Augustus 2014
94
A look at
Diplodia ear
and
stalk rot of maize
and
recently
isolatedmycotoxins
D
iplodia ear rot (
Photo 1
) is possibly the most damaging
and important ear rot disease in southernAfrica causedby
the fungi
Stenocarpella maydis
and
Stenocarpella macro-
spora
.
Stenocarpella macrospora
ear rot symptoms are almost identi-
cal to those of Diplodia ear rot, but occur under different climatic
and epidemiological conditions. Macrospora ear rot appears to be
predominant in areas of KwaZulu-Natal where Diplodia ear rot can
occur throughout the entiremaizeproduction areaof SouthAfrica.
Diplodia ear rot control measures are not always directly applic-
able to Macrospora ear rot and must be treated as two distinct
separatediseaseswhichvarygreatly in termsof their epidemiology.
Macrospora ear rot can easily be confused with Diplodia ear rot,
not onlybyproducers, but by agricultural advisors and agronomists
aswell.
This confusion stems from the inability to determine the correct
identity of the specific disease in a specific area often resulting in
conflicting research results and advice. If a producer is not certain
which ear rot organism is present, they must please contact their
local plant pathologist.
These two pathogens, although visually identical in the field, can
only be distinguished based on isolation onto artificial media and
then identified based on spore size and shape. Diplodia ear rot re-
sults in yield loss (when light weight rotten kernels are discarded in
the harvesting process), grain grade reductions and toxicity prob-
lems for bothhumans and animals.
Diplodia stalk rot (
Photo2
) is causedby the samepathogen, namely,
Stenocarpellamaydis
, however, the epidemiology and climatic con-
ditions that favour diseasedevelopment differs significantly.
Diplodia ear rots are usually noted in seasons with early drought
followed by excessive and extended rainfall during the maturation
stageof themaizeplant. Diplodia stalk rotsbecome common in sea-
sonswhereearly season rainfall is followedby amidseasondrought
periodduringgrainfill.
This unexpected stress on the maize plants at this critical period
predisposes the plants to increased withdrawal of sugars from
the stems which increase their susceptibility to various stalk rot
pathogens.
Diplodia stalk rots may also occur when the roots and crown are
infected by fungi such as Gibberella root and crown rot which
prevents nutrient and water uptake by the plant during grainfill.
This again stresses theplant, forcing it towithdraw sugars andnutri-
ents from the stalk, which in turn predisposes the stalk to infection
by stalk rot pathogens of whichDiplodia stalk rot is one.
Diplodia ear and stalk rot disease produce specific mycotoxins
whichhavebeen shown to affect animalsdifferently invarious stud-
ies,manyofwhichwereoriginallydetermined inSouthAfricawhere
field symptoms of diplodiosiswere initially reported.
Diplodiosis is defined as a nervous disorder of cattle and sheep re-
sulting from the ingestion of mouldy cobs infected by
S. maydis
.
Recent reports of diplodiosis in Argentina, Australia, Brazil and the
USA have resulted in an increase in local and international research
on themycotoxins produced.
Maize infectedwith
S.maydis
has been reported tobe acutely toxic
to ducklings and rats. Reports have indicated that Diplodia-infected
maize used in the chicken broiler and egg laying industries has re-
sulted in reducedperformance inboth industries.
In subsequent studies no correlation was found between the toxi-
city of
S.maydis
isolates inducklings and their ability to inducedip-
lodiosis in cattle and sheep. Some of the
S. maydis
isolates were
acutely toxic toducklings and rats, but were unable to induce diplo-
diosis in either cattleor sheep.
This may indicate that more than onemycotoxinmay be produced
by
S.maydis
isolates and that the isolates of the fungusmay vary in
their ability to produce certainmycotoxins. Their effect on humans
has not yet been determined. Continued research on Diplodia ear
rot, stalk rot and theirmycotoxinswill improveour understandingof
this complexdisease and itsmycotoxicology.
Symptoms
Diplodia ear rot symptoms associated with early infections during
earlyear development areyellowinganddryingof husk leaveswhile
the stalks and leaves remaingreen (
Photo 3
). Infectiongenerallybe-
gins at the ear base and ramifies upwards. The entire ear becomes
overgrownwith awhitemycelial growth.
A cross section of an infected ear shows black spore producing
bodies (pycnidia) at the kernel bases (
Photo 4
). Late season infec-
tionsmayoccurwhen kernelmoisture is low and symptoms are less
obvious. Embryos become infected and slightly discoloured, but no
ramification of the rest of the ear occurs. Such symptomless infec-
tions are locally referred to as
skelmDiplodia
.
Economic importance
Yield losses caused by Diplodia ear rot have not yet been quanti-
fied, because theharvestmethod,makeof harvester, harvest speed,
harvester settings, etc all affect the percentage rotten kernels that
remain in thegrainbinor trailer.
The rotten kernels that are light enough are blown out during the
harvestingprocess, thepercentagehereof dependingonabovemen-
tioned factors. This implies thatwherediplodiaear rot infections are
serious, damage is twofold.
Firstly, if a low percentage of rotten kernels are discarded during
the harvesting process, serious grain quality reductions occur.
Diplodia ear rot /Diplodia stalk rot /Mycotoxins
Pest control
BRADLEY FLETT,
ARC-GrainCrops Institute
ON FARM LEVEL
