Transmission of Maize Lethal Necrosis Disease Viruses and the Effect of Co-infection With Maize Chlorotic Mottle and Sugarcane Mosaic Viruses

Abstract

Maize is the most important staple food for about 90 % of the Kenyan population who depend on the crop directly or indirectly for food, employment and income generation. The outbreak of maize lethal necrosis (MLN) disease caused by of synergistic interaction between Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV) has had a negative effect on the already strained food security. The disease has spread in many parts of Central and Eastern Africa resulting in losses of up to 100%. The study evaluated the transmission of MLN viruses through seeds, vectors, mechanical and infected maize residues. Twelve commercial maize hybrid varieties grown in Nyeri and Bomet Counties were assessed for MLN incidence and severity. Cobs of each variety with percentage disease severity level of 0, 15, 30, 40, 50 and 60 were harvested and the seed were evaluated for physical symptoms of infection, germination, emergence and vigor. Data collected included proportion of shriveled, discolored and mouldy seeds, percentage germination and normal seedlings, seedling length, seedling dry weight, virus titre, cob length and grain weight. Seedlings vigor index was calculated from its length and dry weight. The effectivess of vectors in transmission of MLN viruses was determined by inoculating maize seedlings with adults and nymphs of Western flower thrip (Franklinella occidentalis), corn leaf aphids (Rhapolosiphum maidis) and Russian wheat aphid (Diuraphis noxia) carrying MLN viruses. Data collected included virus titre, plant height, number of plants with disease symptoms and severity score on modified Horsfall-Barrat scale. All the maize varieties grown by farmers had disease incidence of 47.0% to 87.8% and severity of 21.8% to 42.9%. Highest virus titre of 0.55 and 0.61 were recorded in DK8031 seedlings and H614 varieties, respectively. The highest reduction of 62.8%, 32.4%, 62.8%and 61.1% was realized in seedling emergence, plant height, maize cob length and weight of grains, respectively in H614 variety. Hybrid H614 seedlings from seeds harvested from plants with percentage MLN severity of 60,

recorded the highest reduction in seedling emergence at 62.8%. Hybrid 614 had the highest proportion of seeds that were discolored and shriveled at 33.9 and 26.5%, respectively while H513 had the highest proportion of mouldy seeds at 43.5%. Adults of R. maidis were the most efficient in transmitting SCMV. The adults and nymphs of F. occidentalis did not transmit any of the viruses. Disease severity of up to 44.4 % was achieved on plants inoculated with virus using adults of R. maidis. Inoculating H614 seedlings with a combination of MCMV and SCMV using injection had the highest reduction at 39.8%. Highest MCMV titre and disease severity of 0.2 and 21.3%, respectively were recorded in H614 plant which were planted immediately after incorporating the MLN infected maize residues. The highest reduction of plant height and maize grain yield at 33.6% and 44.8%, respectively was realized in plants established in media with freshly incorporated MLN infected maize residues. The study showed that MLN viruses were transmitted through seed resulting in plants with low germination, vigor and yield. Adult of Rhapolosiphum maidis was the most effective in transmission of SCMV. Maize varieties responded differently when inoculated with MCMV and SCMV while MCMV was spread through infected maize crop debris. For effective management of maize lethal necrosis disease, the farmers should use clean certified seeds, manage vectors and observe field sanitation through proper disposal of infected maize residues.