| Corn is one of the major grain crops grown in China and ranked as the thirdmajor one following wheat and rice based on its growth area and yield. Diseasesare the major nature disaster influencing corn production, which cause annualloss of 6%-10%. Corn gray spot causing by Cercospora zeae-maydis wasviewed only as a minor disease ever before in China, however, since 1990's, ithas become one of the serious diseases occurring in growing area in northeasternChina, therefore it has been listed as major disease which demands highlyeffective control measures against it for sustainable growth of corn production. Todate there have been tremendous achievements in many aspects of the diseasestudy particularly on its etiology, epidemiology, localization of resistant gene loci,evaluation of resistant germplasm and IPM measures etc. However still less workhas been reached on the pathogen diversity in biology and genetics as well asphysiological differentiation which would make much sense to realize effectivecontrol of the disease. As a result our work intended to deal with the researchessurrounding pathogen genetic diversity and physiological differentiation whichwould then build robust bases for furthering more effectively resistant breedingand reasonable deployment of resistant varieties.The genetic variation and pathogenicity differentiation in the pathogenstrains were studied in terms of host-pathogen interaction with severaltechnologies or methods involving observation of pathogen culture traits,identification of pathotypes on differential hosts, electrophoresis of isozyme andanalysis of RAPD. The results suggested that pathogenicity differentiation andbiological polymorphism really existed in the population of the pathogen, itspathogenicity differentiation was caused by gene variation or differentiation inDNA level absolutely not by the change of phenotype, in other words, the sort ofpathogen differentiation was supposed to be closely regulated by geneticsubstance, which was especially confirmed by identification of pathotype withstrong pathogenicity.The biological polymorphism of Cercospora zeae-maydis was studiedsystematically in the levels of histology, protein and DNA. The major charactersin polymorphism of pathogen were listed as follows:(1)The great differenceswere seen in shapes and sizes of conidiophores, the growth velocity of myceliumand pigment-productivity of colony among pathogen strains. In addition, analbino strain, a kind of mutant, was casually discovered during isolation ofpathogen from diseased plants, which supported strongly our previous predictionthat there should be some potential genetic variation in Cercospora zeae-maydis.Much significant difference was found in biological characteristics, profiles ofprotein and isoenzyme as well as DNA polymorphism in comparison of wildstrain and the mutant. The mutant grew usually faster on medium than wild strainbut its pathogenicity was not as strong as wild one, thus the virulence of mutantmight not be devoted by its fast growth rate, and more likely by its higherconidiophore-productivity. (2)The different progress levels of symptoms andtypes of lesion caused by pathogen strains with varied virulence verified solidlythe objective existence of polymorphism in histological level. (3)The pathogenpolymorphism was also seen in the process of directional selection of corncultivars over the pathogen, which could be well exhibited by the diversifiedchanges of electrophoresis profile and intense of protein and isozymes includingEST, SOD, CAT, POD, MDH, PPO from strains after each of them beinginoculated on different cultivars, similar results were also obtained when parallelassay was made for their enzyme activities. Furthermore, varied groups clusteredby mycelium anastomosis among strains also exhibited characters of pathogenpolymorphism.(4)The 23 pathogen strains were grouped into 2 by RAPD analysis,the similarity reached 64% between both groups, but 93% (90%～96%) and 74%(64%～84%) within the same group, respectively. Nine corn inbreds (Shen137, 78599-1, Mo17, 748, C8605-2, E28, 598, Va35,K12) and 8 cultivars (Shenshi29, Shenshi31, Tiedan9, Yedan13, Dan413,Danyu18, Shennong87, Dongdan54), which believed to be widely representativefor corn genetic background in China, were successfully screened as differentialand assistant differential hosts, respectively. The pathogenicity types ofCercospora zeae-maydis were difficultly identified only according to lesionreaction types, thus disease rank and frequency of each one appearing ondifferential hosts should be most useful for accurate identification. On basis ofevaluation on the disease rank on the differential/assistant differential hosts incombination with other pathogen traits, 23 pathogen strains collected fromnorthern corn growing area were grouped into 5 pathogenicity types. Though...
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