the Symposium on Wheats for More Tropical Environments - cimmyt

146and resistance to H. sativum. This mayexplain <strong>the</strong> resistance to H. sativumfound in <strong>the</strong> Brazilian lines.After several seasons of field tests withmany lines from abroad and localcrosses. all introductions are nowcompared with susceptible andresistant check varieties. and certainselection criteria are followed.Selections are made. based on foliarinfection. head blight, node infection.black point, maturity period. plan<strong>the</strong>ight. lodging tendency andagronomic characteristics. A single.best growth stage at which resistancecan be identified has not yet beendetermined. Observations in two orthree growth stages are still necessarybecause of fluctuations in diseasepressure. The critical period forinfection is <strong>the</strong> time between earlydough and maturity. Observationsduring flowering provide a goodindicator. but selections made beforeflowering are unreliable.Low percentages of flag leaf necrosisand head blight during <strong>the</strong> soft doughstage are good measurements ofresistance. It may be possible to expressnecrosis by spot type and lesionnumber. Entire spikelets of susceptiblelines are qUickly destroyed byH. sativum: restricted lesiondevelopment and few lesions on <strong>the</strong>spikes are required. Node infectionexpresses itself in stem-breaking atmaturity; 100% stem-break is oftenrecorded on susceptible cultivars likeJupateco.Grain infection varies considerably.even among resistant lines. Latematuringlines may escape infectionand are often wrongly selected asresistant. Therefore. it is necessary torelate infection level to growth stageand to make comparisons to checkcultivars. H. sativum may severelyattack lodged plants. probably due to<strong>the</strong> creation of a favorable microclimatefor disease development after lodging.Dwarf plants do not compete well withweeds. and <strong>the</strong>y risk exposure torelatively high disease pressure.The flag leaf size is ano<strong>the</strong>r criterionthat needs fur<strong>the</strong>r investigation. Largeflag leaves permit more inoculumproduction than smaller ones. andindirectly contribute to higher diseasepressure. Smaller leaves. in general.contribute to a microclimate which isless conducive to disease development.Good tillering provides an effectiveweed-control mechanism. and may alsobe an expression of tolerance toaluminum toxicity and heat.In yield tests in Zambia. H. sativumseverity is scored during flowering andin <strong>the</strong> soft dough stage on a 0 to 9scale. During flowering. <strong>the</strong> height atwhich <strong>the</strong> infection is found on <strong>the</strong>plant is recorded; during <strong>the</strong> soft doughstage. <strong>the</strong> flag leaf and <strong>the</strong> spike arescored; <strong>the</strong> infection is already high upon <strong>the</strong> plant by that time. A severityindex for black point will be included in<strong>the</strong> scoring system.The occurrence of o<strong>the</strong>r diseases. suchas X. campestris and Fusarium spp..complicates <strong>the</strong> scoring of H. sativum.Scoring at flowering is often madedifficult by <strong>the</strong> presence of lesions allover <strong>the</strong> foliage. Fluctuations in diseasepressure by sudden changes in wea<strong>the</strong>r