CYDV-RPV was purified from oat plants (Avena byzantina K. Koch cv. Coast black) inoculated 7 to 8 weeks previously with viruliferous R. padi as described [10] . Infection was determined by yellowing and dwarfing symptoms. Tissue harvested was divided into 200-300 g batches, chopped into 2.5 cm pieces and frozen at 280uC until used. Virus was purified using a modified version of the protocol of Hammond et al. [89] . Tissue was homogenized using 0.1 M phosphate (K 2 HPO 4 ) buffer (pH adjusted to 7 using 0.1 M KH 2 PO 4 ) at 2.5 ml g 21 tissue. Tissue was homogenized with phosphate buffer containing 1% cellulase with and without 0.01 M EDTA and 0.5% sodium sulfite together or individually. Sucrose gradients were fractionated using a density-gradient fractionator (Teledyne-ISCO) at sensitivity 0.5 and chart speed set at 60 cm/h. Two milliliter fractions were collected along the entire gradient for each gradient. The virus fractions were collected as 4 ml gradient fractions and were concentrated by centrifugation for 4 h at 113,6136g in a type 70Ti rotor (Beckman Coulter). The remaining fractions were stored at 280uC until needed. Supernatant was discarded and the pellet was resuspended overnight in 0.01 M phosphate buffer, pH 7. Purified virus was equally divided among several tubes and stored at 280uC. As a control, 500 g of healthy tissue was homogenized in phosphate buffer containing 1% cellulase and purified following the above protocol.