Population studies of Cercospora zeae-maydis and related Cercospora fungi

Grey leaf spot caused by Cercospora zeae-maydis is considered a global threat to maize production. In Africa, the disease was first reported just over 10 years ago, but has rapidly spread to most maize growing countries of sub-Saharan Africa. Being a rapidly spreading new disease in the region, demands for a quick but effective control strategy. Since pathogen populations cause epiphytotics, it is logical that control strategies should target populations rather than individuals. Effectiveness of disease control strategies however, is dependent on the knowledge available about a pathosystem. The studies in this thesis attempted to elucidate the East African C. zeae-maydis pathosystem using various population biology investigative approaches. In study I, the goal was to estimate genetic variability of East African C. zeae-maydis and infer the role of evolutionary forces in the populations. Study II focused on elucidating critical elements of the grey leaf spot disease triangle by examining interactions between biotic and abiotic stresses of the pathosystem. Study III focused on estimation of genetic variability of C. sorghi in order to compare and infer evolutionary responsiveness of C. zeae-maydis. Study IV investigated temporal dynamics in C. zeae-maydis populations in response to disease management. In all the studies, neutral genetic markers were the main genetic tools utilized. Study I showed that East African C. zeae-maydis populations were predominantly of Type II haplotype, with a weak genetic structure between populations. East African C. zeae-maydis was more variable than American Type I isolates. Concordance between neutral genetic markers, provided strong support for gene flow in the populations. Study II revealed that haplotype variability influenced epiphytotics. Presence of susceptible host and poor mineral nutrition also influenced epiphytotic patterns. In study III, no genetic structure of C. sorghi was detected and gene flow from wild hosts appeared to influence epidemics. A host species dependent evolution among Cercospora fungi was inferred by phenetics. Population parameters of C. sorghi were similar to C. zeae-maydis, suggesting similar evolutionary responsiveness of the two fungi. In study IV, the data indicate population stability of C. zeae-maydis. Taken together, these data suggest durability of resistant genotypes once deployed. Quarantine may reduce disease spread to new areas and being one epidemiological unit, suggests that regional efforts to abate epidemics are worthwhile