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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 7, Num. 1, 1999, pp. 91-96
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African Crop Science Journal,
African Crop Science Journal,
Vol. 7 No. 1 1999 pp. 91-96
short communication
Effect Of Plant Age On Sesame Infection By
Alternaria Leaf Spot
P.S. Ojiambo, P.O. Ayiecho1 and J.O.
Nyabundi2
International Potato Centre, SSA, P.O. Box 25171, Nairobi,
Kenya
1Department of Crop Science, University of Nairobi, P.O.
Box 30197, Nairobi, Kenya
2Department of Horticulture, Maseno University College,
Private Bag, Maseno, Kenya
(Received 7 May, 1997; accepted 10 July, 1998)
Code Number: CS99009
ABSTRACT
The effect of plant growth of sesame (Sesamum indicum
L.) on development of seed infection by Alternaria leaf spot, and
seed infection by Alternaria sesami, was monitored in plants
of sesame accession SPS SIK 110. Increase in percentage of leaf
area blighted and percent defoliated fitted the Gompertz model more
closely than the logistic model. Rates of increase in diseased leaf
area and defoliation as well as areas under disease curves (AUDPC)
varied among the five plant ages. Plant ages with larger AUDPC
generally had faster rates of disease progress. Sesame plants
inoculated at 8 and 12 weeks of age were most susceptible to the
disease; plants inoculated at 4 weeks exhibited the least
susceptibility. Seed infection as determined by oat meal agar plate
method was highest in plants inoculated at 8 and 10 weeks and least
in plants inoculated at 4, 6 and 12 weeks.
Key Words: Alternaria sesami, area under disease
progress curve, defoliation, seed infection
RÉSUMÉ
L=effet de la
croissance du sésame dans le développement de l=infection de la semence par la
tâche alternaria sur la feuille et l=infection par l=Alternaria sésami, a
été observé sur l=accession SPS 110 des plants de
sésame (Sesamum Indicum L.). L=augmentation en pourcentage de feuilles
rouillées suivant le milieu et le pourcentage de
défoliation ont cadré avec le modèle Gompertz
beaucoup plus estimé que le logistique. Les taux d=augmentation par milieu de
feuilles atteintes et de défoliation varient aussi bien sous
les courbes de maladie (AUDPC) parmi les âges de cinq plants
que suivant les milieux. Les âges des plants avec une plus
large AUDPC avait généralement eu les taux les plus
rapides de la progression de la maladie. Les sésames
inoculés à 8 et 12 semaines d=âge ont été plus
susceptibles à la maladie; et les plants inoculés
à 4 semaines d=exposition la moindre
susceptibilité. L=infection de la semence telle que
déterminée par la méthode d=Oat meal agar plate, a
été la plus élevée en plants
inoculés à 8 et 10 semaines d=âge et moindre en plants
inoculés à 4, 6 et 12 semaines.
Mots Clés: Alternaria sesami, milieu sous
la courbe de progression de la maladie, défoliation, taux
d=infection
INTRODUCTION
The amount of damage to sesame (Sesamum indicum L.)
plants by Alternaria sesami is dependent on the stage of
growth of the host plant (Kolte, 1985). Lower leaves of sesame
plants have been reported to be more susceptible to C.
sesami (Nyanapah et al., 1995). Changes during growth
have been reported for other host-pathogen systems. Warren et
al. (1971) reported that the age of potato plants and the
position of leaves on the plant influence susceptibility, with
upper and lower leaves of older plants being more susceptible than
those of an intermediate position. In lettuce, susceptibility to
Bremia lactucae decreases with age (Dickson and Crute,
1974), while in onions susceptibility to Alternaria porri
increases with age (Everts and Lacy, 1996).
The effect of infection of host plants at different stages of
growth on seed infection has been studied by several workers
(Bronnimann, 1968; El-shafey et al. 1976). Roy and Abney
(1976) observed that time of infection during the growing season
influenced total seed infection of soybean by Cercospora
kikuchii. Bronnimann (1968) established a clear relationship
between age of sweet-corn at the time of infection with the rate of
seed infection. Early onset of the disease led to increased seed
infection, while plants infected at maturity had seeds entirely
free from Cephalosporium maydis (El-shafey et al.,
1976).
