the Symposium on Wheats for More Tropical Environments - cimmyt

199Loss in yield is directly proportional to<strong>the</strong> time when <strong>the</strong> epidemic starts.Yield losses of <strong>the</strong> cultivar Jupatecowere assessed by controlling <strong>the</strong> start of<strong>the</strong> epidemic, using appropriatefungicides. Whert <strong>the</strong> epidemic startedas early as 30 days after sowing, loss inyield was 70%; when <strong>the</strong> epidemicstarted 51, 58, 64 and 72 days aftersowing, losses were 54, 44, 40 and 4%,respectively (Figure 1).The timing of <strong>the</strong> first spraying is soimportant because, after <strong>the</strong> epidemichas become established, chemicalcontrol of <strong>the</strong> disease is uneconomical.Starting fungicidal spraying soon after<strong>the</strong> start of an epidemic ei<strong>the</strong>r paralyzesit for 30 to 40 days or substantiallyreduces <strong>the</strong> rate of infection, dependingon <strong>the</strong> type of fungicide used. Theobjective is not to obtain 100% controlof <strong>the</strong> disease, but to maintain its levelbelow 50% up to <strong>the</strong> soft dough stage.For this reason, systemic fungicidesshould be used for susceptible cultivarsto delay <strong>the</strong> start of <strong>the</strong> epidemic, andprotectant fungicides for lesssusceptible cultivars (slow-rustingcultivars) to reduce <strong>the</strong> rate of infection.Number of sprayingsNormally three sprayings arerecommended but, with spot blotch,<strong>the</strong>re may be a need for four. Theexperience in Brazil indicates that, if<strong>the</strong> disease epidemic starts 45 to 55days from sowing, at least threesprayings are necessary to obtaineconomic returns. Never<strong>the</strong>less, withprolonged dry periods (40 to 50 days)after tillering, two spraytngs may beenough. Spot blotch is a very fastgrowingdisease, especially underwarm, wet conditions, and it can attack<strong>the</strong> spikes as well as <strong>the</strong> leaves. Undersuch conditions, <strong>the</strong> disease can only becontrolled with at least three to foursprayings with Maneb or Mancozeb orthree sprayings with Propiconazole.Three or four applications not onlyreduce <strong>the</strong> rate of infection of <strong>the</strong>disease on <strong>the</strong> leaves, but protect <strong>the</strong>spikes from becoming infected. Sincewea<strong>the</strong>r conditions are very changeablein Latin America, <strong>the</strong> exact number ofsprayings cannot be predetermined.100Y= 20,415 + 2,885x-O,043x2-~§ eo'0-1IJ>=80 12 =0,984020•32 51 58 64 72Days Mter SowingJl'igure 1. Loss in yield in relation to <strong>the</strong> start of <strong>the</strong> leaf ru.t epidemic on<strong>the</strong> caltlvar Japateco, Brun, 1978