the Symposium on Wheats for More Tropical Environments - cimmyt

113standard date for sowing. In spite of <strong>the</strong>useful guidelines provided in Figure 1.a drought can occur at any stage of <strong>the</strong>crop and for an undetermined length oftime. Although <strong>the</strong> date of seeding canbe adjusted to good moistureavailability, <strong>the</strong> poor water-retentioncapacity of <strong>the</strong> soil can provoke earlydrought. <strong>the</strong>reby causing reduced plantdensity and non-uniform stand. Theconstant mid-season drought around<strong>the</strong> heading stage is crucial in <strong>the</strong>reduction of potential yield. Germplasmof high yield potential and tolerance todrought at this stage of <strong>the</strong> crop cyclewould help stabilize yields to someextent.FrostOne of <strong>the</strong> most feared limiting factorsfor wheat production in <strong>the</strong> region isfrost injury. especially at <strong>the</strong> headingstage. Some adjustment in <strong>the</strong> date ofseeding can serve as an escapemechanism. but this leads to o<strong>the</strong>rproblems at <strong>the</strong> beginning and <strong>the</strong> endof <strong>the</strong> crop cycle. In fact. farmerscustomarily spread wheat seeding overa long period of time to escape frost; <strong>the</strong>various seedings suffer from drought,frost. late rains and severe diseaseepidemics. To solve this problem, <strong>the</strong>earlier-mentioned heat and droughttolerance must be combined with lateheading. through <strong>the</strong> addition of asingle gene for vernalization. in order toescape <strong>the</strong> major frost frequency period.Hlgb rainfallIt is paradoxical to discuss <strong>the</strong> problemof high rainfall and drought at <strong>the</strong> sametime. yet it seems a rule in <strong>the</strong> moretropical areas. In normal years. highrainfall is generally a problem for latevarieties or late-seeded crops. causingdifficulties in <strong>the</strong> harvest process. suchas deterioration in grain quality and. insome cases. preharvest sProutinb' Inabnormally wet years. <strong>the</strong> intensity ofvarious diseases is so severe that evenchemical control is not sufficient tomaintain average productivity.Considering <strong>the</strong>se climatic factors. amodel wheat variety should have hightillering ability. tolerance to early heat.tolerance to drought in <strong>the</strong> mid-season.late heading to escape frost and a shortripening period to mature before <strong>the</strong>onset of rains. To <strong>the</strong>se can be addedsemidwarf plant stature to resist strongwinds and small fertile spikes to ripenqUickly. Such a wheat variety will <strong>the</strong>nneed ample resistance to <strong>the</strong> variousdiseases and insect pests prevalent in<strong>the</strong> region.Biological StressesDiseasesBy far <strong>the</strong> most common factor limitingwheat production in more tropical areasof <strong>the</strong> Sou<strong>the</strong>rn Cone region is disease.In a recent analysis done for CIMMYT'sEconomics Program. <strong>the</strong> average lossesin yield due to wheat diseases in <strong>the</strong>Parana. Matto Grosso do SuI and SaoPaulo regions of Brazil were 11 % peryear. What is more important is that<strong>the</strong>se losses occurred in spite of <strong>the</strong> factthat over 90% of <strong>the</strong> farmers usedchemical control. Where <strong>the</strong>re was nochemical protection. <strong>the</strong>se lossesranged between 20 and 60% (Table 1)and. in some cases. as high as 100%.A combination of high temperature andhigh humidity favors a large complex ofdiseases in this region; <strong>the</strong> importanceof spot blotch (Helminthosporiumsativum) is highlighted in <strong>the</strong> datapresented in Table 2. During <strong>the</strong> 1981to 1983 period. spot blotch aloneconstituted between 50 and 90% of <strong>the</strong>yield loss caused by <strong>the</strong> total diseasecomplex in <strong>the</strong> western Parana region.This picture was quite different during<strong>the</strong> mid-1970s. when leaf rust was <strong>the</strong>major disease.O<strong>the</strong>r important diseases in <strong>the</strong>complex are stem rust. scab. bacterialblight. glume blotch and powderymildew. However. in <strong>the</strong> past few years.