the Symposium on Wheats for More Tropical Environments - cimmyt

Head Scab Screening Methods Used at CIMMYTG.T. Bekele, Wheat Program, CIMMYT, MexicoAbstractOne of<strong>the</strong> most important methodsfor selecting adaptable and resistant wheatplants under adequate planting conditions is <strong>the</strong> disease-screening technique.CIMMYT techniques have been developed and improved in order to reduceunnecessary effort on <strong>the</strong> part ofcooperators andfarmers around <strong>the</strong> worldwhen seed is sent to <strong>the</strong>m. From a wide array ofgermplasm. selections weremadefor scab resistancefrom 220 lines screenedfrom 1980 to 1982 in Tolucaand El Batan. 21 highly resistant linesfrom China and 5 highly susceptible onesselectedfrom 1978 to 1980. Fusarium-infected wheat spikes were collectedforpure spore isolation andfresh inoculum preparation. Two inoculation methods.using cotton or using a spray. have consistently proved effective. The cottonmethod. in which a tuft ofcotton permeated withfusarium spores is placedbetween <strong>the</strong> glumes of <strong>the</strong> spike. is <strong>the</strong> most precise. but both methods areextremely effective and give well-defined results.169Disease screening is one of <strong>the</strong> majortools of plant breeders in selectingdesirable plants with better and Wideradaptation. There are a number ofdifferent disease-screeningmethodologies in operation for handlingdifferent diseases. However. as diseasesthat were once unimportant becomemore important (3). new screeningtechniques need to be developed orexisting ones improved.The question is not only one ofscreening against diseases, but also ofreducing <strong>the</strong> number of breeders'materials. For ensuring better andWider adaptation. <strong>the</strong> selected materialshave to be tested against diseases overa much wider area and under differentconditions. Preliminary testing inbreeders' plots is necessary to reduce<strong>the</strong> amount of germplasm before it issent out to cooperators for testing in hotspots.Although a naturally occurringepidemic is one of <strong>the</strong> tools ofscreening. it frequently fails to occur ina continuous pattern due to changes inclimatic conditions that favor ei<strong>the</strong>r <strong>the</strong>host or <strong>the</strong> pathogen. In this case. <strong>the</strong>flow of disease information received by<strong>the</strong> breeder can lack continuity. Inorder to assure disease developmentannually. conditions must be availableand methods continually created toassure that screening processesfunction properly. Thus. <strong>the</strong> host istested in <strong>the</strong> presence of <strong>the</strong> pathogen.Materials and MethodsOn May 31, 1983.962 advanced breadwheat lines were planted in Toluca in<strong>the</strong> State of Mexico. These same entrieswere planted on June 22. 1983. inPatzcuaro. Michoacan. While <strong>the</strong> bulkof <strong>the</strong> material was from <strong>the</strong> CrossingBlock and <strong>the</strong> 17th International BreadWheat Screening Nursery. <strong>the</strong> groupalso contained 220 lines that wereidentified as scab resistant in tests inToluca and EI Batan from 1980 to1982. Also. 21 highly resistant linesfrom China and five highly susceptiblelines were included in <strong>the</strong> plantings ascontrols.Both at Toluca and at Patzcuaro.plantings were made in two one-meterrows. Five grams of seed were plantedin each plot to assure enough heads forinoculating by two different inoculationmethods. Although <strong>the</strong> wea<strong>the</strong>r patternis similar at <strong>the</strong> two places. soils andpathogens differ greatly. In Patzcuaro.<strong>the</strong> soil has a low pH (J.K. Ransom.personal communication). and <strong>the</strong>re is