Varying forms of crop damage are attributed to Alternaria leaf
spot. A decline in the photosynthetic area due to leaf damage is
often the initial effect. Premature defoliation soon follows which
adversely affects growth and yield (Kolte, 1985). Alternaria
sesami also causes considerable damage to sesame capsules.
Yield losses varying from 18 to 55% have been attributed to the
disease (Barboza et al., 1966).
Sesame is one of the major oil crops currently grown in Kenya
that has the best adaptation to marginal agroecological zones
(Ayiecho and Nyabundi, 1994). However, although damage of sesame
plants by Alternaria leafspot appear to be dependent on the host
age, no study has been conducted to determine the effect of plant
age on disease severity and its effect on seed infection. This
study was conducted to determine the susceptibility of sesame to
A. sesami at different ages and its effect on seed infection
by the fungus.
MATERIALS AND METHODS
Effect of plant age on disease severity. Isolates of A.
sesami were tested for aggressiveness before use in inoculation
tests to evaluate the effect of plant age on Alternaria leaf spot
severity and seed infection. Seeds of sesame plant accession SPS
SIK 110 used in this study were obtained from a germplasm
collection of the Sesame Improvement Project at University of
Nairobi. Sesame plants were grown at different time intervals to
allow for a simultaneous inoculation of plants at 4, 6, 8, 10 and
12 weeks of age. Prior to planting, seeds were treated with 0.1%
Zineb to prevent disease development before inoculation. The
inoculum was prepared from 8-day old monosporic cultures of A.
sesami isolates. The suspensions were mixed in equal
proportions and the resultant suspension standardised to 2 x
103 conidia ml-1. The inoculum was dispensed
in sterile vials immersed in ice to prevent premature germination,
and transported to the Institute of Dryland Research, Development
and Utilisation at Kibwezi in Makueni District of Kenya.
Standard cultural practices for growing sesame were followed at
the experimental site, without application of pesticides. Plots
consisted of five 4-m rows spaced 0.50m apart with plants at a
spacing of 0.2m within the rows. Plots were arranged perpendicular
to the direction of the prevailing wind to reduce inter-plot
interference. They were separated from each other at the ends and
sides by 2m strips of susceptible sesame accession, SPS SIK 013.
Two rows on the right and left side of each plot were inoculated
with sterile distilled water and the inoculum suspension,
respectively, leaving one row untreated in between. The experiment
was analysed as a split-plot with stage of growth and inoculation
as whole-plot and sub-plot treatments, respectively. Whole-plots
were arranged in a randomised complete block design with three
replicates and sub-plots were randomised within whole-plots.
Sub-plot treatments were inoculation and non-inoculation (sprayed
with sterile distilled water).
Average blighted leaf areas in the plots were estimated by
selecting 10 representative plants at random on each sampling date.
Leaf areas (one surface) were calculated by multiplying average
width with length measurements for various linear proportions of
the leaves with the triangular leaf apices calculated separately.
Leaf areas blighted and defoliated were assessed from April, 1996
to June, 1996 and August, 1996 to October , 1996. Lesion counts and
areas were recorded for 20 randomly selected and tagged plants,
every 10 to 14 days during the two assessment periods. Percent leaf
area diseased was calculated by multiplying total lesion number by
average lesion area divided by total leaf area. To determine
percent defoliation, each row within the plot was divided into 0.6
m segments per row prior to each assessment date. One segment per
row was randomly selected every 10 to 14 days in each plot and
subdivided into four 0.15m sub-segments. One such sub-segment was
selected at random and the number of nodes and missing leaves
counted on each main stem. The percent leaf area blighted or
defoliated were used in determination of the area under disease
progress curve due to leaf area blighted (AUDPC-DL) and defoliation
(AUDPC-DF) using the formula of Shaner and Finney (1977).
