the Symposium on Wheats for More Tropical Environments - cimmyt

138upright, are given a qUick spraying,from <strong>the</strong> top to <strong>the</strong> bottom of <strong>the</strong> leafand <strong>the</strong>n from <strong>the</strong> bottom to <strong>the</strong> top.The rest of <strong>the</strong> procedure is <strong>the</strong> same asthat for <strong>the</strong> seedling test.Field trialsAlthough greenhouse tests are reliable,it is often necessary to conduct fieldtrials to confirm varietal reaction undernatural conditions. In institutionswhere controlled environmentalconditions are not present, screeningtests can be perfonned in <strong>the</strong> field withartificial inoculations. All of <strong>the</strong>resistant lines selected in <strong>the</strong>greenhouse are tested under fieldconditions over a period ofat least twoyears. It is important that tests beperfonned at three to four hot-spotlocations each year. It is worthwhile toinclude resistant and susceptible checksin <strong>the</strong> nursery, and it should besurrounded by a susceptible spreader.Inoculum is prepared in <strong>the</strong> same wayas for greenhouse tests, and <strong>the</strong>conidial suspension sprayed on <strong>the</strong>spreader as many times as necessary toobtain a severe epidemic.Disease ratings should be taken at leasttwice, once at <strong>the</strong> heading stage andonce at <strong>the</strong> soft dough stage. However,if time pennits, weekly readings arevery useful. From <strong>the</strong> disease progresscurve, <strong>the</strong> rate of infection can becalculated; <strong>the</strong> lower <strong>the</strong> rate ofinfection, <strong>the</strong> higher <strong>the</strong> degree ofresistance (12). Agronomically desirablelines with low infection ratings are usedas sources of resistance in <strong>the</strong> breedingprogram.Through this process, some lines havebeen selected (Table 1). However, <strong>the</strong>yneed to be tested fur<strong>the</strong>r at o<strong>the</strong>rresearch stations before being used forgeneral breeding purposes. Table 2indicates some lines resistant toB. soroktntana, based on screeningsmade in Zambia, Brazil and Bangladeshin 1980-81 (13). In India, Adlakha et al.(1) tested 625 wheat lines and reported16 lines resistant to B. soroktntana;HD1927 was considered to be <strong>the</strong> best.They believe that sources of resistanceare readily available in wheat.Table 1. Resistant lines to spot blotch (8. sorokiniana) based on field screenings, Londrina,Brazil, 1982-83Resistance reaction~/Variety/cross Londrina Palotina S. MiguelPamir"S"CM20834-A-7Y-501 Y-502Y-DB 8 MR, 30 MS 30 MS 10 MS, 20 MRPF7339=Sonora 64-SKe xL R64A/1 AS49HorizonVeery"S"CM33027-F-12M-1Y-3M-1Y-OM12S,45MS 8MR,20MR 25MS,5MR1 MS,40MS 5MR,20MS 5MR,20MRTMR, 20 MS 8 MS 20 MS, 15 MS