Grapevine leafroll-associated virus 3 (GLRaV-3) is the
most serious virus in New Zealand and South African
vineyards. Its negative influence on berry development is
reflected on wine quality, thus making GLRaV-3 control
a priority. In red berry cultivars, changes in leaf colour
could be useful for the visual identification of GLRaV-3-infected
vines with a view to roguing (removing) such vines.
We tested the efficacy of visual diagnosis as a potentially
cost-effective alternative option to the enzyme-linked immunosorbent
assay (ELISA) that is usually used for this
purpose. All the vines, or a subsection of vines, in multiple
vineyards in New Zealand or South Africa where annual
roguing was being performed, were evaluated with the two
methods. Of the 114,782 vines assessed visually for symptoms
and tested by ELISA, the two methods were in agreement
for 114,701 (99.9%) vines, with only 81 vines showing
differing results. In 11 of the 44 annual vineyard analyses,
visual detection of symptoms was perfectly correlated with
ELISA results (sensitivity 100%). The specificity of visual
symptom identification compared with ELISA was higher
than 99.7% in 43 of the 44 annual vineyard analyses.
Symptoms as a predictor of negative ELISA proved to be
above 97.5% in all 44 annual vineyard analyses but as a
positive predictor, was 100% in 10 of 19 annual vineyard analyses where this could be determined. We conclude
that for the red-berried cultivars in this study, visual assessment
of foliar symptoms should be adopted as a costeffective
alternative to ELISA during implementation of
roguing for GLRaV-3 control.
This work formed part of the New Zealand Grape
and Wine Research programme.