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Februarie 2019
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Management of sorghum diseases
G
rain sorghum may be affected by a wide range of diseases.
The incidence and severity of these diseases are growth
stage and weather dependent. The current fluctuations in
rainfall distribution and intensity, as well as increasing tem-
peratures could result in a shift in the importance of diseases over
the next few years.
A disease that may fall into this category is charcoal rot (
Photo 1
)
which is defined as a stress-related disease where stress implies
low soil moisture and high temperatures. Still, stress can also be
defined in terms of low temperature, for example where low soil
temperatures associated with early planting dates, predispose seed-
lings to seedling blights and damping-off (
Photo 2
).
The later rains experienced over the last few seasons and the lat-
er plantings when soils have warmed up sufficiently, may result in
this disease complex and diseases such as head smut (
Photo 3
)
which is also dependent on colder soils during the seedling stage,
becoming less important.
At the other end of the spectrum are diseases such as ergot
(
Photo 4
), grain moulds (
Photo 5
) and the risk of associated myco-
toxins, which are associated with critical plant growth stages such as
flowering, coinciding with late season cooler and wetter conditions
due to later plantings. Thus, the changing weather patterns could
impact on the incidence of currently important diseases and new
threats may also arise.
Disease management
In the pursuit of disease management, reference is often made to
‘resistant cultivars’. Although this is the ideal and cheapest manage-
ment strategy, the size of the current local sorghum market as well
as the need to close the yield gap between sorghum and maize has
resulted in disease resistance being secondary to yield and quality.
Cultivars may have varying degrees of disease resistance and many
producers pursue yield with the idea of chemical intervention when
deemed necessary. Leaf blight (
Photo 6
) and anthracnose (
Photo 7
)
fall within this category. This too requires careful planning and man-
agement in order to be effective and economical.
In Europe for example, only 34% of chemical applications on canola
and soybean for the control of Sclerotinia stem rots are deemed
necessary or effective. Local sorghum data are not available, but
observations suggest this could fall into a similar category.
The Epidemiology sub-division of the Plant Pathology Division of
the Department of Plant Sciences at the University of the Free State
(UFS) has been focusing on the identification and management of
disease driving variables. Much of this research is funded by the
Sorghum Trust and aims at providing practical advice to producers
which could reduce the risk of disease reaching crop loss thresholds.
A number of advisories are posted below as well as an indication
of studies currently being pursued. Producer participation in trials
is also being encouraged so as to increase the diversity of weath-
er, soil and agronomic variables under which diseases are studied.
Interested parties are encouraged to contact the unit.
Disease management strat gies
Disease management strategies are aimed at favouring sorghum
growth and development while attacking vulnerable stages in
the life cycle of the pathogen so that its development is either pre-
vented or restricted. Internationally it is accepted that there are six
general strategies for disease management, five of which are rel-
evant to sorghum. Their application in sorghum production is sum-
marised below.
Exclusion and avoidance as
disease control strategies
Exclusion refers to preventing the introduction of a pathogen to a
target area while avoidance refers to preventing disease by selecting
a time of the year or a site where there is no inoculum or where the
environment is not favourable for the initiation of infection. Exclu-
sion in the context of sorghum production would primarily be the
use of certified or disease-free seed to prevent the introduction of
pathogens into areas where they do not already occur. Exclusion
could also include preventing the movement of soil and debris on
implements from an infected area to a disease-free area.
Avoidance of sorghum diseases can be construed as misnomers in
the strict sense, but practices aimed at reducing the risk of disease
favourable environments include the choice of planting date and cul-
tural practices. Sorghum mortality due to a complex of soilborne
pathogens during the first three weeks after planting is a recurring
challenge in many areas, requiring replanting at considerable seed,
fuel and labour cost.
Sorghum seed showing high viability (90%) in standard germination
tests often exhibit drastically reduced emergence in the field. Most
soilborne fungi are opportunistic pathogens and any condition that
reduces seedling vigour can predispose sorghum to infection, thus
providing these pathogens the required opportunity.
As alluded to above, a major stress factor is low soil temperature
associated with early planting. Prolonged night temperatures of
less than 13°C subsequent to planting can reduce seedling growth
rates and are particularly favourable for infection. A simple adjust-
ment of planting dates to when soils are warmer can reduce the risk
of seedling blights and damping-off. Similarly, low soil pH stresses
seedlings and predisposes them to infection by soilborne seedling
pathogens.
It is essential to ensure that soil pH does not decrease to pH(H
2
O)
<5,2 if the risk of predisposition is to be avoided. Localised acidifica-
tion of soil in a band within the seeding zone may occur when band
placement of certain fertilisers coincides with cooler, wet weather
which in turn predisposes seedlings to infection. Other stresses in-
clude excessive planting depths, planting densities and the use of
certain pre-emergence herbicides under sub-optimal conditions.
Sphacelotheca reiliana
is better known as the causal organism of
cob and tassel smut of maize which was very common during the
late 1960s and early 1970s. However, the introduction of more sus-
ceptible germplasm into the local market has resulted in a significant
ON FARM LEVEL
Identification / Preventing / Protection
Integrated pest control
PROF NEAL MCLAREN
and
LISA ROTHMANN,
Department of Plant Sciences:
Plant Pathology Division, University of the Free State