Effect of plant age on seed infection. The percent seed
infection by A. sesami in harvested seed was assessed using
the oat meal agar plate method (Neergaard, 1979). Bacterial growth
was minimised by adding 200 ppm streptomycin sulphate to the molten
oat meal agar (OMA) cooled to 45ºC just before dispensing into
individual plates. Four hundred seeds surface sterilised with 1%
NaOCl for 5 minutes were plated in 20 plates (20 seeds per plate)
at a spacing of 1cm between the seeds. Identification of A.
sesami was based on characteristic growth colonies produced on
OMA according to Lee (1978). Each seed with the characteristic
growth colony was recorded for each of the plant ages and percent
infection computed on the basis of 400 seeds.
Data analysis. Gompertz and logistic models were fitted
to percent leaf area blighted and percent defoliation data.
Apparent rates of disease increase were obtained by regressing
transformed data against time (expressed as days after
inoculation). The most suitable model for assessing infection and
defoliation rates was identified by comparing the coefficients of
determination, the best fit and residuals obtained using each model
(Campbell and Madden, 1990). Student=s t-test was carried out separately for
AUDPC-DL, AUDPC-DF, infection and defoliation rates and seed
infection levels to determine the existence of differences between
treatments and also seasons. For each season, AUDPC-DL, AUDPC-DF,
infection and defoliation rates and seed infection levels from each
treatment were compared using analysis of variance. Significant
differences among the treatment means were identified using the
Waller-Duncan range test (Steel and Torrie, 1980).
RESULTS
Effect of plant age on disease severity. Areas under disease
progress curves for percent leaf area blighted (AUDPC-DL) due to
Alternaria leaf spot were similar during the first and the second
seasons (t = 0.65, P < 0.05). There were, however, highly
significant differences in AUDPC-DL among the five plant ages of
sesame in either season (Table 1). Sesame plants inoculated at 10
weeks of age exhibited the largest AUDPC-DL in both seasons. Unlike
AUDPC-DL, areas under disease progress curves for percent
defoliation (AUDPC-DF) were significantly larger during the first
than in the second season (t = 4.13, P < 0.01) (Table 1).
There were also highly significant differences in AUDPC-DF among
the five plant ages in both seasons. Sesame plants inoculated at 12
weeks had significantly larger AUDPC-DF than did other plant ages
except for plants inoculated at 8 and 10 weeks in season one and
plants inoculated at 10 weeks in the second season.
The Gompertz model produced slightly higher coefficients of
determination (R2) and better fit than the logistic
model. Rates of increase in percentage leaf area affected and
defoliated by Alternaria leaf spot were, therefore, estimated and
compared using the Gompertz model (Table 2). Mean rates of increase
in percent leaf area blighted (infection rates) were similar in
both seasons (t = 1.10, P < 0.05). There were, however,
highly significant differences in infection rates among the five
plant ages in both seasons. The highest infection rate was attained
on plants inoculated at 10 weeks in both seasons (Table 2), and
were significantly lower on the other plant ages studied except on
plants inoculated at 12 weeks of age in the first season and 8 and
12 weeks in the second season.
Mean rates of increase in percent defoliation (defoliation
rates) were higher during the first than second season (t = 4.69, P
< 0.01). Differences in defoliation rates among the five
plant ages in either season were highly significant
(P<0.01), with plants at 12 weeks of age having the
fastest defoliation rates in both seasons (Table 2). The lowest
rate of increase in percent defoliation was observed on plants
inoculated at 4 weeks, although defoliation rate at this plant age
did not differ significantly from those inoculated at 6 and 8
weeks.
Effect of plant age on seed infection. Seed
infection was statistically similar for both seasons (t = 0.20, P
< 0.05), with significantly higher seed infection levels
in inoculated than in non-inoculated plants (t = 2.70, P = 0.05).
Infection of sesame seed by the fungus increased with plant age up
to 8 and 10 weeks, and then declined at 12 weeks (Table 3). Seed
infection was highest when plants were inoculated at 8 and 10 weeks
old.
