Effect of mulching with cereal straw and row spacing on spread of bean yellow mosaic potyvirus into narrow‐leafed lupins (Lupinus angustifolius)

Abstract

SummaryTwo field experiments examined the effect of straw spread on the soil surface on the incidence of bean yellow mosaic potyvirus (BYMV) in plots of narrow‐leafed lupin (Lupinus angustifolius) sown at narrow (17.5 cm) vs wide (35 cm) row spacing and low (25–30 kg/ha) vs medium (50–60 kg/ha) seeding rates. Virus ingress was by vector aphids flying from adjacent pastures dominated by subterranean clover. In Expt 1, in which BYMV infection was extensive, straw greatly decreased the rate and amount of virus spread regardless of row spacing or plant density, decreasing infection more than 70% by the final assessment date. This effect of straw was attributed to decreased landing rates of incoming vector alates. In the plots without added straw, narrow row spacing decreased BYMV % infection by 38% by the last assessment date. Sowing at the medium seeding rate also decreased infection. The effect of wide row spacing seemed due to delayed canopy closure between rows which is likely to have increased the landing of aphids while the effect of medium seeding rate was attributed partly to the dilution effect of greater plant numbers and partly to the effects of partial canopy development in decreasing landing rates. In Expt 2, in which the incidence of BYMV infection was low, added straw again decreased BYMV spread, but by only 25–27% at final assessment; there were no effects of row spacing or seeding rate. In both experiments, an additional “reference” treatment was included which had a high (90–100 kg/ha) seeding rate, narrow rows and no straw. The dense canopy it developed also decreased BYMV incidence but less than in the plots with added straw in Expt 1.In Expt 1, adding straw and the resulting decrease in plants killed by BYMV, were associated with an overall increase in lupin grain yield of 20%. The greater plant densities resulting from the medium seeding rate also increased grain yield but row spacing did not affect it significantly.These results indicate that retaining stubble on the soil surface at seeding will assist in management of BYMV infection in lupin crops but that wide row spacing in the absence of retained stubble is undesirable.