the Symposium on Wheats for More Tropical Environments - cimmyt

171Wheat spikes are spray-inoculatedfrom a distance of 10 to 15 cm until<strong>the</strong>y are soaked with sporesuspension.Crop stage for inoculationfor maximum disease developmentFor maximum scab development.wheat heads are inoculated at floweringor at an<strong>the</strong>r protrusion stage. Young.plump and yellow an<strong>the</strong>rs (not whiteand dry) are good indicators of <strong>the</strong> rightstage for inoculation as are floweringwheat heads with one to threeprotruded an<strong>the</strong>rs at <strong>the</strong> mid-point of<strong>the</strong> spike(s). In this study. a cultivarwas considered ready for inoculationwhen 5 to 10% of <strong>the</strong> extruded an<strong>the</strong>rsappeared in a plot of wheat plants. It iscritical that this growth stage is used atall times. Any inoculation at pre orpost-flowering (especially at postfloweringwhen extruded an<strong>the</strong>rs arewhite and dry) results in poor scabdevelopment.Inoculating <strong>the</strong>wheat plant in <strong>the</strong> fieldWheat plants in plots of two rows onemeter long were inoculated withfusarium spore suspension by both <strong>the</strong>cotton and <strong>the</strong> spray methods atToluca; only <strong>the</strong> spray method wasused in Patzcuaro. In Toluca. ten spikesin one row of each plot were inoculatedwith <strong>the</strong> cotton method. and eachinoculated spike covered with a glassinebag. The remaining single row inToluca and one row in <strong>the</strong> plots inPatzcuaro were spray inoculated.Spray-inoculated wheat spikes were notcovered with bags. Mter inoculation.plants were checked periodically fordisease development.Disease development and scoringScab disease developed differently onearly and late-maturing cultivars. bothat Toluca and Patzcuaro. In spite ofdifferences in inoculation time. diseasedevelopment occurred between 35 and45 days after inoculation. Diseasedeveloped more rapidly in earlymaturinglines than in later ones.Therefore. disease scoring was done42 days after inoculation. using ao to 5 scale.Disease development was excellent withboth inoculation methods. With <strong>the</strong>cotton method. spikes remained cleanand bright (straw color). since <strong>the</strong>ywere protected by <strong>the</strong> glassine bags.The movement of fUsarium infectionwas well-defined: it started at <strong>the</strong> pointof inoculation and progressed ei<strong>the</strong>r upor down <strong>the</strong> spike from where <strong>the</strong>cotton was placed. Since <strong>the</strong>re was nodripping of <strong>the</strong> liquid inoculum. <strong>the</strong>disease remained localized and <strong>the</strong>nspread to o<strong>the</strong>r spikes in a continuouspattern. without skipping spikelets.This type of disease development wasnot obtained in previous studies when<strong>the</strong> syringe injection-inoculationtechnique was used (unpublished data).With <strong>the</strong> spray method. head scabdevelopment was also excellent at bothToluca and Patzcuaro. Differencesbetween susceptible and resistant linesand between inoculated anduninoculated portions of <strong>the</strong> plots werewell defined. Susceptible lines showedbright pinkish-orange sporodochia filledwith fUsarium conidiospores. Infectedspikes (rachis. glumes and seeds) ofsusceptible lines were permeated withfusarium mycelium and filled withspores. In most cases. seed formationwas aborted or replaced bysporodochia. Symptom developmentwas good with both techniques.However. with <strong>the</strong> spray method. <strong>the</strong>rewere a large number of infection pointsin each spray-inoculated spike anddisease development was much greater.spreading over <strong>the</strong> full length of <strong>the</strong>infected spikes.Both inoculation methods were foundvaluable in identifying 198 highly scabresistantlines from <strong>the</strong> 962 lines tested.All of <strong>the</strong>se 198 selected lines showedresistance both in Toluca and in