Napier grass stunt: a new disease associated with a 16SrXI group phytoplasma in Kenya

Abstract

Napier or Elephant grass (Pennisetum purpureum) is a native clumping grass of tropical Africa that grows 3–5 m tall and is mainly propagated from cuttings of three to four nodes in length. It is widely used as a fodder crop and is also planted for environmental protection, to stabilize soils and act as windbreaks. In Kenya, it has been used in a novel ‘push-pull’ pest management system for cereal stem borers (Khan et al., 2001). Since 2000, symptoms have been seen on Napier grass in western Kenya that include foliar yellowing, little leaves, proliferation of tillers and shortening of internodes to the extent that clumps appear severely stunted. Tests for viruses and root-infecting fungi that might cause these symptoms were all negative. Leaf samples from both yellowed and apparently healthy Napier grass were sent to Rothamsted, as were rooted plants which were grown-on under quarantine. Total DNA extracted from each leaf sample was used as a template in a nested PCR using phytoplasma ribosomal DNA primer pairs P1/P7 and then R16F2n/R16R2. A 1250-bp rDNA product was amplified from all yellowed leaves and from two of three apparently healthy leaves. All rooted plants grown under quarantine conditions had yellowed leaves and stunted growth and were phytoplasma-positive. RFLP analysis of the rDNA amplimers showed the same pattern for all samples, so four samples were chosen for cloning and sequencing. Analysis of rDNA sequences (deposited in GenBank as accession numbers AY377874-AY37787) identified these phytoplasmas as members of the 16SrXI (rice yellow dwarf) group. Strains were most similar (96%) to Bermuda grass white leaf phytoplasma (accession number Y16388). No information is available about putative insect vectors, but the potential for spread of the phytoplasma in this part of Africa, by clonal propagation of infected cuttings, is considerable.