Maize (Zea mays L.) is ranked as the third most important food crop by production globally, after rice and wheat. Several biotic (diseases, pests) and abiotic (unfavorable climatic conditions) factors affects its production. Maize lethal necrosis (MLN) disease outbreak within East Africa threatens production of maize. Information on interactions of viruses causing MLN with plant parasitic nematodes is lacking. This study was carried out to determine i) the effect interaction of of plant parasitic nematodes with viruses causing MLN on disease development in maize fields and ii) the effect of lesion nematodes (Pratylenchus spp.) on MLN disease development in the greenhouse. For the field study, four counties in Kenya were visited, farms selected at random, MLN scored and both maize leaf and soil samples collected and analyzed for presence of viruses causing MLN disease and for parasitic nematodes. Snowball sampling or chain-referral sampling technique was used to sample MLN infected farms across the selected regions. Variance analysis was used to measure significant differences (P< 0.05) in MLN disease incidence and severity due to interaction between viruses and nematode populations. In the greenhouse study, two maize varieties, were used H614D and Emph 1101. Variety H614D is known to be susceptible to both MLN and Pratylenchus spp. whereas maize variety Emph 1101 is susceptible to MLN but resistant to Pratylenchus nematodes. The two maize varieties were subjected to three distinct treatments: single inoculation with MCMV and SCMV; combined MCMV + SCMV inoculation; the third treatment was the addition of Pratylenchus nematodes to the previous two treatments. Disease severity and incidence were recorded weekly over a period of two months. xiii Survey results indicated no significant effect of combined infestation of parasitic nematodes (Pratylenchus spp., Tylenchus spp., Meloidogyne spp. and Helicotylenchus spp.,) on MLN disease severity in the field. However, there was significant effect of Pratylenchus to MLN severity in the greenhouse experiment. The development of MLN disease in maize varieties Emph 1101 and H614D infected with Pratylenchus spp. nematodes was studied under a greenhouse experiment. MLN disease severity was higher in H614D than in Emph 1101. Plants inoculated with MLN+Pratylenchusrecorded a significant difference across the two varieties on area under disease progress curve (AUDPC). There is need for nematodes management even though the field experiment indicated no significant effect of parasitic nematodes on MLN disease developement. There is also need for an open field study to evaluate the effect of Pratylenchus spp. on the development of MLN disease.

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