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Februarie 2019
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ON FARM LEVEL
Integrated pest control
Management of sorghum diseases
Avoidance as a strategy also applies to diseases of the inflorescence
and grain. Ergot and grain m ulds are associated with cool, wet
conditions during early flowering and grain development, respec-
tively. Cooler conditions normally occur late in the season, after
mid-February, and are normally accompanied by extended dew
periods. Planning of flowering dates, either with planting date or
cultivar growth period, can reduce the risk of flowering and grain
development during these disease-favourable conditions.
Avoidance can also include other risk reduction activities such as
population stress often associated with root and stalk rots. Char-
coal rot caused by the fungus
Macrophomina phaseolina
is a major
disease where sorghum is produced under hot, dry conditions. The
disease generally becomes evident on the maturing crop when
plants lodge.
Sclerotia of
M. phaseolina
become incorporated into the soil with
host debris where they can survive for a number of years. Stress re-
duction can play a major role in reducing the risk of infection. Main-
taining soil moisture during grain fill can reduce infection. In higher
risk areas, plant populations should be adjusted to reduce the risk of
competition and moisture stress.
Eradication as a disease management strategy
Eradication refers to activities that eliminate, destroy, or inactivate
inoculum. The disease management strategy of pathogen eradi-
cation applies to most sorghum diseases. Soilborne inoculum as-
sociated with the seedling disease complex and root rots can be
managed with tillage practices and crop rotations.
Both of these practices aim at reducing the nutrient base and thus
reduce pathogen growth and proliferation. Conventional ploughing
to promote the rapid breakdown of crop debris and prevent the con-
centration of crop stubble in the upper soil layers is more effective in
reducing inoculum than minimum tillage or no-till.
The advantages of the latter as far as factors such as water retention
and reduced wind erosion are concerned, should be borne in mind
though. Similarly, studies have shown that rotation systems with in
particular legumes, can reduce inoculum levels of critical pathogens.
Legume rotations have the advantage of stimulating root volume in
subsequent sorghum crops which also compensates for root infec-
tions, providing for improved water and nutrient uptake and result-
ant yields.
The strategy of eradication applies to foliar diseases of sorghum.
Exserohilum leaf blight caused by the fungus
Exserohilum turci-
cum
(syn.
Helminthosporium turcicum
) and anthracnose caused by
the fungus
Colletotrichum graminicola
are particularly prevalent in
warm, wet conditions.
Disease development is favoured by moderate temperatures (18°C -
27°C), heavy dews and high humidity. Dry weather retards disease
development. Losses are dependent on the degree of foliar damage
prior to flowering. If the disease is well established before panicle
emergence, yield losses can be as high as 50%.
If infection is moderate or delayed until after panicle emergence,
yield losses are reduced. Foliar disease can occur at any growth
stage, although it tends to be more prevalent on mature leaves. The
pathogens overwinter as mycelia and conidia in infected leaves,
grain and other plant debris. Spores are windborne and can be car-
ried over long distances to host plants.
Secondary infection occurs as a result of spore production within
leaf lesions. These spores are spread to new hosts by water and
wind. Crop rotation with non-susceptible hosts aids in the destruc-
tion of infected crop residues and helps to minimise sources of
inoculum for subsequent crops. Similarly, tillage practices aid the
breakdown of crop debris.
Biological control by means of organisms such as
Trichoderma
spp.
that suppress pathogen activity or kill pathogens by means of me-
tabolites and direct parasitism, although not widely applied to sor-
ghum, also falls within the strategy of eradication.
Host protection
Host protection refers to the prevention of infection by means of a
toxicant or some other barrier to infection. In the sorghum disease
context this generally applies to the use of fungicides, although, as
above, biological agents may also be included. Particularly in the
case of seedling diseases, the use of fungicide seed dressings that
reduce the activity of both seedborne and soilborne pathogens and
enhance seedling vigour is of paramount importance. Seed dress-
ings can also be specific, for example metalaxyl which can be ap-
plied for the prevention of downy mildew (a sporadic disease in
South Africa) and carboxin for head smut.
On maize a number of triazole seed dressings have been reported to
effectively control smuts, but these are not registered on sorghum.
The over-use of metalaxyl in Texas has given rise to fungicide re-
sistant strains of the downy mildew pathogen implying that these
chemicals need to be used with caution.
As indicated above, the need to close the yield gap in sorghum has
increased the dependence on chemical control of in particular fo-
liar diseases such as leaf blight. Although a number of efficacious
fungicides have been identified, only azoxystrobin + difenoconazole
and azoxystrobin + epoxiconazole are registered on sorghum for
leaf blight control in South Africa while azoxystrobin is registered
for anthracnose control.
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Producer participation
in trials is also being
encouraged so as to
increase the diversity
of weather, soil and
agronomic variables
under which diseases
are studied.