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RUBRIEKE/ FEATURES
4 Graan SA Standpunt: Hoe nou vorentoe?
5 Grain SA Point of View: How do we go forward?
7 Uit die Woord
7 Op die kantlyn
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3
Voorblad/ Cover
Desember 2017
16
Research on and control
of Diplodia in maize
Diplodia stalk and ear rot is caus-
ed by the fungus Stenocarpella
maydisand can be encountered
throughout theworld.Diplodia ear
rot is usually noted in seasons with early
drought followed by excessive and extend-
ed rainfall during the maturation stage of
themaize plant.Diplodia stalk rots become
common in seasons where early season
rainfall is followed by amidseason drought
periodduringgrain fill.
Diplodia ear and stalk rot disease produc-
es specific mycotoxins which have been
shown to affect animals differently in vari-
ous studies,many ofwhichwere originally
determined in South Africa where field
symptoms of diplodiosis were initially re-
ported.
Diplodiosis is defined as a nervous disor-
der of cattle and sheep resulting from the
ingestion of mouldy cobs infected by
S.maydis.
The past
The first record of diplodiosis in South
Africa is a report by Van der Bijl in 1914 in
the regionof theMooiRiver in theKwaZulu-
Natal province. He reported an outbreak of
‘sickness’ in cattlewhichwas characterised
by paralysis following grazing on harvested
maize fields.
Over thepast threedecades inSouthAfrica,
various reports of diplodiosis in livestock
were received from veterinarians and pro-
ducers. However these outbreaks have not
been confirmed to be caused by Diplodia
contaminated residues.
The present
Stenocarpellamaydis
(Diplodia) ear rot
Drought during the early season, followed
by rain during the late season, can lead to
Diplodiaear rotepidemics,especiallywhere
high inoculum sources are present on stub-
ble covering soil.
This fungushas the ability toproduce spore
producing structures that can survive on
maize stubble through thewinterwhilepro-
ducing sporesduring spring.
These spores then infect plants through-
out the growing season. After rain or
during high humidity, these structures re-
lease spores in the air,which land onmaize
plants and infect the base of the ear/leaf
junction and ramifies upwards into the ear.
The entire ear becomes overgrown with a
whitemycelialgrowth (Photo 1).
If a cross section ismadeof an infected ear,
black spore-producing bodies at the kernel
basescanbe seen (Photo2).Late season in-
fectionsmay occurwhen kernelmoisture is
low, but these symptoms are less obvious.
Infections that show little or no symptoms
are locally referred to as ‘skelmDiplodia’.
Diplodia ear rot can re-occur (epidemic) in
certain areas and infected grain is then har-
vested with the healthy grain, thereby re-
ducing grain quality. Reduced grain quality
will have negative financial implications as
this reduces theprice theproducer receives
forhisgrain.During suchanepidemicwhen
early infectionsarepresent,yield lossescan
beofgreat economic importance.
Stenocarpellamaydis
(Diplodia) stalk rot
This fungus is common in allmaize produc-
ing areas and in seasons with early rains
and persisting late season droughts, this
disease becomes very damaging, resulting
in lodging andpoorgrain fill.
Diplodia stalk rot reduces yield by reducing
nutrient andmoisture uptake to ears during
grain fill. This sink (the ear) extracts sugars
from thestalkwhich furtherpredisposes the
stalk to fungal growth and further reduces
nutrientuptake.
This continual sink-source cycle reduces
yield. The onset of windy conditions whilst
plants aredrying results in lodging (Photo 3)
and further economic losses as ears have
to be picked by hand. Estimated annual
yield lossesof 5% to 20%mayoccurdue to
Diplodia stalk rot and lodging.
The fungusoverwinters inamycelial form in
maize stubble (buriedoron the soil surface)
throughout the winter. Under warm,moist
conditions, pycnidia developwhich release
sporeswhich are spreadbywind and rain.
Infections of plants occur mainly through
the crown and roots andoccasionally at the
nodesbetween the crown andear. Infection
usually takes place two to three weeks af-
ter silking under favourable conditions.Dry
early season conditions followed by rain
during silk formation favour Diplodia ear
rot,whereas awet early season followedby
drier conditionsorheat stress is likely to re-
sult inmore severeDiplodia stem rot.
Stalk rot symptoms appear several weeks
after silking. Leaves of infected plantswilt,
become dry and appear greyish-green.
Lower internodes become brown and
spongy. Small, black fruiting bodies (pyc-
nidia) clusternear thenodesof the rind.
The rind may also be covered by a white
mycelial growth. The stalk pith discolours
and disintegrateswith vascular bundles re-
maining intact. Thisweakening of the stalk
predisposesplants to lodgingduring strong
winds and rainprior toharvest.
Control measures for
Diplodia ear and stalk rots
It is critical that Diplodia ear and stalk rot
control is seen holistically and that other
control measures are included in an inte-
grated control programme tomanage both
Diplodia ear and stalk rots.
Stubble reduction/retention
Control ofDiplodia ear rot includes surface
stubble reduction by means of grazing,
burning, baling or ploughing in of surface
maize stubble. As the fungus (
S. maydis )
survives on maize stubble and survives
poorly in soil, any management practice
that reduces levels of infected surface
stubble will reduce inoculum concentra-
tions in the field.
The removal of stubble for a single season
and then resortingback to stubble retention
practices, only reduces Diplodia ear rot for
that specific season.Where stubble is pre-
sent the following season, the risk ofDiplo-
dia ear rotwill increase to its original level,
shouldweather conditionsbe favourable.
Stress reduction
Avoid planting unrealistically high plant
populations onmarginal soils and in areas
where there isahighprobabilityofdrought,
leaforalternate stalk rotdiseaseconditions.
