Disease control
Inputs/Production
More on take-all of wheat
CATHY DE VILLIERS, ARC-SMALL GRAINS
INSTITUTE, AN INSTITUTE OF THE ARC FIELD
CROPS DIVISION, BETHLEHEM
Root diseases are typical of diseases that
occur under irrigation practices – or areas with
high rainfall and low temperatures. One of
the most prevalent root diseases on wheat is
take-all (
Photo 1
) and it is caused by the fungus
Gaeumannomyces graminis
var.
Tritici
or
Ggt
.
In Afrikaans the common term
vrotpootjie
has
been adopted as a name for this disease.
The fungus also causes disease on triticale,
barley and rye in the wheat production areas
of the Eastern Free State and the Western
Cape, as well as in the cooler irrigation areas
in Vaalharts, Sandvet, Riet River and the
Orange River.
In the Western Cape, the fungus survives on
grasses and volunteer wheat. Crop residues
are important for the survival of take-all,
since it will be the source of infection for the
following crop season. Take-all is the result
of the
Ggt
-wheat-environment interaction
and occurs only when favourable conditions
of these three factors of plant, pathogen and
environment overlap.
Infection starts when the roots of the wheat
plant come in contact with the fungus. It
spreads between and within the roots by
means of dark runner hyphae. Once inside the
roots, the phloem is destroyed, cutting off the
supply of water and nutrients to the plants.
These are the symptoms usually used in
assessing diseased plants. The plants will
be shorter in length and lighter in colour,
compared to healthy plants. Typical bleached
ears will develop during the flowering stage in
patches in the field and ears will die before the
grains are fully developed.
As such, yield losses can be expected,
depending on the amount of wheat infected.
The best way to determine if it is take-all, is
to pull out the plants to examine the stems
and roots. Infected plants will be easy to pull
out. Brown to black discolouration of roots,
crowns and lower internodes of the plant will
be visible. The roots are also brittle, thickened
and break easily (
Photo 2
).
The nitrogen and magnesium content of the
soil is of great importance in the development
of take-all in plants, since low levels of the
abovementioned can lead to an increase of
infected plants. Phosphorus and potassium
promote secondary root development and if
these new roots grow vigorously, the plant will
sustain less serious damage from take-all.
One of the main cultivation practices to control
the fungus is crop rotation. The life cycle
of the fungus is broken by cultivating a non-
host crop, and the amount of inoculum from
the previous crop is reduced and fewer or no
infection occurs in the following year. A one
year fallow period could also be practiced to
reduce the amount of inoculum in the field.
The disease intensity in the following wheat
season will be determined by the amount
of decomposition of the stubble from the
previous year. Crop rotation with soybeans
and lucern may indirectly increase the disease
occurrence, since it reduces the manganese
levels in the soil that promotes take-all. Oats
is one of the crops that are recommended to
reduce take-all.
Take-all will occur in areas that are over-irrigated
and/or where soil is compacted (low drainage
areas), which may lead to flooded areas in the
field and where soil temperatures are between
5°C and 30°C. At lower temperatures, take-all
infections may become more severe. The pH
of soil is also very important since take-all is
able to develop where the soil pH is between
5,5 and 8,5.
Drainage or structural problems that may
restrict rooting should be corrected. High
seeding rates which reduce root development
by interplant competition and favour the disease
should be avoided. An elevated soil pH is likely
to give rise to deficiencies on minor nutrients,
such as copper and manganese. Liming is
important to prevent the development of acid
patches in the field, but overliming should be
avoided. In South Africa there are currently no
resistant cultivars available to control take-all,
due to the irregular occurrence of this disease.
Take-all is not the only reason why bleached
heads occur. Other reasons may also include
Fusarium head blight, cold damage, insect
damage and drought and mistakes are easily
made in identifying the problem.
Fusarium head blight needs a warm and humid
climate whereas take-all prefers cooler and
humid conditions. The differences between
take-all and Fusarium head blight symptoms
are displayed in
Table 1
.
For more information regarding take-all,
contact Cathy de Villiers at the ARC-Small
Grain Institute, Bethlehem on 058 307 3400 or
.
33
February 2014
Plant section
Take-all
Fusarium head blight
Roots
Roots have a dark colour, are brittle
and break easily.
No discoloration.
Crowns and
lower internodes
Dark brown to black crowns and low-
er internodes are visible.
No infection.
Plants
Plant height can be uneven. Infect-
ed ears are bleached, shorter than
healthy ears and with little or no
grain. Plants can be removed easily.
No difference in plant height.
Infected ears are bleached, will show
a pink to orange fungal growth with
little or no grain. Plants can be re-
moved with effort.
TABLE 1: DIFFERENCES BETWEEN TAKE-ALL AND FUSARIUM HEAD BLIGHT.
Photo 1: Take-all plants identified by bleached
heads in a wheat field.
Photo 2: Root symptoms of a take-all infected
wheat plant.
Photo 3: Fusarium infected wheat head.
