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Thursday, 26 February 2015

Understanding Maize Lethal Necrosis Disease

By Bob Aston
Maize Lethal Necrosis Disease (MLND) is a serious disease of maize which has adversely affected maize production in Laikipia County. A government survey conducted in Laikipia West in September 2014 indicated that the disease had destroyed 2,952 out of 35,604 hectares of maize in the sub county.
MLND is said to be caused by a mixed infection between Maize chlorotic mottle virus (MCMV, genus Machlomovirus) and potyviruses infecting maize. In Kenya and other countries, most frequently it is Sugarcane mosaic virus (SCMV) in synergism with MCMV causing MLND. 
Initial stages of MLND
MLND is mainly spread by a vector, transmitting the disease from plant to plant and field to field. The most common vectors are maize thrips, rootworms and leaf beetles. Hot spots appear to be places where maize is being grown continuously.
The disease is known to naturally affect varieties of maize resulting in chlorotic mottling of the leaves, severe stunting and necrosis, often leading to plant death. In mixed infections, early infected plants appear stunted and show a general chlorosis, leaf bleaching and necrosis.
Virus diagnosis
Identification of MLND and the viruses involved in the disease complex is generally by observation of symptoms in the field. However, because single infections of the viruses and early stages of the disease are often inconspicuous and resemble physiological disorders, specific diagnostic tests are to be applied to confirm virus presence and to adequately detect/identify the viruses in the mixed infection.
Control of MLND
The most effective control of MLND is through integration of cultural practices, control of vectors, host resistance and best farming practices.
Advanced stage of MLND
Insect vector control can normally be done by seed treatment that covers the plants at the seedlings stage and application of foliar sprays with systematic pesticides from three weeks.
Adopting good agricultural practices like use of certified seeds, avoiding use of recycled seeds, early planting and applying adequate soil amendments including manure, basal, foliar and top dressing fertilizers to boost plant vigor and hence tolerance to pests and diseases.

Enhancing crop diversification to improve soil fertility and reduce risks caused by over reliance to one crop is also an effective agricultural practice. Crop rotation with non-maize (Graminae) crop has also been shown to reduce the incidence of MLND.
Practicing cultural control like ensuring field hygiene by removing affected plants and cultivating nearby weedy areas before maize emerges to reduce potential of thrips build up is also important. It is also important to implement a closed season programme by documenting planting calendar for each area. Beyond this period it is advisable to plant an alternative crop.
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