Determining the role of seed and soil in the transmission of viruses causing maize lethal necrosis disease

Abstract

Maize is an important food crop in sub-Saharan Africa (SSA) and is consumed by over 90% of Kenyan people. The outbreak of Maize lethal necrosis (MLN) disease in Kenya in 2011 resulted in drastic reduction in maize production. The disease, a result of synergistic interaction between Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV), is also reported to be present in Uganda, Tanzania, Rwanda, DRC-Congo and Ethiopia. The two viruses causing MLN are reported to be transmitted by vectors, through mechanical means, and by seeds though at very low rates. However, the rate of spread of the disease in the eastern Africa region has been very high, indicating a potential role of seed transmission. There is also little information on the role of contaminated soil in the transmission of the two viruses. The aim of this study was therefore to determine the rate of MCMV and SCMV transmission through seed and soil and to determine the combined effect of mechanical and seed transmission in MLN disease development. In order to determine the rate at which the two viruses are transmitted through seed and soil, fortyeight inbred lines were evaluated, where seeds from infected plants were planted in clean soil, and seeds from non-infected plants (clean seeds) planted in soil where MCMV or SCMV-infected maize plants were harvested. Seeds harvested from infected plants were first tested for both viruses using DAS-ELISA. Soil was collected in pots from fields where infected susceptible plants were grown and all debris removed before planting clean seeds. To determine the combined effect of seed and mechanical transmission in MLN disease development, maize were grown from clean seeds which were not inoculated with any virus; other plants were grown from clean seed but inoculated with both viruses, and other plants were grown from seeds obtained from infected plants and inoculated with one virus first and later with the other virus. The experiments were laid out in a completely randomized design with three replications in the greenhouse. Plants were evaluated for incidence and severity, and tested with DAS-ELISA to determine the presence of MCMV and SCMV. All seeds harvested from MCMV-infected plants tested positive for MCMV while all seeds tested negative for SCMV, even in those obtained from SCMV-infected plants. There were also no observable symptoms in plants grown from infected seeds or in contaminated soil. However, DASELISA results confirmed MCMV to be present in leaves from plants grown from infected seeds at a rate of 4.17% in CMCMV111, a susceptible inbred line, and at a rate of 8.34% in CMV066 (tolerant inbred line), and CMCMV111 (susceptible inbred line) grown on contaminated soil. Transmission of SCMV by seed or through soil could not be confirmed by use of DAS-ELISA. Results also indicated that all combinations of artificial inoculations with MCMV or SCMV resulted in development of typical symptom associated with the viruses. Resistant genotypes resulted in low rates of infection, particularly where MCMV was artificially inoculated. However, no symptoms were observed in plants from infected seeds but not inoculated with either of the viruses. MCMV was detected in all genotypes inoculated with MCMV or both viruses, while there was no detection of SCMV or MCMV in non-inoculated plants, even when seeds were grown from infected plants. Based on these findings, it can be concluded that MCMV is transmitted through seed and contaminated soils though at low rates, information that could be of importance to farmers and all stakeholders interested in maize production; while SCMV was not. The results also indicate that mechanical inoculation (and probably any form of secondary infection such as by vectors) of viruses, causing MLN plays a significant role in disease development. While mechanical transmission of viruses is naturally rare in maize, the findings are helpful to farmers in that prevention of secondary infection in the field may help in reducing MLN incidence and severity. Further studies need to be conducted to determine the localization of the virus in seed.