the Symposium on Wheats for More Tropical Environments - cimmyt

156dry weight per ear was caused mainlyby a severe reduction in <strong>the</strong> number ofkernels per ear.ConclusionsEarly foliar infection byhelminthosporium reduced <strong>the</strong> kerneldry weight per ear of both Africangenotypes. Foliar infection betweendevelopment stages 30 and 50 DC of<strong>the</strong> plant inhibited kernel production.but did not appear to influence dryweight per kernel. Foliar infectionbetween development stages 40 and 65DC of <strong>the</strong> plant. however. could affectboth number of kernels per ear and dryweight per kernel. The results suggestthat <strong>the</strong> development stage at which<strong>the</strong> infection occurs is of importance for<strong>the</strong> number of kernels produced. Thedegree of inhibition was high.particularly at <strong>the</strong> moderately high airtemperature and humidity. Underirrigated conditions. as in Zambia.helminthosporium occurs on <strong>the</strong> leavesbefore an<strong>the</strong>sis; <strong>the</strong> experimentssuggest that helminthosporium may<strong>the</strong>n cause significant yield loss. Theamount of yield loss found in <strong>the</strong>seexperiments justifies yield loss studieswith fungicide applications beforean<strong>the</strong>sis. and screening for geneticvariation in resistance to earlyinfection.At high air temperatures andmoderately high humidity. <strong>the</strong> effect oftemperature on kernel dry weight perear was larger than was <strong>the</strong> effect ofhelminthosporium. This indicates that.in areas with high air temperature andhumidity, breeding for tolerance to hightemperature should have priority overbreeding for resistance tohelminthosporium.In both trials. kernel dry weight per eardiffered between genotypes; it washighest for MIL04-21. which wasselected under warm. rainy-seasonconditions in Zambia. Under thoseenvironmental conditions, MIL04-21had a higher yield than Tokwe. Geneticvariation appears to allow selection forincomplete resistance to helminthosporiumbut. Without good tolerance tohigh temperature. this resistance mayhave little practical value.AcknowledgementsThe authors wish to thank CIMMYT forproViding <strong>the</strong> opportunity forpresenting this paper. <strong>the</strong> Departmentof Phytopathology of <strong>the</strong> StateAgricultural University. Wageningen.Ne<strong>the</strong>rlands. for proViding <strong>the</strong> facilitiesto perform <strong>the</strong> trials. M. Kelfkens andM. Krot for carrying out <strong>the</strong> 1982 and1983 trials. respectively. and W.Hoogkamer for prOViding technicalassistance.Table 3. The effect of foliar infection by Helminthosporium sativum on yield componentsof first tillers for two African wheat genotypes grown in a growth chamber. Ne<strong>the</strong>rlands.1982 and 1983YieldcomponentTria/1,1982(genotype and effect)Effect ofEffect ofTokwe inoculatign!.' Mll04·21 inoculatio.,!.'Trial 2. 1983(genotype and effect)Effect ofEffect ofTokwe inoculation!' Ml104·21 inoculatio.,!.'Kernel dry weightper ear (g)No inoculation 0.14Inoculated 0.14 0Number of kernels HNo inoculation 5Inoculated 6Kernel dry weight (my)No inoculation 29Inoculated 25 .14!' Effect of inoculation =1.00-(+/-10.53 0.68 1.280.33 .38 0.25 .63 0.80 .3719 21 4315 .21 8 .62 28 .3528 35 3024 .14 35 0 30 0