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.
Seed Transmission of Bean Yellow Mosaic Virus in Broad Bean (Vicia faba).