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

English: Root rot is an important disease of maize (lea mays l.). Colonization of roots by fungi has been studied to a limited extent in field soils because of difficulty of visibility and quantification. Several fungal species are involved in maize root rot, occurring in a complex of causal fungi separated by time and space. Different fungal species occur throughout the season on and in the roots, making it difficult to determine the primary pathogens. In addition, fungi associated with roots differ between localities. Root rot therefore, requires study at more than one locality and the spectrum of fungi involved is to be determined (Chambers, 1987). According to him pathogenicity tests should include different plant growth stages. Mixtures of fungi should also be used, as well as different environmental 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 rot of maize. The primary criteria used to measure root rot are root discolouration and root development. The use of these criteria is however, questionable since other factors may affect root discolouration and development. Furthermore, maize plants have a considerable ability to compensate for injury to root systems. A further complication is that general symptoms such as early senescence and lodging, only occur with extremely severe infection, or late in the season when the plants have reached physiological maturity. Normally the disease is characterized by an absence of distinct above ground symptoms, and subtle yield losses. A possibility also exists that fungal toxins may be involved in the root rot complex. Control of root rot has received limited attention. Interactions between practices and root rot incidence seem promising and need to be investigated with regard to developing a disease control system. The possibility of pesticides such as chlorpyrifos, which degrade to release a fungicidal 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 fungal species will ease control of genetic manipulation as well as enable the stability of resistance to be quantified particularly with regard to isolate differences or races. The inheritance of resistance in maize is not stable and the fact that the resistance is quantitative rather than qualitative suggests that there are many genes involved. Resistance has also been associated with many different resistance mechanisms and factors. In order to understand the principal aspects related to the root rot of maize, a literature 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 on genetic control. To determine pathogenic variability and the heritance of resistance for root rot eight 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 isolate Fusarium oxysporum. Root rot discolouration, plant length, root volume, root efficiency and yield were measured. A diallel analysis was used to analyse the data and determine the combining abilities, genetic correlations, heritabilities and correlated response. Significant differences in F l-hybrids for root discolouration, plant length, root volume, effective root volume and yield were found. Effective root volume was highly significantly correlated with root volume and plant length and a similar correlation was recorded between root volume and plant length. Root rot discolouration 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 GCA and SCA effects existed for root rot discolouration. Highly significant GCA effects were found for plant length, root volume, effective root volume and high significant SCA effects existed for plant length and yield. Highly significant SCA effects were found for root volume and effective root volume. Highly significant genetic correlations were found between root volume and effective root volume and between root volume and plant length. Genetic correlations between effective root volume and plant length were also significant. Yield was genetically negatively correlated with plant length, root volume and effective root volume and positively correlated with root rot discolouration. To assess field resistance to root rot in maize, 34 hybrids were planted and evaluated over five environments at Bethlehem and Potchefstroom from 1997 to 2000. The Additive Multiplicative Interaction (AMMI) statistical model was used to describe genotype x environment (G X E) interactions. Highly significant G X E interactions were recorded for root discolouration and yield. No significant G X E interactions were found for root volume and effective root volume. Additive main effects and multiplicative interaction (AMMI) model analysis clearly showed that different genotypes were identifiable with low potential environments predominating and other were identifiable with high potential environments predominating for root rot disease. The AMMI model can summarise patterns and relationships of genotypes and environments successfully, as well as provide a valuable prediction assessment of disease resistance. In general, stability of root rot is very complex due to numerous fungal species associated with infection of maize roots. Breeding for resistance to specific pathogens rather than to a complex of fungal species should ease control of genetic manipulations as well as enable the stability of resistance to be quantified particularly with regard to isolates and different races. This study will hopefully serve as an important source of information for future research or root rot resistance in maize.