthosporium sativum is by far <strong>the</strong>most common species in this genus.and much less is known about it than isknown about <strong>the</strong> rusts. GeneticVariability for resistance eXists. but nocomplete resistance is known. Genes forresistance from several sources.including species o<strong>the</strong>r than wheat.may have to be combined to achievesufficient resistance for <strong>the</strong> tropics.Currently. <strong>the</strong> most resistant lines arenot necessarily those of highest yieldpotential.Resistance to seedling blight causedby Sclerotium rolfsii-This fungus canbe particularly devastating in poorlydrained fields. Until recently. it wasassumed that <strong>the</strong>re was almost nogenetic resistance to it. and that seedtreatment or agricultural practices were<strong>the</strong> only solutions. However. recentpreliminary research in <strong>the</strong> Philippinesindicates some variability for resistance(D.A. Lapis. personal communication).Resistance tofusarium headscab-This disease is not restricted tohot environments. but can be verydevastating under warmer conditions.Genetic variability exists. andconsiderable research to pyramid genesfor resistance is being conducted inChina. Brazil and Mexico.These last three diseases are typical ofhot. humid conditions. Knowledgeabout <strong>the</strong>ir life cycle. host range andepidemiological behavior is limited. asare <strong>the</strong> techniques for working with<strong>the</strong>m. Artificial inoculation proceduresand storage and artificial multiplicationof spores must be fur<strong>the</strong>r studied beforemass screening of breeding materials ispossible.Resistance to aphids. stem borers andwaterlogging-These characters aremore important in hot climates than intraditional wheat areas. Aphids cancause direct feeding damage. and alsotransmit virus diseases. They havepreferences for certain varieties inexperimental plots. but it is not knownwhe<strong>the</strong>r <strong>the</strong>re is true resistance tofeeding. Little is known aboutresistance to stem borers and toleranceto waterlogging.This short overview reveals <strong>the</strong>existance of genetic Variability for mostcharacters. The breeder must combine<strong>the</strong>se characteristics into germplasmwith acceptable agronomic background.so that <strong>the</strong> farmer is assured ofreasonable yields every year. It is easyto breed for a single character whileneglecting <strong>the</strong> o<strong>the</strong>rs, but such a linewill never reach farmers' fields. Thecountry reports presented here shouldhelp in <strong>the</strong> understanding ofcombinations of characters needed for<strong>the</strong> various environments of <strong>the</strong> tropics.