Status of Maize Lethal Necrosis in Seed Production Systems and Interaction of Viruses Causing the Disease in Kenya

Abstract

Maize is cultivated in different climatic and ecological conditions in Africa. In Kenya, it’s a principal crop with an average of 90% of residents in maize production areas relying on it for food and employment. Production of maize has been on decline due to infection by Maize lethal necrosis (MLN) disease that is triggered by infestation of MCMV and a potyvirus, mainly Sugarcane mosaic virus (SCMV). Transmission of MCMV and SCMV through seed and soil may occur though at a very low rate. This research was done to determine the status of MLN disease in maize growing farms in Kenya and the effects of interactions of MCMV and SCMV in maize plant. A survey was conducted in seed production fields in 5 agro-ecological regions in 13 counties that produce maize seed in 2015, 2017, 2018, and 2019. Collection of samples was done for non-symptomatic and symptomatic maize by applying a standard protocol with testing for the MCMV virus using immunostrips and both viruses by real-time quantitative reverse transcription polymerase chain reaction (qRT- PCR). A total of 2550 ha was surveyed where 21% of maize was found to having varied degrees of severity of MLN infection. High MLN disease incidences and severities were recorded in Sub-humid agro ecological regions constituting of Embu, Elgeyo Marakwet, Uasin Gishu and Nakuru counties. Out of the total samples analyzed using qRT PCR, 38% had MCMV alone, 14% having SCMV alone and 18% with dual MCMV and SCMV. Out of the 185 samples analyzed with immunostrip from 2017-2019, 29 (16%) were positive for MCMV. To determine the effects of interactions, maize plants were infected with MCMV and SCMV single infections and in combination of the two viruses (M+S). Two varieties Duma 43 and DK 8031 were used in the experiment which was carried out in two seasons. Disease assessment for severity and heights were recorded 7 days post inoculation (7 dpi) and after every 7 days for a period of 56 days. Sampling of leaves was also done in each treatment in the same interval and leaves preserved at -80°C. Stored leaves were tested for MCMV using q-PCR and CT values recorded for each sampling point for each treatment. Treatment M+S recorded the lowest mean heights while SCMV recorded higher mean heights compared with MCMV except at 7 dpi. The M+S treatments recorded the highest disease severity score throughout the data collection period. There was significant difference (P < 0.05) in Ct values between the treatments for all the days of data collection. Treatments M+S recorded the lowest Ct values which is inversely proportional to the virus titer in the infected maize. The concentration of the MCMV increased in mixed infections compared to single inoculations. Similarly growth was retarded in mixed infections and disease severity was increased compared to single infections. Disease incidence was high in mixed infection in the initial stages of plant growth. The above findings show MLN disease is still a threat and measures need to be put in place to help minimize the introduction and spread of the disease. Monitoring of disease causing viruses through inspection and testing is effective in reducing the disease impact which is a result of the synergistic interactions of the viruses. Potyviruses play a major role in increased MLN outbreaks and hence require attention as part of management of the disease and development of resistant or tolerant varieties.