Potatoes… - Bayer CropScience

Life cycle of Phytophthora infestans
Sporangia
release zoospores
Movement of zoospores on
the leaf surface
Sporangium
Infected tubers
as an inoculum
source for primary
infections
Direct
germination
of sporangia
Zoospores
encyst and
germinate
Mycelial growth
Infection of tubers
via sporangia
Formation of a sporangial lawn
on the underside of the leaf.
shade (Solanum sarrachoides) and rocket
leaf (dense-thorn bitter apple Solanum
sisymbriifolium), which can then act as
host plants for inoculum.
More action against
Phytophthora necessary
Keeping Phytophthora infestans under
control will require more vigilance and
action towards prevention and eradication
than before, says Schepers. He emphasises
that prevention always begins with the
elimination of sources of early infection,
for example by covering cull piles early on,
and by removing volunteer plants. Moreover,
early foci of Phytophthora infection
must be suppressed as rapidly as possible.
In the Netherlands, a system of presenting
farmers with yellow and red cards has been
in use for several years to encourage this:
potato growers who do not have their
cull piles covered by April 15 are given a
yellow card; the same applies to growers
with many volunteer plants in their fields,
and to growers who do not control the
early foci, or do so only inadequately. If
the growers have not acted within 3 days
of receiving the yellow card, the consequences
are a red card and a fine.
A second factor that can curb the
disease is extended crop rotation. Research
has shown that oospores can survive in the
soil for 3 to 4 years. If a second crop of
potatoes is grown within this period (or if
volunteer plants are allowed to grow on
the same plot of land), early infection can
develop from oospores.
Choice of variety is a further important
factor in keeping Phytophthora in check.
Infection occurs considerably less rapidly
in highly-resistant varieties. With regard to
crop protection, the spraying strategy must
be tightened up where mixed populations
are present, in order to counter the increased
risk of infection. Among other
things, this can mean starting spraying
earlier, with shorter intervals between
applications, so that the pathogen has less
chance to develop. In addition, where there
is high disease pressure, more attention
will have to be paid to combining different
active substances (modes of action) than
is the case at present. Schepers draws
attention to the critical issue that active
substances increasingly have a limited
number of applications. “With the more
aggressive Phytophthora being present,
greater knowledge is needed in order to
develop a watertight strategy.” ■
Written by: Han Hammink
20 COURIER 1/08