Abstract:

The major threat to maize production in Uganda since 2012 is maize lethal necrosis (MLN) disease. MLN is a viral disease caused by double infection of maize plants with Maize chlorotic motile virus (MCMV) and a cereal potyvirus notably Sugarcane mosaic virus (SCMV). Although maize lethal necrosis disease was first reported to be present in eastern Uganda in late 2012, to date there is limited information on farmer knowledge, distribution, vector specificity and virus alternative host plants of maize lethal necrosis disease in Uganda. This study was undertaken to generate this information. A socioeconomic survey covering 396 farmers from 14 major maize growing districts in five agroecological zones of Uganda was conducted aimed at assessing farmers’ awareness of MLN and its management. In addition, biological field surveys were carried out in 16 major maize growing districts from five agro-ecological zones (AEZ) of Uganda to determine incidence and severity of MLN, weeds as potential natural hosts and potential insect vectors of MLN causing viruses. Samples from maize plants, weeds, cultivated crops and potential insect vectors were collected and tested for the presence of MLN causing viruses by Double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) and Reverse Polymerase Chain Reaction (RT-PCR). Screen house transmission studies were conducted to determine susceptibility of identified host plants (weed species and cultivated crops) to MLN viruses and the ability of selected insects to transmit the viruses. From the socioeconomic survey, most farmers (58.1%) had not heard or experienced MLN. A good proportion of respondents (56.6%) became aware of MLN in 2014. A Chi square test revealed that the level of MLN awareness was the five agroecological zones (x2 = 66.428, df = 4, P significantly different across 0.001). Eastern highland agroecological zone had the highest level of awareness (66.9%) while Western Highlands had the lowest levels of awareness (17.5%) among the respondents. The farmers who recognized MLN symptoms were able to identify the diseased plants at one month after planting. Up to 53.9% of farmers attempted to control MLN mostly by uprooting and burning diseased plants. From biological surveys, MLN incidence and severity significantly (P<0.05) varied across agroecological zones. Eastern Argo-ecological zone had highest incidence and severity during the three seasons of this study. Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV) was the main MLN disease causing viruses identified in Uganda. A total of 16 species of weeds representing 8 families and 17 cultivated crops species representing 9 families were found growing in association with maize in the surveyed farms. Digitaria abyssinica, Amaranthus spinosus, Cyperus rotundus, Pennisetum purpureum and Commelina benghalensis were the weed species found to be hosts of Maize chlorotic mottle virus. Cultivated crops such as beans (Phaseolus vulgaris}, cassava (Manihot esculenta) and simsim (Sesamum indicum) were natural hosts of MCMV. No weeds were found to be natural hosts of Sugarcane mosaic virus. However, sorghum {Sorghum bicolor) and sweet potato (Jpomoea batatas) were natural hosts of SCMV. Digitaria abyssinica (93.3%) was most susceptible to MCMV while Pennisetum purpureum (93.3%) was most susceptible to SCMV. A total of I 16,255 individual insects from 22 families were collected. Known aphid vector species of SCMV were found present. Frankliniella occidentalis (Thripidae) a known thrips vector species of MCMV was confirmed present. Among the insects collected, Rhopalosiphum padi, Lagria villosa, Rhopalosiphum maidis, Oxya hyla, Eschistus quadrator Hysteroneura setariae were carriers of MCMV under natural field conditions. Rhopalosiphum padi (Aphididae) and Rhopalosiphum maidis (Aphididae), Oxya hyla (Acrididae) and Hysteroneura setariae (Aphididae) were carriers of SCMV under natural field conditions. Screen house transmission studies under controlled conditions confirmed that Rhopalosiphum padi, Rhopalosiphum maidis, Oxya hyla, Hysteroneura setariae, Eschistus quadrator and Lagria villosa had the ability to transmit MCMV; while Rhopalosiphum padi and Rhopalosiphum maidis, Oxya hyla and Hysteroneura setariae transmitted SCMV. The studies have increased our knowledge of the farmer awareness levels of MLND, MLN causing viruses, MLN viruses’ alternate host plants and vectors driving the epidemic in Uganda. This knowledge is important in designing weed host, vector and disease management strategies for containing the spread of MLN disease in Uganda.