TABLE 1. Effect of plant age on mean areas
under disease progress curvesa for percent leaf area
blighted (AUDPC - DL) and percent defoliation (AUDPC - DF) of
Alternaria leaf spot on sesame acession SPS SIK at Kibwezi,
Kenya
Plant age (Weeks) |
Season I |
Season II |
AUDPC-DL |
AUDPC-DF |
AUDPC-DL |
AUDPC-DF |
4 |
1.13cd |
3.97bcd |
0.98cd |
2.16d |
6 |
1.68cd |
4.00bcd |
1.20bcd |
2.24bcd |
8 |
3.86abc |
4.73abc |
2.55abc |
3.34bc |
10 |
6.29a |
4.83ab |
5.08a |
3.38ab |
12 |
5.45ab |
5.30a |
4.40ab |
3.81a |
Mean |
3.68 |
4.56 |
2.84 |
2.98 |
c.v. (%) |
35.10 |
29.70 |
45.30 |
32.50 |
a Average of 3 replications; within
each experimental season, means followed by the same letter do not
differ significantly at P = 0.05
TABLE 2. Effect of plant age on rate of
increasea in percent leaf area visually infected and
percent defoliation due to Alternaria leaf spot on sesame accession
SPS SIK at Kibwezi, Kenya
Plant age (Weeks) |
Season I |
Season II |
Infection
rateb |
Defoliation
rateb |
Infection
rateb |
Defoliation
rateb |
4 |
0.068bcd |
0.015cd |
0.054bcd |
0.008c |
6 |
0.082bcd |
0.016cd |
0.018bcd |
0.009bc |
8 |
0.091bc |
0.020abc |
0.084abc |
0.009bc |
10 |
0.169a |
0.023ab |
0.141a |
0.015ab |
12 |
0.140ab |
0.024a |
0.132ab |
0.016a |
Mean |
0.110 |
0.019 |
0.098 |
0.011 |
c.v. (%) |
25.50 |
21.70 |
33.89 |
28.90 |
a Rates of increase were obtained by regressing
Gompertz-transformed disease/defoliation data against time (days
after inoculation) using the equation K = [gompit (Ymax) -
gompit (Ymin)]/ (t2 - t1) in which gompit =
-In [-ln(Y)], (Ymin) and (Ymax) being proportion of
disease/defoliation observed at the beginning (t1) and
the end (t2).
bValues shown represent averages of 3 replications;
within each season, means followed by the same letter do not differ
significantly at P = 0.01
TABLE 3. Effect of plant age on
establishment of Alternaria sesami in seed of sesame
accession SPS SIK 110 at Kibwezi, Kenyaa
Plant age (Weeks) |
Percent seed
infectionb |
Inoculated |
Non-inoculated |
4 |
9.0bc |
3.0bc |
6 |
12.0b |
4.0b |
8 |
40.0ab |
10.0a |
10 |
45.0a |
13.0a |
12 |
8.0bc |
2.0bc |
Mean |
22.8 |
6.4 |
a Seed infection did not differ significantly between
the two seasons. Infection levels shown are thus means for the
combined data.
bAverage of 3 replications as determined by the agar
plate method; means followed by the same letter do not differ
significantly at P = 0.05. LSD at P = 0.05 for comparing means
across plant age is 2.5%
DISCUSSION
This study has shown that sesame is most susceptible to
Alternaria leaf spot at 8 to 10 weeks of growth. As such, control
of Alternaria leaf spot on sesame is most critical between 8 and 10
week of age. Rotem et al. (1989) reported that
susceptibility of cotton to Alternaria macrospora was
affected by age; older plants being more susceptible. Like
Alternaria species in other pathosystems, the infection of
sesame may be one of the age-conditioned proneness to infection.
Such a phenomenon has been described for Phytophthora
infestans in potatoes (Rotem and Sari, 1983).
The Gompertz model provided good fit to the Alternaria
leaf spot disease progress data, for all the sesame plant ages.
However, coefficients of determination of Gompertz transformed
values varied among the plant ages. Area under disease progress
curve appeared to be a better indicator of relative susceptibility
of the host age than estimates of infection or defoliation rates as
it facilitated the avoidance of problems associated with imperfect
fits of disease progress data. It also resulted in the detection of
more differences among the plant ages.
