The significance of Napier Grass Stunt Phytoplasma and its transmission to cereals and sugarcane

Production of Napier grass, Pennisetum purpureum, the most important forage crop in Africa, is severely constrained by Napier grass stunt (NGS) disease. Wild grasses have been identified as alternative hosts for NGS, but there is no record regarding the ability of this phytoplasma to infect cultivated food crops. To assess this threat, six crop species were placed around phytoplasma-infected Napier grass with the vector Maiestas banda (Hemiptera: Cicadellidae) in transmission cages for a period of 30days. The crop plants were observed for 3months after removal from the cages for disease development. Polymerase chain reaction based on the conserved 16S gene, primed by P1/P6-NapF/NapR nested primer sets, was used to diagnose phytoplasma in test plants, before and after transmission experiments. Pennisetum purpureum showed the highest infection level (81.3%), followed by Saccharum officinarum (56.3%), Eleusine coracana (50%), Sorghum bicolor (43.8%), Oryza sativa (31.3%) and Zea mays (18.8%). All the test plants remained symptomless except sugarcane that exhibited mild to moderate symptoms comprising yellow leaves and bright white or yellow midribs. Napier grass, however, was stunted with small yellow leaves. The study showed that food crops could be infected, suggesting their possible role as sources of inoculum for NGS and their contribution in the spread of the disease in regions where these crops are cultivated. This implies an additional disease cycle could occur for NGS disease, highlighting the challenges to development and implementation of management strategies for the disease.