Thesis (M.Sc. (Crop Science)) --University of Limpopo, 2008 / Stenocarpella ear rot caused by Stenocarpella maydis (Berck) Sutton is the most important disease of maize in South Africa. It is a sporadic disease which makes it difficult for farmers to prepare for its occurrence and consequently of the control measure. The objectives of this study were to genetically characterise the resistance of Stenocarpella maydis ear rot and to identify agronomically suitable Stenocarpella ear rot resistant inbred lines with good combining ability for grain yield. The experimental design was a randomised complete block design with three replications. Studies were conducted at Bethlehem, Cedara and Potchefstroom. To facilitate the comparison, separate trials were established for inbred lines and top cross hybrids. Fifty-four inbred lines were compared against four inbred lines vs. E739, DO620Y, H111 and Mo17 that are well adapted and stable yielders possessing variable resistance to Stenocarpella maydis ear rot. Fifty-four top crosses were compared against one open pollinated variety (SAM 1066), which was used as a tester line as well as three commercial hybrids vs. PAN 6124BT, PAN 6026 and CRN 3505. At Potchefstroom there was an inoculation trial using both inbreds and top crosses. The inbred and top cross materials were obtained from 2004/05 breeding nursery under natural infestation of Stenocarpella maydis at Agricultural Research Council-Grain Crops Institute, Potchefstroom. Data collected were number of days to 50% silking, plant and ear height in centimetres, husk cover, ear position, stand count, total number of ears, number of diseased ears and lodging resistance,. Entries 43 and 4 were the most stable inbred lines with a beta close to 1, while entries 9 and 25 had the smallest deviation from regression. Among the tested inbred lines entry 47 was superior over other inbred lines for grain yield followed by entry 4. Entry 47 showed grain yield of 2.84 tons ha-1 at Bethlehem and 4.42 tons ha-1 at Potchefstroom. While entry 4 had a grain yield of 2.19 tons ha-1 at Bethlehem and 4.58 tons ha-1at Potchefstroom. The two lines, however, are poor combiners for both grain yield and Stenocarpella maydis ear rot resistance. Using SAM 1066 as a tester the grain yield observed for top crosses at Bethlehem, Cedara and Potchefstroom were 5.94, 7,15 and 9.95 tons ha-1, respectively. Entries 57 and 14 were the most stable top cross hybrids with a beta close to one, while entries 46 and 47 had the smallest deviation from regression. Entries 56 and 28 were the most superior top cross hybrids. Entry 56 showed grain yield of 5.58 tons ha-1 at Bethlehem, at Cedara it showed the yield of 5.90 tons ha-1 and at Potchefstroom it was 9.95 tons ha-1 and for the average of three sites it was 7.14 tons ha-1. Entry 28 showed grain yield of 5.80 tons ha-1 at Bethlehem, at Cedara it was 5.80 and at Pothefstroom it was 9.35 tons ha-1 and the combined average was 6.98 tons ha-1. These values compared favourably with the commercial standards. The checks entries 58 and 57 had proved to be resistant over locations. The best combiners for Stenocarpella maydis resistance were entries 29 and 52. Stenocarpella maydis ear rot was found to be of polygenic resistance with additive genetic effects. / The government of Lesotho, Agricultural Research Council-Grain Crops Institute (ARC-GCI), and the Maize Trust

Diplodia ear rot (DER) of maize (Zea mays L.) caused by the fungus, Stenocarpella maydis (Berk.) Sutton has increased in incidence in localized fields over the past decade. My research focused on screening for resistance by examining the development of DER following inoculations prior to flowering, analyzing a diallel cross for DER resistance, and examining the genetic variability of the fungus from isolates collected from the U.S. and the Republic of South Africa. DER developed in maize following inoculations with a spore suspension prior to flowering in both greenhouse and field evaluations. A spore suspension gave a better differentiation of resistance responses than dried preparations of colonized millet, colonized ground popcorn, or kernels from a diseased maize ear, all applied in the whorl 10 to 15 days prior to flowering (V12 for inbreds), and natural occurrence of disease. General combining ability was significant for both 1994 and 1995 growing seasons in an analysis of the F₁ of the diallel cross, indicating that additive gene action may be responsible for resistance and could be introduced into commercial cultivars. Specific combining ability was significant in 1995 and indicates that dominant gene action or epistasis may play role in DER resistance. There were minimal numbers of isozyme polymorphisms found in my S. maydis collection. Two isolates were polymorphic for esterase, two isolates were polymorphic for hexokinase and malate dehydrogenase and one isolate was polymorphic for hexose kinase. Fungi that have limited isozyme polymorphisms often are biotrophs or fungi with formae speciales which are usually limited to one host. These groups of fungi usually have races and this may indicate that a gene-for-gene interaction exists. These findings suggest that i) the whorl inoculation separates genotypes into resistant, intermediate, and susceptible groupings; ii) additive gene action is predominant form of inheritance, and iii) there are few isozyme polymorphisms in the population of S. maydis sampled. / Ph. D.