For healthy potatoes - Bayer CropScience

Birds-eye view of a trials field: the untreated plots can be clearly
distinguished from the treated ones.
A glance at the untreated plots shows the devastating effect of
Phytophthora infestans on potato plants.
After infection, the further development
of the fungus is predominantly determined
by climatic conditions. Sporangium formation
requires high humidity, and has a temperature
optimum in the range 18°-23°C.
The sporangia are dispersed by wind. If a
sporangium lands on a potato plant, 6-12
zoospores can be released. This process
can only occur in water droplets on the
plant surface. The zoospores quickly germinate
to form a germ-tube, which penetrates
into the plant’s tissues. Under optimal
conditions, a successful infection can
usually occur within 2 hours. The infection
process can take place on either side of the
leaf.
The incubation period, i.e. the period
between penetration of the host and the
first appearance of lesions, lasts 2-3 days.
The infection cycle continues when the
pathogen sporulates. Sporangia are
released into dew or raindrops and can
enter the upper soil layers when this water
runs off the leaf. Here, they release their
zoospores, which are able to penetrate into
very young tubers through the epidermis.
In older tubers, in contrast, the pathogen
can only penetrate into the tuber’s starchy
interior through open lenticels, via stomata,
or through the eye. The pathogen has a
further opportunity to infect tubers during
the harvesting process, during which
tubers come into contact with infected
foliage or with earth contaminated with
sporangia. Inoculum in the form of sporangia
can remain viable in soil for some time
(ca. 30 days). Even small injuries to tubers
are sufficient to allow infection.
Symptoms
Because potato leaves tend to develop
various brown spots during the vegetative
period, it is important that advisors and
farmers are able to identify late blight
symptoms clearly.
Primary infections are seen on the stem
and the petioles (leaf-stalks). These plant
parts become brown, and eventually nearly
black, and the associated leaves die off.
Surface growth of the fungus is rarely seen
on stems.
Initial symptoms of leaf infection are
small, yellowish to dark-green spots.
Infection usually first occurs on leaf margins
and leaf tips, because this is where
water droplets are retained longest. Under
favourable environmental conditions, the
spots quickly enlarge and become darkbrown
to black. A whitish-grey zone of
downy fungal growth can be seen on the
underside of the leaf at the border between
infected and healthy tissue. This is particularly
obvious to the eye during periods of
high air humidity, in the early morning
(dew period) or after rainfall. The white
zone is the unmistakable sign of late blight
infection.
Infected tubers have more-or-less large,
irregular and slightly sunken, blue-grey
spots on their surfaces. Below the areas
showing these symptoms, large parts of the
starchy tissues are discoloured rustybrown;
there is no sharp distinction
between the rusty-brown areas and healthy
tuber tissue.
12 COURIER 2/06