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 knowlege,
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
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.
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five agroecological zones of Uganda was conducted aimed at assessing farmers’
I
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
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 Agro-ecological zone had highest incidence and
severity during the three seasons of this study. Maize chlorotic mottle virus (MCMV) and
Sugarcane mosaic virus (SCMV) were 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,
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square test revealed that the level of MLN awareness was
the five agroecological zones (x2 = 66.428, df = 4, P
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
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.