Page 42 - Grapevine NovDec 2020
P. 42
Around The Vineyard
However, symptoms vary depending on the grape transmit the GRBV in greenhouse and laboratory
variety, rootstock, and climatic conditions. At conditions. Although, the three cornered alfalfa
the moment, two different clades of GRBV have hopper has been found in vineyard blocks where
been reported but no differences in their biolo- red blotch disease has spread, transmission exper-
gy or effect on symptoms in the vineyards have iments in the field have not been completed to
been observed so far. Just as seen with leafroll, date. It is interesting that grapevine is not the
the symptom expression of GRBV infected vines is preferred host for Spissistilus festinus that prefers
affected by climatic conditions and the author has to feed on legumes, grasses, and shrubs. While
noted differences in the effect on sugar reduction research continues to determine if other vectors
in sunnier and warmer areas (i.e., California coast- are capable of transmitting GRBV it is clear that the
al areas with more fog and lower sunshine yield rapid expansion of this virus in vineyards was due
fruit with lower sugar concentration than the same to propagation and grafting of cuttings from infect-
grape varieties grown inland with more sun expo- ed vines. This also explains the arrival of GRBV to
sure). many countries in Asia, Europe, and South America
where GRBV had not been previously reported.
Transmission and Spread of the Viruses In summary, both, GLRaVs and GRBV are graft
transmissible and predominantly propagated by
Ampeloviruses (GLRaV-1, -3 and -4) are trans- producing cuttings of infected rootstock and scion
mitted by sap-sucking insects (mealybugs and material.
soft scale insects) in a non-specific manner. This
means, different mealybug and soft scale insect Diagnosis and Status of
species can transmit any leafroll virus. Research Foundation Plant Material
has shown that the citrus (Planococcus citri),
grape (Pseudococcus maritimus), long-tailed The distribution and concentration (titer) of lea-
(Pseudococcus longispinus), obscure (Pseudococcus froll and red blotch viruses is different in infected
viburni) and vine (Planococcus ficus) mealybugs plant material. While leafroll detection appears
as well as the soft scale insects Pulvinaria vitis to be seasonal (best detected late in the grow-
and Ceroplastes rusci are able to transmit GLRaVs. ing season), detection of red blotch virus can be
Mealybugs and soft scale insects feed on the vine’s performed any time of the year. Further, work
sap by inserting their sucking mouthparts into the performed in my lab showed that red blotch virus
plant’s vascular system (phloem). The honeydew can be detected in high titers in any part of the
excreted during the feeding process attracts ants vine. The work showed that red blotch virus can
that nurse and aid mealybugs to be transported to be detected in any tissue tested, new or mature
different positions of the vine or a different vine in leaves, petioles, green or lignified canes, as well
the row. as cordons and trunks. In contrast, leafroll viruses
are generally found in low concentrations and are
Mealybugs may be difficult to observe as they can best detected in mature leaves, canes, cordon, and
hide beneath the bark. In these cases, ant activity trunk. If a vine has been infected through cuttings,
and the growth of a black fungus (sooty mold) are the older the plant material is, the easier it is to
good indicators of the presence of mealybug vec- detect GLRaVs.
tors in the vineyard. No insects able to transmit
GLRaV-2 or GLRaV -7 have been reported to date Keeping both leafroll and red blotch viruses out
and their propagation (just like all other GLRaVs) is of the productive vineyards relies on clean plant-
performed by humans who produce and distribute ing stock programs. Sadly, a few years ago the
cuttings from infected vines. University of California at Davis Foundation Plant
Services (FPS) scientists announced the finding of a
Work by researchers at Cornell University and few vines infected with GRBV in the Russell Ranch
the University of California reported that the three foundation block. The block was planted with vines
cornered alfalfa hopper (Spissistilus festinus) can produced with a tissue culture technique that is
Page 40 The Grapevine • November - December 2020 877-892-5332
Grapevine Main Pages G111220_Layout 1-1 .indd 40 10/27/20 1:37 PM