Roy and Abney (1976) reported that infection of soybean seeds by
C. kikuchii decreased as plants were inoculated in later
growth stages. In the present study, seed infection by A.
sesami was least in plants inoculated at maturity, and highest
in plants inoculated between 8 and 10 weeks. These two plant ages
correspond to flowering and capsule formation. The high seed
infection levels associated with the flowering period of sesame
plants suggest that flowers may be the main infection court for
A. sesami. This period also corresponded to the highest
Alternaria leaf spot severity. These results suggest, therefore,
that the flowering period of sesame may be associated with the
natural incidence of A. sesami infection. It may be possible
to reduce infection by A. sesami by reduction of the
duration of the flowering period of sesame possibly through
breeding.
REFERENCES
- Ayiecho, P.O. and Nyabundi, J.O. 1994. Sesame Production
Handbook for Kenya. IDRC, Nairobi. 47pp.
- Barboza, C.N., Mazzani, B. and Malaguti, G. 1966. Effect of
leaf spots caused by Cercospora sesami and Alternaria
species on the yield of 10 sesame varieties. Review of Applied
Mycology 47:258.
- Bronnimann, A. 1968. Zur Keetrus von Septoria nodorum
Berk., dem Errger der Spelzerbraune ind einer Blattdurre des
Weizens. Phyto-patholgische Zeitschrift 61:101-146.
- Campbell, C.L. and Madden, L.V. 1990. An Introduction to
Plant Disease Epidemiology. John Wiley & Sons, New York.
532pp.
- Dickson, C.H. and Crute, J.R. 1974. The influence of seedling
age and development on the infection of lettuce by Bremia
lactucae. Annals of Applied Biology 76:49-61.
- El-shafey, H.A., Abdel-Rahim, M.F., Abedel-Azim, O.Z. and
Abedel- Hamid, M.S. 1976. Carryover of maize stalk-rot fungi in
seed. Agricultural Research Review 54:29-42.
- Everts, K.L. and Lacy, M.L. 1996. Factors influencing infection
of onion leaves by Alternaria porri and subsequent lesion
expansion. Plant Disease 80:276-280.
- Kolte, S.J. 1985. Disease of Annual Edible Oil Seed Crops.
Volume II: Rapeseed, Mustard, Safflower and Sesame
Diseases. CRC Press Inc., Boca Raton, Florida. 232pp.
- Lee, D.Y. 1978. Growth habits of Alternaria species on
naturally infected seeds. Korean Journal of Mycology 6:15-
20.
- Neergaard, P. 1979. Seed Pathology. Vol I. MacMillian
Press Ltd, London. 648pp.
- Nyanapah, J.O., Ayiecho, P.O. and Nyabundi, J.O. 1995.
Evaluation of sesame cultivars for resistance to Cercospora
leaf spot. East African Agriculture and Forestry Journal
60:115-121.
- Rotem, L. and Sari, A. 1983. Fertilization and age-conditioned
predisposition of potatoes to sporulation of and infection by
Phytophthora infestans. Zeitschrift Pflanzekrakein
Pflarzenschutz 90:83-88.
- Rotem, J., Blickle, W. and Kranz, J. 1989. Effect of
environment on host and sporulation of Alternaria macrospora
in cotton. Phyto-pathology 79:263-266.
- Roy, K.W. and Abney, T.S. 1976. Purple seed stains of soybeans.
Phytopathology 66:1045 - 1049.
- Shaner, G. and Finney, R.E. 1977. The effect of nitrogen
fertilization on the expression of slow-mildewing resistance in
knox wheat. Phytopathology 67:1051-1056.
- Steel, R.G.D. and Torrie, J.H. 1980. Principles and
Procedure of Statistics: A Biometric Approach. 2nd
Edition, McGraw Hill, London. 633pp.
- Warren, R.C., King, J.E. and Colhoun, J. 1971. Reaction of
potato leaves to infection by Phytophthora infestans in
relation to position on the plant. Transactions British
Mycological Society 57:501-514.
Copyright 1999, African Crop Science Society
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