Wine Science: Principles and Applications, Third Edition - Vinum Vine

198 4. Vineyard Practice
Because grapevine roots do not form natural grafts,
vine-to-vine transfer does not occur. Although transferable
to other plants, the disease is limited to grapevines
under fi eld conditions. This may result from the limited
host range of its nematode vectors.
The impact of infection varies widely, depending on
cultivar tolerance and environmental conditions. Tolerant
cultivars are little affected by infection, whereas susceptible
varieties show progressive decline. Yield losses
may be up to 80%, with the fruit of poor quality.
Infected vines have a shortened life span, increased sensitivity
to environmental stress, and reduced grafting
and rooting potential. Three distinctive syndromes have
been recognized, based on particular strain–cultivar
combinations. These are malformation, yellow mosaic,
and vein-banding.
Fan-shaped leaf malformation is the most distinctive,
but not only, foliage expression. Chlorotic speckling and
a leathery texture commonly accompany leaf distortion.
Shoots may be misshapen, showing fasciation (stem
fl attening), a zigzag pattern at the nodes, atypically variable
internode lengths, double nodes, and other aberrations.
Fruit set is poor and bunches are reduced in
size. The yellow mosaic (chromatic) syndrome develops
as a strikingly bright-yellow mottling of the leaves,
tendrils, shoots, and infl orescences in the early spring.
Discoloration can vary from isolated chlorotic spots to
uniform yellowing. The third expression, veinbanding,
develops as a speckled yellowing on mature leaves, bordering
the main veins in mid- to late-summer. In both
discoloration syndromes, leaf shape is normal, but fruit
set is poor, with many shot berries (“hen and chickens”
appearance). Fanleaf degeneration also shows a characteristic
intracellular development of trabeculae. These
appear as strands of cell-wall material spanning the
lumen of xylem vessels.
Where both vector and virus are established, planting
vines grafted on rootstock resistant to the virus is usually
required (tolerance to the vector can still permit infection).
Fumigation of the soil with nematicides can, to
varying degrees, reduce but not eradicate nematode populations.
Allowing the land to lie fallow can be useful in
reducing nematode populations, but the strategy often
requires 6–10 years. The long requisite fallow period
probably results from nematode survival on undislodged
roots. These occasionally remain viable for up to 6 years
following vine uprooting.
Leafroll Leafroll is a widespread virus-like disease
associated with one or more of up to eight or nine grapevine
leafroll-associated viruses (GLRaVs) (Boscia et al.,
1995). GLRaV-1 and GLRaV-3 are the most widespread
and economically signifi cant. They are all members of
the closterovirus group. Two vitiviruses (GVA and GVB)
have also been associated with leafroll, although they are
more commonly associated with Kober stem grooving
and corky bark, respectively. Symptomatic expression
varies considerably, but it does not generally lead to vine
degeneration. Many scion and rootstock cultivars are
symptomless carriers of the infectious agent(s). As with
fanleaf degeneration, the pathogens probably originated
in Europe or the Near East; feral North American grapevines
are not infected.
The spread of the causal agents depends primarily
on graft transmission. Nevertheless, insect vectors may
occasionally be involved. For example, GLRaV-1, -3, -5
can be transmitted by mealybugs or soft scale insects
(Sforza et al., 2003). Several species of mealybugs have
been reported to transfer grapevine viruses A (GVA)
and B (GVB). Nevertheless, healthy and infected vines
often coexist side by side without transmission. That
the mealybug, Planococcus fi cus, has been found on
the roots of grapevines (Walton and Pringle, 2004) is of
concern since most control measures have been aimed
at aboveground insects. It is a key pest in South Africa,
the Mediterranean, Argentina, and now occurs in some
regions of California.
The disease complex derives its name from a marked
down-rolling of the basal leaves that occurs late in the
season in some varieties. The interveinal areas of leaves
also may turn pale yellow or deep red, depending on the
cultivar, whereas the main veins remain distinctly green.
Infected vines can occasionally be detected by their
retention of leaves much longer than adjacent healthy
vines. In addition, leaf blades may fall, leaving petioles
still attached to the cane. Whole vines, as well as individual
shoots and leaves are dwarfed in comparison to
healthy plants. Fruit production may be depressed by
up to 40%, and berries may show delayed ripening,
reduced sugar levels, and altered pigmentation.
Control is dependent on the destruction of infected
vines and replacement by disease-free stock. Disease-free
nursery stock may be generated by thermotherapy or
by micropropagation. Because the identity of all causal
agents still remains unestablished, confi rmation of elimination
of the infectious agent(s) is performed by grafting
to sensitive cultivars (indexing).
Yellow Speckle Yellow speckle is a widespread but
relatively minor viroid disease of grapevines. Other
viroids occur in grapevines, but their economic signifi -
cance and relationship to recognized grapevine diseases
are unclear. Symptoms of infection by grapevine yellow
speckle viroids (GYSVd1 and GYSVd2) are often shortlived
and develop only under special climatic conditions.
Foliar symptoms generally develop at the end of the
summer and consist of leaf spotting. When suffi ciently
marked, the scattered chlorotic spots may resemble the