- A survey of major bean (Phaseolus vulgaris) breeding programs and
seed producing areas in Idaho, Washington, and Oregon was conducted to
determine the occurrence of cucumber mosaic virus (CMV), recently reported
to be seedborne in beans. Some 259 plants, sampled from breeding
lines of diverse germplasm, were found by enzyme-linked immunosorbent
assay (ELISA), bioassay, or gel double-diffusion serology to be
free of seedborne CMV. CMV was, however, detected in 1 of 32 Plant
Introduction (PI) bean accessions tested by the same methods.
Seed-transmission rates of selected isolates (isolates will hereafter
refer to those reportedly seedborne and one not reported to be
seedborne in beans) were determined under field and greenhouse conditions.
An isolate from PI No. 271998 (originating in Spain) was found
to be seed-transmitted in up to 49% of the progeny of infected 'Top-crop'
bean plants. Seed-transmission depended on early inoculation of
plants, i.e., less than four weeks after planting. Rates of transmission
under our conditions differed from values reported by others.
Most notably, seed-transmission of CMV-F, reported at a rate of 54% in
seeds of 'Toperop' (61), was found in our studies to be 3.3% in this
cultivar. CMV was distributed uniformly throughout seedlings arising
from infected seed but relative virus concentration was most consistently
higher near the growing point. Symptoms were mild or masked in
plants arising from infected seed.
Cucumber (Cucumis sativus 'Lemon') and tobacco (Nicotiana tabacum
'Xanthi') plants infected with CMV were detected by gel serology using
standard procedures. To obtain gel serological reactions with infected
bean or cowpea tissue, a 0.5 M citrate buffer, pH 6.5 with 0.1%
sodium thioglycollate and 0.1% Triton X-100, was employed. CMV was
readily detected in infected plant tissues by ELISA. Purified CMV was
detected by gel serology at concentrations of 0.05-0.1 mg/ml and by
ELISA at concentrations of 6 ng/ml.
Isolates were compared for differences in host range, infectivity,
serological relatedness, electrophoretic mobility of protein and nucleic
acid species, and presence of minor or satellite RNA components.
No differences among isolates were found in any of the above properties,
except that CMV-Pg seemed to produce lower yields in beans and tobacco
and that its major RNA components appeared to fragment into unique electrophoretic
species. Otherwise, the greater seed-transmission rate of
CMV-Pg was the principal distinction among these isolates.
The nucleic acid components of CMV-Pg (seedborne) and CMV-Le (nonseedborne)
were separated by sucrose density gradient centrifugation.
Pseudorecombinants were prepared among RNA components of these isolates
for continuing studies of the genomic determinants of CMV seed-transmissibility