Effects of groundnut sowing date and plant spacing on rosette virus disease in Malawi.
In a series of field trials at Chitedze, Malawi, groundnuts were sown at spacings of 30 cm, 7.5 cm and 2.5 cm within rows 90 cm apart. Contrasting sowing dates were approximately 1 December and 1 January. Immigration of alatae of Aphis craccivora Koch, monitored on observation plots, commenced 5-6 weeks after the emergence of early-sown groundnuts. Numbers of immigrant alatae were significantly greater on late-sown or open-spaced than on early-sown or close-spaced groundnuts, and were correlated with numbers of randomly distributed primary rosette virus infections. During observations, more alatae landed on 56-60-day-old plants than on 85-112-day-old plants. Similar numbers landed on 11-26-day-old plants at 30-cm and 2.5-cm spacing, but significantly more aphids settled on 30-cm spacing. The incubation period of rosette virus was consistently longer in 2.5-cm than in 30-cm spacing, and increased with plant age at inoculation. The virus was transmitted from a low proportion of infected plants before the appearance of rosette symptoms, and from a high proportion of plants showing symptoms. The rate of spread of virus transmissions from inoculated sources within the crop varied significantly with spacing as 30 cm > 7.5 cm > 2.5 cm. Secondary spread of rosette virus varied, approximately, with the numbers of aphids per infected plant.<new para>ADDITIONAL ABSTRACT:<new para>In a series of field trials in 1964-7 at Chitedze, Malawi, groundnuts were sown at spacings of 30, 7.5 or 2.5 cm within rows 90 cm apart on approximately 1 Dec. or 1 Jan. Details are given on transmission and spread of rosette virus by Aphis craccivora. Early-sown close-spaced plants had least infection.