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Cultivar screening,
the answer
to charcoal stalk rot of maize
C
harcoal stalk rot is caused by the
fungus
Macrophomina phaseolina
and is favoured by soil tempera-
tures of 30°C to 42°C and low soil
moisture. Initial symptoms in maize can be
observed after flowering with the abnormal
drying of upper leaf tissues, stalk lodging
and premature death.
When plants approach maturity, the lower
stalk nodes (usually limited to the first five
nodes) show a typical charcoal, grey-black
discolouration. When the stalk is cut open,
numerous black specks (microsclerotia) are
visible (
Photo 1
) in the shredded vascular
bundles and on the inside of the stalk.
The charcoal rot pathogen (fungus) has a
broad host range that includes soybean,
sunflower, sorghum, maize, tobacco and a
range of vegetable crops, making crop rota-
tion ineffective as management option. The
most effective and environmentally friendly
management option remains the use of
cultivars with resistance to charcoal rot.
No information was available regarding the
resistance of commercial cultivars to char-
coal stalk rot and therefore, in a project
funded by the Maize Trust, 30 commercial
maize cultivars were screened for resist-
ance during the 2017/2018 maize growing
season.
The 30 cultivars were selected from the na-
tional cultivar trials and planted in a field tri-
al in Potchefstroom, with three replications.
Maize stalks were inoculated with
M. pha-
seolina
infected toothpicks (
Photo 2a
) and
the controls were inoculated with sterilised
clean toothpicks.
To conduct disease ratings of the stalks
from the individual cultivars, the stalks
were split open and the lesion size captured
(
Photo 2b
) according to a disease severity
scale (Shekhar and Kumar, 2012) ranging
from 1 (healthy or slightly discoloured at the
site of infection) to 8 (discoloration of five
or more internodes and premature death
of plant).
Cultivar disease scores in this study ranged
from 1 to 5, where a score of 1 indicates a
healthy stalk or slightly discoloured stalk at
the site of inoculation. A score of 2, 3 or 4,
means that up to 50%, 51% to 75% and 76%
to 100% of the inoculated internode is dis-
coloured, respectively.
A score of 5 means that 100% of the inocu-
lated internode is discoloured, with less
than 50% of the adjacent internode.
The data were analysed by Ms Nicolene
Cochrane (ARC-Biometry Services) using
a split-plot ANOVA with cultivars as main
treatments and treatments (inoculated and
control) as sub-plots. Significant cultivar
differences were observed (P = <,0001
– see
Graph 1
).
LS 8541 BR, PAN 5R-591 R and PAN 4A-156
had the lowest disease ratings (healthy or
slightly discoloured at the site of inocula-
tion) and cultivars SC 506 and US 9616 the
highest (100% of the first internode discol-
oured, with less than 50% discolouration of
the adjacent internode).
M. phaseolina
resistance information gen-
erated can now for the first time assist pro-
ducers with cultivar choices
for the management of char-
coal stalk rot.
Reference
Shekhar, M and Kumar, S. 2012.
Inoculation methods and
disease rating scales for maize diseases
. Directorate of
Maize Research, Indian Council of Agricultural Research.
Pusa Campus: New Delhi, India.
25
December 2018
ON FARM LEVEL
Integrated pest control
Charcoal stalk rot / Resistance
DR BELINDA JANSE VAN RENSBURG,
ARC-Grain Crops, Potchefstroom
Graph 1: Mean
M. phaseolina
stalk rot disease severity (%) in 30 commercial cultivars planted in a
field trial in Potchefstroom during the 2017/2018 maize growing season.
Different letters indicate significantly different reactions of cultivars to
M. phaseolina
infection.
1: Microsclerotia visible as black specks in the
shredded vascular bundles.
2a and 2b: Maize stalk inoculated with a
M. phaseolina
infected toothpick and lesion size at
infected site.
1
2a
2b