The Pathosystem Of Napier Stunting Disease In Western Kenya

ABSTRACT

Napier stunting (Ns) disease, caused by Rice Yellow Dwarf (RYD) phytoplasma, is a big threat to Napier grass production and food security in Kenya. The disease destroys fodder completely, with a devastating impact on livestock farmers. To control the disease, this study described the four components of Ns-disease pathosystem; phytolasma itself as the causative pathogen, the host plant susceptible to phytoplasma, the insect as a competent vector, and the alternative sources of phytoplasma inoculum in western Kenya. The study also developed quicker, more economic and robust methods for diagnosis of Ns-disease. The vector was identified through laboratory transmission experiments with field collected insects, while the alternative host grasses were discovered by molecular detection and characterization of natural phytoplasmas in symptomatic grasses. The local Napier grass germplasm characteristics were studied for genetic diversity and Ns-disease response using Amplified Fragment Length Polymorphism (AFLP) technique and laboratory transmission experiments. Molecular diagnosis of Phytoplasma in test plants and insects was performed by nested polymerase chain reaction (PCR) and loop mediated isothermal amplification of DNA (LAMP), based on phytoplasma 16S and ITS gene sequences. The study discovered that Ns-disease is transmitted by insect vector Maiestas (=Recilia) banda (Homoptera: Cicadellidae). The vector showed feeding preference to closely related pennisetum species: Napier grass and Pearl Millet. The RYD phytoplasma was transmissible to Cereals under laboratory conditions. It was also discovered in wild Thatching grass Hyparrhenia rufa, where it is pathogenic. A closely related phytoplasm; Bermuda grass white leaf (BGWL), was also discovered in pasture grass Cynodon dactylon. The phytoplasma therefore circulates between Napier grass and other wild grasses corresponding to a polycyclic epidemic. AFLP analysis divided the local Napier grass germplsm into 4 genetically distinct groups; however, the quality of this germplasm as source of resistance to Ns-disease was poor, and none of the varieties showed resistance to phytoplasma infection, which explains the high prevalence of Nsdisease in western Kenya. Based on Ns-disease pathosystem, this study recommends the following disease control strategies: elimination of infected plants (Napier grass, Bermuda grass and Thatching grass), certification of planting material and control of the insect vector M. banda. There is also need to extensify resistance screening to a much wider germplasm, both wild and improved varieties.