Reaction Of Maize Germplasm To Common Foliar Diseases And Variability Of Maize Streak Virus Isolates
Maize is an important food crop that is grown in most regions of Kenya and it is consumed in various forms by over 80% of the population. Among the biotic constraints, foliar diseases cause heavy yield losses thus compromising food security in the country. This study was undertaken to determine the major foliar diseases infecting maize in different agro-ecological zones in Kenya and assess the reaction of various germplasm to these diseases. Isolates of Maize streak virus (MSV), a causal agent of one of the main maize diseases in the country was also characterized using molecular techniques. A survey was conducted to determine the occurrence, incidence, severity and distribution of different diseases infecting the crop in different agro-ecological zones in Kiambu, Embu and Nakuru counties. The study focused on six diseases which were northern leaf blight (Exserohilum turcicum), common rust (Puccinia sorghi), maize streak disease (Maize streak virus, MSV), gray leaf spot (Cercospora zea maydis, (GLS), head smut (Sphacelotheca reiliana) and common smut (Ustilago maydis). Twenty maize varieties were also evaluated for their reaction to the different maize diseases in a field experiment at University of Nairobi Kabete Campus. During the survey, maize leaves with maize streak disease (MSD) symptoms were collected to study the variability of MSV. Degenerate primers for Geminiviruses were used to amplify C1/C2 regions of different isolates of MSV. Polymerase chain reaction (PCR) products were sequenced and nucleotides used to compare the Kenyan isolates within themselves and with other sequences from the GenBank. Northern leaf blight, common rust and maize streak disease were found to be the three most prevalent and severe diseases in the different agro-ecological zones of the three counties. The same diseases were recorded in all the genotypes in the field evaluation. More efforts are therefore needed to manage the three diseases. Gray leaf spot, head and common smuts were also present, but were not widely distributed and had low incidence and severity both in the survey and field evaluations. These diseases should however not be ignored as their status may change with changes in climatic conditions. The Kenyan isolates were highly similar to one another with 99 to 100% nucleotide and 95 to 100% amino acid sequence similarities. They were also closely related to others from the rest of the world with 98 to 100% nucleotide and 94 to 100% amino acid sequence similarities. They all belonged to the MSV-A strain, the main subtype infecting maize. The high percent sequence similarities indicate low variability within the sequenced C1/C2 region of the virus. This information is important to breeders since low virus diversity indicates that maize genotypes showing resistance to MSV may have wider areas where they can be grown without risk of infection by different virus strains.