Napier grass stunt disease in East Africa: Farmers' perspectives on disease management

Plant-pathogenic phytoplasmas found in wild grasses in East Africa could pose a serious threat to the cultivation of Napier grass, Pennisetum purpureum, the most important livestock fodder in the region. To asses this threat, leaves from plants of 33 grass species were sampled from Mbita, Bungoma, and Busia districts in western Kenya; Tarime district in northern Tanzania; and Busia and Bugiri districts in the eastern Uganda to determine which species host phytoplasmas, the identity of the phytoplasmas, and their relationship with disease symptoms. Phytoplasmas were detected using universal primers based on conserved phytoplasma-specific 16S rDNA sequences from 11 grass species collected. Sequence and phylogenetic analysis revealed the presence of Napier grass stunt-related phytoplasmas in 11 grass species, ‘Candidatus Phytoplasma cynodontis’ in three, and goosegrass white leaf phytoplasma in 2 wild grass species. This study showed that the geographical distribution, diversity of phytoplasmas, and their grass host species in East Africa is greater than antecedently thought and that typical disease symptoms, including white leaf or stunting alone, are not reliable indicators of the presence of phytoplasma. It also shows the need to identify insect vectors responsible for phytoplasma transmission from native grasses to Napier grass or other cereals present in the region.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-11-14-1173-RE

Detection, Identification, and Significance of Phytoplasmas in Wild Grasses in East Africa.

Weeds and wild plants as hosts of phytoplasmas play an important role in the epidemiology and emergence of phytoplasma diseases of economically important crops In this chapter phytoplasmas detected in weeds and wild plants, their geographic origins, symptoms, identification, and their role in natural dissemination of phytoplasmas are described

Phytoplasmas in Weeds and Wild Plants

Phytoplasmas are phytopathogenic bacteria from the class Mollicutes. Cereal diseases caused by these microorganisms contribute to serious economic losses. Infected plants change the colour of their aboveground parts and are stunted. Bacteria also damage the root system and cause plant necrosis. The ears are deformed, which leads to sterility and reduced grain yield. The disease can cause on average about 50%, and in some cases up to 90% loss in yield. Wheat, oats and barley are susceptible to phytoplasmas from the ribosomal group 16SrI. Bacteria from ribosomal groups 16SrI and 16SrXII infect maize. Phytoplasmas are transmitted to cereals by insects, mainly species representing the Cicadellidae family, e.g. Psammottetix striatus (L.), Dalbulus maidis (DeLong & Wolcott), Macrosteles laevis (Ribaut) and the Cixiidae family e.g. Reptalus panzeri (Low). In addition, many studies have suggested that wild Poaceae plants may be a reservoir of phytoplasmas in the environment. The occurrence of diverse phytoplasmas in Poaceae species worldwide indicates an ongoing threat towards cereal crops in Europe.

Phytoplasmas in Poaceae species: a threat to the most important cereal crops in Europe

Napier grass (Pennisetum purpuruem) production for zero grazing systems has been reduced to rates of up to 90% in many smallholder fields by the Napier stunt (Ns) disease caused by phytoplasma sub-group 16SrXI in western Kenya. It is hypothesized that several other wild grasses in Kenya could be infected by phytoplasmas that would otherwise pose a significant threat to Napier, other important feeds and food crops. This study therefore sought to detect and identify phytoplasma strains infecting wild grasses in western Kenya using 16S ribosomal RNA (ribonucleic acid) gene as well as identify wild grass species hosting phytoplasmas in 646 wild grass samples that were collected in October 2011 and January 2012 during a random crossectional survey conducted in Bungoma and Busia counties of western Kenya. DNA was extracted and nested polymerase reaction (nPCR) used to detect phytoplasmas. Two sub-groups of phytoplasmas were detected in eight grass species observed to grow near infected Napier fields. Only one of the two phytoplasmas reported was related to the Ns phytoplasma. There was a strong association between proportions of phytoplasma infection and the grass species collected (p = 0.001). C. dactylon, D. scalarum, B. brizantha, poverty grass and P. maximum had high proportions of infection and were abundantly distributed in western Kenya hence considered wild phytoplasma hosts. E. indica and C. ciliaris were scarcely distributed and had low infection rates. There was statistically significant difference in proportions of infection per location of survey (p = 0.001). Phytoplasma subgroups 16SrXI and 16SrXIV were the only phytoplasma genotypes distributed among wild grasses in western Kenya. Phytoplasma subgroup 16SrXIV predominantly infects only C. dactylon and B. brizantha grasses while phytoplasma subgroup 16SrXI is broad spectrum and infects a large number of wild grasses. In general, there is a diversity of wild grasses hosting phytoplasmas in western Kenya. These host grasses may be the reason for the high rates observed in the spread of Ns disease in western Kenya by acting as reservoirs for Ns phytoplasma.

https://www.longdom.org/open-access/molecular-determination-and-characterization-of-phytoplasma-16s-rrna-gene-in-selected-wild-grasses-from-western-kenya-2157-7471-1000274.pdf

Molecular Determination and Characterization of Phytoplasma 16S rRNA Gene in Selected Wild Grasses from Western Kenya

Hyparrhenia rufa is a thatching grass commonly found throughout the tropics where it also serves as valuable cattle fodder and border grass to prevent soil erosion (Skerman & Riveros, 1990). In June 2010, we observed some H. rufa …

/pdf/hyparrhenia-grass-white-leaf-disease-associated-with-a-59qdz8uukf.pdf

Hyparrhenia grass white leaf disease, associated with a 16SrXI phytoplasma, newly reported in Kenya

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Hyparrhenia grass white leaf disease, associated with a 16SrXI phytoplasma, newly reported in Kenya