Abstract

A new strain of bean yellow mosaic virus (BYMV), a non-necrotic strain, was found in south-west Western Australia. It differs from the original necrotic strain of BYMV in that it does not kill Lupinus angustifolius (narrow-leafed lupin) plants, but causes symptoms of mottle and stunting, or dead growing points, fleshy expanded leaves, and stunting. A survey of L. angustifolius crops during September and October 1997 compared the distribution and incidence of the necrotic strain with that of the non-necrotic strain. Based on 1000 plants inspected at the edge of each crop, the necrotic strain was found in 100 of 102 crops while the non-necrotic strain was found in 64 of them. Incidences ranged from 0.3 to 56% (necrotic strain) and 0.1 to 7% (non-necrotic strain) of plants counted. Both strains were present over the whole range of the survey. Wild L. angustifolius and L. luteus (yellow lupin) populations were also inspected. The necrotic and non-necrotic strains were found in 31 and 9 of the 34 L. angustifolius populations examined, respectively. Incidences ranged from 0.1 to 28% (necrotic strain) and 0.1 to 3% (non-necrotic strain) of plants counted. BYMV was found in 9 of 11 wild L. luteus populations with incidences ranging from 0.3 to 7% of plants counted. In a separate survey in which samples of L. angustifolius crops, with necrotic symptoms suspected of being caused by Colletotrichum gloeosporioides (lupin anthracnose disease), were examined, 37 of 130 samples had typical necrotic BYMV symptoms. Samples with these necrotic symptoms also came from northern and eastern wheatbelt areas not normally associated with BYMV infection. When 8 BYMV isolates cultured by sap inoculation in Trifolium subterraneum (subterranean clover) were tested by aphid transmission to L. angustifolius plants in 1994 and again in 1997, the isolates of the 2 strains behaved the same on both occasions causing only necrotic (3 isolates) or non-necrotic (5 isolates) symptoms. Thus, despite repeated subculture by sap inoculation over a 3.5-year period, the 2 BYMV strains still remained distinct. An isolate collected from wild L. luteus in 1997 produced only non-necrotic symptoms in L. angustifolius. The non-necrotic strain caused symptoms typical of BYMV in hosts other than L. angustifolius, reacted strongly with BYMV antiserum, and failed to react with antiserum to clover yellow vein virus. In a BYMV-infected lupin crop, grain yields of individual L. angustifolius plants infected early with the non-necrotic strain were decreased by 95%. Shoot weights, seed number, and seed size were also greatly decreased. Widespread occurrence of the non-necrotic strain of BYMV is cause for concern for the lupin industry.