Mechanisms of resistance to groundnut rosette

Rosette (caused by rosette assistor virus, groundnut rosette virus and satellite RNA) resistance in 3 groundnut genotypes (ICGV-SM 90704, ICG 12991 and JL 24) was evaluated, using a vector or mechanical transmission of the virus complex. Branches from rosette-infected plants (groundnut cv. Malimba) were grafted onto 23-day-old healthy stocks of the 3 genotypes, grown in pots in a greenhouse. Eighteen days after grafting, all the new shoots of ICG 12991 and JL 24 stocks showed severe rosette symptoms. The differences in rosette incidence recorded from the graft transmission and field observations may involve resistance to Aphis craccivora. Thus, an experiment was carried out to assess the vector performance on the 3 genotypes. Thirty days after sowing the 3 genotypes in pots in a greenhouse, young leaves were exposed to 5 viruliferous A. craccivora alatae (winged). Aphids were counted 10 days after infestation (DAI) on each plant. Exposed plants were left in a greenhouse up to 60 days after infestation to record rosette symptoms. Results indicated highly significant differences in aphid population counts between the 3 genotypes. At 10 DAI, increased numbers of aphids (alatae plus nymphs) were observed on ICGV-SM 90704 and JL 24, with an average of 93 and 96 aphids per plant, respectively. In contrast, aphid number on ICG 12991 fell from 5 to 3 per plant. There were also significant differences in disease expression at 60 DAI, since JL 24 showed 100% disease incidence, while no symptoms were noted on ICG 12991. Only mild symptoms were observed on ICGV-SM 90704