Rosette and Early Leaf Spot Diseases: A Review of Research Progress, 1984/85

Observations made over 2 years on the patterns of spread of groundnut rosette virus (GRV) within fields suggested that primary infections originated from a migration of infective vectors that occurred within days of plant emergence, and that significant radial spread was from these point sources of infection only. Such conditions were simulated in the establishment of a GRV resistance screening nursery in which a disease incidence of about 90% was induced. Preliminary studies of the inheritance of resistance to GRV confirmed that it is governed by recessive genes and an approximate ratio of 15 susceptible plants to 1 resistant plant in progenies of resistant x susceptible crosses. Grafting experiments indicated that the GRV-resistant cultivar RG 1 is immune to vector inoculation of GRV and also demonstrated symptomless systemic infection. Aphis craccivora Koch vector of GRV was trapped in low numbers throughout the dry season, suggesting the presence of local resident populations. Dry-season aphid activity was greater in central than in southern Malawi. At one site in October, GRV occurred before the onset of the rains. In preliminary studies of early leaf spot (Cercospora arachidicola), the number of leaflets retained were counted at 70 and 88 days after sowing on a range of genotypes. Highest yielding line of sequentially branched types retained more leaflets than control varieties, but so did a small number of poorer-yielding entries. Leaf retention among alternately branched types was apparently more uniform