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Animals, particularly cattle, being fed Diplodia-infected rations
must bemonitored.
As soon as symptoms of reluctance to move, standing with wide-
based stance, poor co-ordination, walking stiff legged with a high
stepping gait, falling, paralysis, constipation, salivation and tremors
are observed, animals must be removed immediately and fed on
healthy rations. Recovery rates are high, but animals will die if kept
on the infected ration.
Incidence and severity
The incidence and severity of Diplodia ear and stalk rots are largely
seasonal whichmeans climatic factors play amajor role in the epi-
demiologyof these twodiseases. Epidemicsof Diplodiaear rot have
been associated with early season droughts and late season rains,
particularlywhere infected stubblehasbeen retainedon the soil sur-
face and susceptiblehybrids havebeenplanted.
Theepidemicof themid80s inSouthAfricahas sincebeen followed
by smaller, more localised epidemics. These areas are generally
where susceptible hybrids havebeenplanted inmonoculturewhere
surfacestubble retentionwashigh followingapreviousyet oftenun-
noticedDiplodia stalk rot epidemic.
The reason that Diplodia stalk rot epidemics often go unnoticed
is that breeding programs have focussed on resistance to lodging
which does not equate to resistance to the stalk rot pathogens, but
rather to rind (outer layer) strength. Thismeans that plantsmay still
be infectedby the stalk rot pathogens, but not lodge as easily.
Life cycle and epidemiology
S. maydis
spores are transmitted by air, seed and soil. Airborne
spores result in heavy infections up to 10 m from the inoculum
source and the number of successful infections is reduced with
distance. Single spores travelling long distancesmay lead to trace
infections which may then develop into an epidemic focal point.
Infected maize seed is an important inoculum source which may
result in seedling and crown rot diseases. However, themajority of
S.maydis
-infected kernels do not germinate.
Spores land behind leaf or ear sheaths where they germinate and
infect stalks or ears. Stalk infections may occur through root and
crown infectionswhere viable spores in the ground infect under fa-
vourable conditions and the fungus ramifies up through the crown
into the lower internodes when the maize plant becomes stressed.
Spore germination is inhibited by exposure to sunlight and desicca-
tion. Free water is necessary for germination and an infectionmay
take up to seven days after spore germination to flame up. Germi-
nated spores may enter tissue and lie dormant until conditions are
favourable for fungal growth through the tissues.
Mycelial colonisation is accompanied by cell wall degradation
ahead of the growing pathogen. This is due to enzymes that are se-
creted. Fungal ramification of maize ears begins at the shank. Cob
tissue colonisation begins at the attachment with embryonic tis-
sues and proceeds into the endosperm. Similarly stalk ramification
may alsooccur. These infected tissues develop fruitingbodies (pyc-
nidia) whichproduce spores during the subsequent season.
Control measures for Diplodia ear and
stalk rots
Stubble reduction/retention
Control of Diplodia ear rot includes surface stubble reduction by
means of grazing, burning, baling or ploughing in of surface maize
stubble. As the fungus (
Stenocarpella maydis
) survives on maize
stubble and survives poorly in soil, any management practice that
reduces levels of infected surface stubblewill reduce inoculum con-
centrations in the field.
The removal of stubble for a single season and then resorting
back to stubble retention practices, only reduces Diplodia ear rot
for that specific season.Where stubble ispresent the following sea-
son, the risk of Diplodia ear rot will increase to its original level,
shouldweather conditions be favourable.
There is however, a slight contradictionwhen taking Diplodia stalk
rot into consideration as conservation tillage practices improve
soil water retention and in doing so reduce plant stress levels
which reduces predisposition to Diplodia stalk rot. These are two
separate diseases as the climatic conditions influencing the epi-
demics of the two diseases, but the same pathogen and host, are
however involved. Stubble retentionmay on the one hand increase
Diplodia ear rots should the pathogens’ inoculum survive on this
stubble the previous season; it may however reduce possible stalk
rot incidencewhich in turnmay reduce inoculum levels for the fol-
lowing season.
To reduce the possible effects of this contradiction, it is critical that
Diplodia ear and stalk rot control is seen holistically and that other
control measures are included in an integrated control programme
tomanagebothDiplodia ear and stalk rots.
Stress reduction
Avoid planting unrealistically high plant populations on marginal
soils and in areas where there is a high probability of drought, leaf
or alternate stalk rot disease conditions. Ensureplant nutrition is ad-
equateandbalanced relative to theyieldpotential of the landor area
tobeplanted.
Unnecessary stressors on the crop may increase the potential for
Diplodia stalk rot which indirectly in the long termmay increase in-
oculum levels on the land and under conditions favourable for Dip-
lodia ear rots an epidemicmay occur. To date no direct relationship
betweenplant stress andDiplodia ear rot has been reported.
Crop rotation
Crop rotations reduceDiplodia ear rots by reducing inoculum levels
in twoways. Firstly, a non-host for the funguswill not allow the fun-
gus to persist for the seasonwheremaize is not grown. Secondly, a
greater period (a season or two) betweenmaize crops allows for a
natural breakdown of maize stubble, which again reduces the sur-
vival of the fungus.
Leguminous crops, such as soybeans, dry beans, groundnuts, cow-
peas, etc are very good rotation crops. Other rotation crops that
reduceDiplodia ear rots, arewheat, oats etc. Sunflowers donot sig-
nificantly reduce Diplodia ear rots under experimental conditions,
but, the reason thereforehas not yet been found.
Early harvesting
Early harvestingwill reduce Diplodia ear rots as it reduces the time
available for the fungus to grow on the ear. The fungus (
Stenocar-
pella maydis
) can grow on maize ears in the field until 11%mois-
ture. Late or winter rains keep earswet and increase the chance for
fungal growth. In certain cases it would pay to harvest early at
higher moisture levels and artificially dry grain. This is possibly
why Diplodia ear rot is not a major problem in the USA where
maize is harvested early anddried artificially.
Hybrid resistance
Selectionof cultivars is very important in the control of Diplodia ear
Pest control
ON FARM LEVEL
A look at
Diplodia ear
and
stalk rot of maize
and recently
isolatedmycotoxins
