Sorghum Diseases in India

severities. Standard area diagrams were prepared
and used to aid in estimating disease severities.
Leaf length and width were taken from
the central leaves.
An average of 45 plants selected to cover all
disease-severity levels at the site were selected,
and the panicles tagged and harvested. They
were later weighed and threshed individually.
Humidity readings were then taken and seed
mass per panicle determined.
These data were used to develop empirical
mathematical models to explain variations in
yield at each site and to estimate yield losses due
to these diseases. Data were also combined to
produce a general mathematical model based on
principal components of plant height, panicle
length, and disease severities.
Results
Yield estimates for minimum and maximum disease-severity
levels at mean plant height and
panicle length reflect the effect of the change in
disease severity, independent of possible effects
of disease severity on panicle length and plant
height (Table 4). Only gray leaf spot showed a
real difference in yield levels due to disease effects
alone; a 7.2% difference between the yield
estimates for plants with minimum and with
maximum disease levels, at mean plant height
and panicle length values, was recorded. Yield
estimates for rust, oval leaf spot, and smut had
no real differences between low and high severity
levels, since confidence limits overlapped.
When yield differences were estimated allowing
plant height and panicle length to vary
along with disease severities, there was a 14.6%
difference in estimated yields between plants
with less than 20% gray leaf spot and plants
with 100%. There was a 3.6% difference between
estimated yields for plants with less than 5%
rust and with 25% rust. A difference of 5.5% was
calculated between plants with less than 20%
oval leaf spot severity and plants with 100%
(Table 4). Observed versus estimated yields for
the general model had a correlation coefficient
of 0.792.
Foliar Disease Severities and Yields in
Fungicide-treated and Nontreated Plots
During the 1983 cropping season, zonate leaf
spot was the predominant disease at the La
Lujosa Experiment Station in Choluteca. This
disease is caused by the fungus (Gloeocercospora
sorghi). In an experiment to study the effect of
this disease on sorghum yields; comparisons
were made between plots protected with fungicide
applications and nonprotected plots. A
similar study was done at the same location during
the 1984 cropping season. In this season, the
predominant foliar disease was gray leaf spot
caused by the fungus Cercospora sorghi.
Table 4. Sorghum yield loss estimates in farmers' fields at three sites in southern Honduras, 1985.
Disease
severity
CERC 1 20%
CERC 100%
Rust 5%
Rust 25%
OVAL 2 10%
OVAL 100%
1. CERC = Gray leaf spot
2. OVAL = Oval leaf spot.
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Plant height and ' panicle length
Yield
estimate
[g (plant) -1 ]
116.3
107.8
113.2
109.8
107.5
106.7
Yield
difference
(%)
7.2
Plant height and panicle length
Yield
estimate
[g (plant) -1 ]
121.4
103.1
114.2
110.0
110.0
104.0
Yield
difference
(%)
14.6
3.6
5.5