the Symposium on Wheats for More Tropical Environments - cimmyt

178Inheritance of resistance to scabSchroeder and Christensen (11)reported that <strong>the</strong> resistance of wheat tohead scab was a quantitative charactercontrolled by polygenes. In recentyears. Chinese workers have alsoshown that resistance of wheat tocolonization by G. zeae is controlled bypolygenes (27). The resistance of Fistends to resemble <strong>the</strong> resistant parent.and exhibits obvious heterosis.Backcrossing with resistant parentstends to increase resistance. XiaSui-shing. Zhou Chau-fei and Quia Cunming(personel communication) studied<strong>the</strong> inheritance of scab resistance inSu-mai 3 and Wang-shui-bai andobtained similar results. Yu Yu-jin (26)studied <strong>the</strong> inheritance of resistance ofSu-mai 3 by monosomic analysis;results indicated that at least five pairsof genes which determine resistance tohyphal spread were located onchromosomes lB. 2A. 5A. 6D and 7D.Chen Chu-huo (2). using a 4 x 4 halfdiallelscheme with two resistant andtwo susceptible parents to study <strong>the</strong>inheritance of resistance in wheat.suggested that Su-mai 3 and WangzhouHong-he-shang (Red Monk) might havemore dominant genes for controllingresistance: <strong>the</strong>y demonstrated highgeneral combining abll1ty for redUcing<strong>the</strong> rate of diseased spikelets in <strong>the</strong>irprogenies. It appears that <strong>the</strong>inheritance of resistance to scab ismainly governed by additive genes. butnon-additive genes also have asignificant effect. The genes controllingresistance were partially dominant. Itwas found in diallel crosses that <strong>the</strong>rewas a significant positive correlationbetween resistance and plant height orspike length. and a significant negativecorrelation between resistance andspikelet density in F2 populations.Various workers have suggested that<strong>the</strong> heritabll1ty of resistance of wheat toscab is low.Sources of resistance to scabThe studies of <strong>the</strong> cooperative researchteam have demonstrated that sources ofscab resistance come mainly fromcommon wheats. Some Chinesevarieties with high and stable resistancehave been <strong>the</strong> best available sources ofresistance among <strong>the</strong> world collection;<strong>the</strong> resistance in exotic varieties has notexceeded <strong>the</strong> resistance of <strong>the</strong>sevarieties. Su-mai 3 has been introducedinto many countries of <strong>the</strong> world. and<strong>the</strong>re has been no report that itsresistance is equal to or better thanlocal varieties in those countries.Nanjing 7840. a new source ofresistance. is derived from crosses of(Aurora x Anhui 11) F2 x Su-mai 3. andmany resistant varieties or lines withgood agronomic characters have beenderived from it (29).Crosses between moderately resistantand susceptible varieties may yieldhybrids with higher resistance than<strong>the</strong>ir parents. The well-known resistantvariety Su-mai 3 was bred through <strong>the</strong>hybridization of <strong>the</strong> moderatelysusceptible varieties. Funo and Taiwanwheat. The crossing of <strong>the</strong> moderatelyresistant varieties. Jingzhou 1 and Sumai2. also produced progenies withhigh and stable resistance.Some materials of <strong>the</strong> genus Secale areano<strong>the</strong>r source of resistance. Jingzhou1 was derived from <strong>the</strong> hybrid of Nanda2419 and Jingzhou rye. Jingzhou 66(MS-MR) was derived from <strong>the</strong>combination (Funo x durum) (Nanda2419 x rye). Jingzhou 1 has beenadopted as a common parent in <strong>the</strong>breeding programs for resistance toscab in South China.EpidemiologyOccurrence of scabcUsease in <strong>the</strong> fieldA rice-wheat cropping system is foundin most of <strong>the</strong> regions of <strong>the</strong> middle andlower reaches of <strong>the</strong> Yangtse River. Theinitial inocula of scab usually comesfrom <strong>the</strong> peri<strong>the</strong>cia on rice stubble.