Peanut Clump virus transmission by Polymyxa graminis under controlled conditions

More than fifteen soil-borne viruses belonging to the Beny-, Bymo-, Furo- or Pecluvirus, causing diseases on cereals and groundnut, are transmitted by the soil-borne protist root endoparasite Polymyxo graminis. Five special forms are distinguished within this species on the basis of their specific ecological and molecular characteristics, but the specificity of the transmission of the viruses by these forms has been Little investigated. In order to analyse the virus-vector interaction, the transmission assay of the Peanut clump virus by P. graminis f.sp. tropicalis has been conducted under controlled conditions. The major difficulty to be overcome was to combine high levels of PCV and vector infection in the same living plant. This was achieved by using cuttings of PCV-infected sugarcane (variety CP-89327) originating from Burkina Faso showing the typical red leaf mottle symptom as a source for the virus. A culture of P. graminis f.sp. tropicalis isolated from a PCV-infested soil from Niger was used as the vector. Systemic PCV infection in sugarcane at 25-30 degrees C allows plants with new young roots infected by the virus to be produced by placing cuttings of PCV-infected sugarcane in Hoagland nutrient solution for 21 days. These plants were inoculated with aviruliferous zoospores of P. graminis f.sp. tropicalis produced on infected pearl millet roots and maintained in an automatic immersion system. After 21 days' incubation at 25-30 degrees C with a 12 hour photoperiod, zoosporangia were observed in the roots of sugar cane. The zoospores released from these roots were used to infect healthy young pearl millet plants. Twenty days postinoculation, PCV was detected by RT-PCR in roots of inoculated plants. The vector was clearly identified in the roots by microscopy. This result reveals that a PCV isolate from red leaf mottle sugar cane is transmissible by a P. graminis f.sp. tropicalis isolate from a peanut clump infested field. This method should be helpful in the analysis of the specificity of pecluvirus transmission by P. graminis f.sp. tropicalis and P. graminis f.sp. subtropicalis.