Ensure plant nutrition is adequate and bal-
anced relative to the yield potential of the
landor area tobeplanted. 17December 2017
ON FARM LEVEL
Diplodia stalk and ear rot Integrated pest control
DRBELINDA JANSEVANRENSBURG,
ARC-GrainCrops,Potchefstroom and
PROFVINESHMAHARAJ,Department ofChemistry,University ofPretoria
Unnecessary stressors on the crop may
increase the potential for Diplodia stalk
rot, which indirectly in the long term may
increase inoculum levels on the land and
underconditions favourable forDiplodiaear
rots an epidemicmayoccur.
Crop rotation
Crop rotations reduce Diplodia ear rots by
reducing inoculum levels in twoways. First-
ly, a non-host for the funguswill not allow
the fungus to persist for the seasonwhere
maize is not grown.Secondly, a greater pe-
riod (a seasonor two)betweenmaize crops
allows for a natural breakdown of maize
stubble,which again reduces the survivalof
the fungus.
Leguminous crops, such as soybeans, dry
beans, groundnuts and cowpeas are very
good rotation crops. Other rotation crops
that reduceDiplodia ear rots, arewheat and
oats.Sunflowersdonot significantly reduce
Diplodia ear rots under experimental condi-
tions. The reason for this has not yet been
found.
Earlyharvesting
Early harvesting will reduce Diplodia ear
rots as it reduces the time available for the
fungus to grow on the ear. Late or winter
rains keepearswetand increase the chance
for fungal growth. In certain cases, itwould
pay to harvest early at higher moisture
levelsandartificiallydrygrain.This ispossi-
blywhyDiplodiaear rot isnot amajorprob-
lem in the USA where maize is harvested
early anddried artificially.
Hybrid resistance
Selection of cultivars is very important in
the control of Diplodia ear and stalk rots.
In general, a resistant hybrid will always
have less Diplodia ear rot than susceptible
hybrids relative to prevailing conditions.
This effect is an interaction between the
available inoculum, the host and prevailing
weather conditions.
However, there are many maize hybrids
that react consistently over all localities,
but there are some that do not. In a project
funded by theMaize Trust, the local South
AfricanNational Cultivar Trial entries of the
ARCareused to screenmaizehybridsannu-
ally for resistance/susceptibility toDiplodia
ear rots under various weather and inocu-
lum conditions. Diplodia inoculum is also
supplied to companies for screening.
Resistance toDiplodia stalk rot isdifficult to
quantify as plant standability or resistance
to lodging, does not necessarilymean the
stalk is not infectedwith Diplodia stalk rot.
Itmeans that even though the stalkmay be
infected with Diplodia stalk rot, it will not
lodge. Resistance therefore is not against
the fungus, but by improving stalk rind
thickness.
A thicker rindmay still have Diplodia stalk
rot which results in the breakdown of the
stalk pith tissue within the rind. Thicker
rindsmay in the long term have the unin-
tendedeffectofactually increasingDiplodia
inoculum as it ismore resistant to decom-
position and may improve survival of the
inoculumwhich from extensive studies has
been shown to survive successfully in intact
maize residues, particularly those retained
on the soil surface. It is therefore important
that Diplodia stalk rot resistance is seen in
the correct context.
Diplodiosis and its
associated toxins
Diplodiosis,anervousdisorderofcattleand
sheep, results from the ingestion of ears
infected by Diplodia. Cases of diplodiosis
occur from six days to twoweeks after the
animals are placed on fields with infected
maize cobs.
The disease is characterised by reluctance
of the animals to move, a wide-based
stance, inco-ordination, tremors, paralysis
and death. The disease also causes abnor-
mal foetaldevelopment and foetaldeath.
Field outbreaks of diplodiosis in southern
Africa are favoured by late, heavy rains and
occurduring the latewintermonths (July to
September). The practice of using harvest-
edmaize fields for winter grazing is ama-
jor contributing factor to outbreaks of this
mycotoxicosis. In addition to diplodiatoxin,
new metabolites, namely dipmatol, diplo-
nine and chaetoglobosins K and L, have re-
cently been isolated fromDiplodia infected
crops.
To date, none of these pure metabolites
have been administered to ruminants in
order to reproduce the disease. Laboratory
analytical test methods that quantify and
establish the presence and distribution of
these toxins in infectedmaize commodities
are also lacking.
The future
In a current Maize Trust funded project,
the University of Pretoria (Departments
of Chemistry and Veterinary Science), the
Department of Biotechnology and Food
Technology (TUT), South African Grain
Laboratories (SAGL) and ARC-Grain Crops
are currently collaborating in producing the
various Diplodia mycotoxins in sufficient
quantities to develop an analytical test for
the detection of themetabolites in infected
maize as well as to confirm Diplodiosis of
thesemetabolites in target animals.
Whowill benefit from
this research?
Little research has been done internation-
ally and this technology will give South
Africanmaize producers, livestock produc-
ers, regulatory authorities and final consu-
mers a distinct advantage. The availability
of an analytical testmethodwill ensure that
maize products supplied to thewide range
of consumers are free of the mycotoxins
rendering them safe.
Animal feed samples can be routinely test-
ed for the presence ofDiplodiamycotoxins
and furthermore casesofdiplodiosis canbe
confirmed or disproved enabling interven-
tion methods. The technology will benefit
theSouthAfricanand international research
community, enabling new avenues of re-
search thatwillgivepractical
solutions to all beneficiaries
involved.
1:Amaize earovergrownwithwhite
mycelialgrowth.
2:A cross sectionof an infectedmaize ear,
showingblack spore-producingbodies at the
kernelbases.
3: Lodgin e toDiplodia
stalk rot. Flett
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INTEGRATED PEST TROL 16