Agriculture Reference
In-Depth Information
when the vine is stressed for water. Infected roots swell and become club-like, with
many yellow galls (figure 5.12). Rootlet growth stops, and the uptake of water and
nutrients is impaired. Progressively, there is a decline in vigor, stunted growth, and
finally vine death after several years.
Because phylloxera starts from a point source and spreads outward, the organ-
isms are most easily found on mildly affected vines at the edge of an infestation,
usually in the top 30 cm of soil. Although remote sensing can detect weakened
areas, these must be checked by the use of traps and soil inspection in summer
(phylloxera are dormant in winter and shelter under bark on the roots). The sever-
ity and rate of spread of an infestation depend on the virulence of the phyllox-
era strain; there are 83 known genotypes with different degrees of lethality. In
Victoria, Australia, for example, where phylloxera has existed since 1877, the larg-
est range of genotypes occurs in the older infected areas. Less virulent genotypes
may be present for 30+ years without obvious symptoms, even though the organ-
isms are numerous on vine roots.
Phylloxera is a greater problem in poorly drained, heavy clay soils than in
sandy soils. Phylloxera probably suffers from desiccation in sandy soils. For exam-
ple, in the Nagambie Lakes subregion of central Victoria, a small plot of Shiraz
vines survived phylloxera at the end of the 19th century because they were grow-
ing on a sandy ridge in an alluvial plain of clay loam soils (figure 5.13).
Figure 5.12
Phylloxera adults and eggs infesting a
Vitis vinifera
root. Note the swelling and
yellow galls. (White, 2003. Copyright © The State of Victoria, Department of Primary
Industries. Reproduced by permission of the Department of Primary Industries and therefore
not an official copy. Photo by Greg Buchanan.) See color insert.
