the Symposium on Wheats for More Tropical Environments - cimmyt

176F. gramtnearum from Jiangsu differedsignificantly in virulence. as didresistance of wheat varieties toF. gramtnearum. but interactionbetween <strong>the</strong> virulence of <strong>the</strong> pathogenand <strong>the</strong> resistance of <strong>the</strong> wheat varietywas not present. Wang Yu-chung (14)confirmed that isolates of F.graminearum from Jiangsu differedsignificantly from one ano<strong>the</strong>r invirulence. but that <strong>the</strong>re was nosignificant difference in averagevirulence among isolates from differentcounties. Xu Yong-gao and FangZhang-da (23) fur<strong>the</strong>r suggested thatsingle ascospores isolated from a .culture differed as to virulence. as didthose from an ascus. OWing to <strong>the</strong> factthat <strong>the</strong> variation of G. zeae is verycomplicated. it has been difficult toclassify <strong>the</strong> isolates of G. zeae in Chinainto different physiological races.Based on <strong>the</strong> comparative study ofvirulence of <strong>the</strong> four isolates collectedfrom Mexico. Sri Lanka. Indonesia andChina on 7 Chinese and 28 Mexicanwheat varieties. Xu Yong-gao (personalcommunication) concluded that <strong>the</strong>Chinese isolate of G. zeae was morevirulent than <strong>the</strong> Mexican isolate onboth Chinese and Mexican varieties.Resistance of WheatVarieties to ScabTypes of resistanceIn <strong>the</strong> early 1960s. Schroeder andChristensen (11) and Takegami (12)reported that <strong>the</strong> resistance of wheatvarieties could be divided into twotypes. based on whe<strong>the</strong>r <strong>the</strong> resistancewas to initial infection or to <strong>the</strong> spreadof <strong>the</strong> infection. Many Chinesepathologists have confirmed scabresistance to be to hyphal spreadingwithin wheat. but <strong>the</strong>re has not beenagreement as to resistance to initialinfection (15.23.27). Through fieldobservations over many years. manyChinese workers have indicated that<strong>the</strong> difference in <strong>the</strong> rate of diseasespread within <strong>the</strong> spike when infectedat <strong>the</strong> same flowering stage indicates<strong>the</strong> difference in resistance to initialinfection. Twenty wheat varieties weretested for resistance to initial infectionat <strong>the</strong> heading stage by keeping singlespikelets moist for two days byspraying with a spore suspension. XuYong-gao (personal communication)found that <strong>the</strong> variety Zhen-mai 7495showed a lower level of diseased heads(23 to 40%); <strong>the</strong> rest of <strong>the</strong> varietiesreached 100%. It was suggested thatZhen-mai 7495 might be resistant toinitial infection under certainconditions. but that it did not exhibitany resistance to initial infection in <strong>the</strong>flowering stage or after being keptmoist for four days. Resistance to initialinfection seemed to be affected byenvironment.Testing for resistanceThe follOWing methods are generallyused for testing resistance to wheatscab in China:• Scattering diseased wheat grains on<strong>the</strong> soil surface-This method ofindUcing epidemics in <strong>the</strong> field issimilar to natural conditions (19).The fields are kept moist by sprinkerirrigation.• Dripping spore suspension into <strong>the</strong>floret-With a syringe. <strong>the</strong> fusariumspore suspension is dripped into asingle floret in <strong>the</strong> middle of <strong>the</strong>spike (15.17,23). This method isprecise and reliable. because <strong>the</strong>inocula can be appliedquantitatively.• Cutting <strong>the</strong> glume with scissorsdipped in spore suspension-Withthis method. developed by Wang-Yuchungand Yang Xin-ning (15). <strong>the</strong>inocula is introduced into one of <strong>the</strong>spikelets in <strong>the</strong> middle of a spike bycutting <strong>the</strong> glumes with scissors thathave been dipped in <strong>the</strong> spore