EFFECTS OF SOWING SEED WITH DIFFERENT LEVELS OF INFECTION, PLANT-DENSITY AND THE GROWTH STAGE AT WHICH PLANTS FIRST DEVELOP SYMPTOMS ON CUCUMBER MOSAIC-VIRUS INFECTION OF NARROW-LEAFED LUPINS (LUPINUS-ANGUSTIFOLIUS)

During 1990-92, narrow-leafed lupins (Lupinus angustifolius) cv. Gungurru, were grown in six field experiments in which plots were sown with healthy seed or seed that carried 0.5-5% infection with cucumber mosaic virus (CMV). Seed-infected plants were the primary source for subsequent virus spread by aphids. The rate of CMV spread and the extent of infection in plots depended on the level of infection in the seed sown, the proportion of infected seeds that successfully developed into established plants and the time of arrival and number of aphid vectors. In general, higher seed infection levels, better establishment of seed-infected plants and early aphid arrival favoured greater virus spread, yield loss and infection in the harvested seed. In 1991, in two experiments, late arrival of aphids resulted in minimal CMV spread, no effect on yield and much less infection in the harvested seed than in the seed sown. In the other four experiments, significant yield losses (25-42%) always resulted when seed with 5% or 3% infection was sown. Sowing 1% infected seed resulted in significantly decreased yields in two experiments, while 0.75% and 0.5% infected seed caused significant losses in one experiment (16-19% losses). Yield decreases were due to both fewer seeds being formed and decreased seed size. Levels of infection in harvested seed did not exceed 15%. In a further field experiment in 1991, individual plants that developed CMV symptoms before initiation of flowering or during flowering had significantly smaller yields than those that developed symptoms after flowering had finished. In four field experiments during 1990-91, 10-15% CMV-infected lupin seed was sown at rates of 20-150 kg ha(-1) to generate a range of plant densities. The numbers of seed-infected plants were recorded at different times. The extent of loss of seed-infected plants due to shading by neighbouring healthy plants depended on plant density and the time when canopy closure developed. Where no canopy formed there was little or no loss of seed-infected plants, while early canopy formation at high plant density was most effective in removing them. Current-season spread was also decreased by early canopy formation and high plant density, but this effect was smaller when aphids arrived early. This work supports recommendations for management of CMV infection in lupins by sowing seed with low infection (<0.5%) to minimize infection sources, and sowing early at high seeding rates to remove seed-infected plants through improved canopy formation.