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September 2017
FOCUS
Integrated pest control
Special
Yield loss associated with northern corn
leaf blight
–
is it that bad for all cultivars?
N
orthern corn leaf blight is probably
the most widespread leaf disease
on maize in South Africa. It tends
to be especially severe in the
eastern parts of the country, e.g. KwaZulu-
Natal and Mpumalanga. It is also common
and causes serious yield losses, even in
the drier western parts, particularly under
irrigation systems along the Vaal, Modder
and Orange river systems.
International literature estimated yield
losses attributed to northern corn leaf
blight to vary between 2% and 8% for every
10% increase in northern corn leaf blight
(
Photo 1
) disease severity. In general, yield
losses that most commonly occur are be-
tween 15% and 30%.
As part of a larger study currently being
conducted on northern corn leaf blight,
two field trials were planted by ARC-
Grain Crops at Potchefstroom during
mid-November and mid-December 2016
respectively.
Three irrigation maize hybrids were
included, representing hybrids with differ-
ent resistance or susceptibility. The aim of
these trials was to create varying degrees
of disease severity in order to establish the
yield impact as experienced by the three
cultivars as a result of infection. All treat-
ments were replicated three times.
Due to the first trial being planted dur-
ing mid-November, the establishment of
northern corn leaf blight was slow, as op-
posed to the second planting date, where
lesions were observed even before inocula-
tion. From the results, it was clear that three
cultivars differed regarding their inherent
yield potential, with Cultivar 1 having the
highest yield, followed by Cultivar 3 and
lastly Cultivar 2.
Graph 1a
and
Graph 1b
represents the
eventual regressions plotted between
yield (as percentage of control) and dis-
ease severity at dent stage obtained for the
three cultivars at the two planting dates.
An initial observation was that the disease
was only established with the first planting
after inoculation was conducted, whilst it
was already present in the second trial when
inoculation commenced.
Where the disease was slow to establish
(planting 1), Cultivar 1 suffered a 0,32% loss
in yield for every 1% increase in disease
severity as opposed to Cultivar 3 which had
a 0,39% yield loss per 1% disease increase.
With the later planting (mid-December), we
observed that Cultivar 1 now experienced a
yield loss of 0,67% per 1% disease increase
as opposed to Cultivar 3 which experienced
a 0,45% yield loss per 1% disease increase.
DR MARYKE CRAVEN,
ARC-Grain Crops, Potchefstroom
Graph 1a and Graph 1b: Yield obtained as expressed as a percentage of an untreated control at
various disease severities at dent stage as observed in a November planting date (a) and December
planting date (b).
1a
1b
1: Northern corn leaf blight.
1