Inheritance of root rot resistance in maize (Zea Mays L.)

[摘要] English: Root rot is an important disease of maize (lea mays l.). Colonization of rootsby fungi has been studied to a limited extent in field soils because of difficulty ofvisibility and quantification. Several fungal species are involved in maize root rot,occurring in a complex of causal fungi separated by time and space. Differentfungal species occur throughout the season on and in the roots, making itdifficult to determine the primary pathogens. In addition, fungi associated withroots differ between localities. Root rot therefore, requires study at more thanone locality and the spectrum of fungi involved is to be determined (Chambers,1987). According to him pathogenicity tests should include different plantgrowth stages. Mixtures of fungi should also be used, as well as differentenvironmental stresses factors.Although very little attention has been given to the study of root rots of maize,one of the major areas requiring urgent attention is the quantification of root rotof maize. The primary criteria used to measure root rot are root discolourationand root development. The use of these criteria is however, questionable sinceother factors may affect root discolouration and development. Furthermore,maize plants have a considerable ability to compensate for injury to rootsystems. A further complication is that general symptoms such as earlysenescence and lodging, only occur with extremely severe infection, or late inthe season when the plants have reached physiological maturity. Normally thedisease is characterized by an absence of distinct above ground symptoms, andsubtle yield losses. A possibility also exists that fungal toxins may be involved inthe root rot complex. Control of root rot has received limited attention.Interactions between practices and root rot incidence seem promising and needto be investigated with regard to developing a disease control system. Thepossibility of pesticides such as chlorpyrifos, which degrade to release afungicidal component, should also be studied.Breeding for resistance is a long term control measure for root rot disease.Breeding for resistance to specific pathogens rather than to a complex of fungalspecies will ease control of genetic manipulation as well as enable the stability ofresistance to be quantified particularly with regard to isolate differences or races.The inheritance of resistance in maize is not stable and the fact that theresistance is quantitative rather than qualitative suggests that there are manygenes involved. Resistance has also been associated with many differentresistance mechanisms and factors.In order to understand the principal aspects related to the root rot of maize, aliterature survey concerning to the host range, host-pathogen interactions,control and economic importance of the disease was conducted. Furthermore,different methods of disease management are reviewed, with emphasis ongenetic control.To determine pathogenic variability and the heritance of resistance for root roteight inbred lines were crossed and planted in a 8 x 8 full diallel (Modeil)during the 1999/2000 season. The plants were infected with root rot isolateFusarium oxysporum. Root rot discolouration, plant length, root volume, rootefficiency and yield were measured. A diallel analysis was used to analyse thedata and determine the combining abilities, genetic correlations, heritabilities andcorrelated response.Significant differences in F l-hybrids for root discolouration, plant length, rootvolume, effective root volume and yield were found. Effective root volume washighly significantly correlated with root volume and plant length and a similarcorrelation was recorded between root volume and plant length. Root rotdiscolouration was positively but not significantly correlated with plant length,root volume, effective root volume and yield.Analyses of variance were done for GCA and SCA effects. Non-significant GCAand SCA effects existed for root rot discolouration. Highly significant GCAeffects were found for plant length, root volume, effective root volume and highsignificant SCA effects existed for plant length and yield. Highly significant SCAeffects were found for root volume and effective root volume.Highly significant genetic correlations were found between root volume andeffective root volume and between root volume and plant length. Geneticcorrelations between effective root volume and plant length were also significant.Yield was genetically negatively correlated with plant length, root volume andeffective root volume and positively correlated with root rot discolouration.To assess field resistance to root rot in maize, 34 hybrids were planted andevaluated over five environments at Bethlehem and Potchefstroom from 1997to 2000. The Additive Multiplicative Interaction (AMMI) statistical model wasused to describe genotype x environment (G X E) interactions. Highlysignificant G X E interactions were recorded for root discolouration and yield.No significant G X E interactions were found for root volume and effective rootvolume.Additive main effects and multiplicative interaction (AMMI) model analysisclearly showed that different genotypes were identifiable with low potentialenvironments predominating and other were identifiable with high potentialenvironments predominating for root rot disease. The AMMI model cansummarise patterns and relationships of genotypes and environmentssuccessfully, as well as provide a valuable prediction assessment of diseaseresistance.In general, stability of root rot is very complex due to numerous fungal speciesassociated with infection of maize roots. Breeding for resistance to specificpathogens rather than to a complex of fungal species should ease control ofgenetic manipulations as well as enable the stability of resistance to be quantifiedparticularly with regard to isolates and different races. This study will hopefullyserve as an important source of information for future research or root rotresistance in maize.

[发布机构] University of the Free State