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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
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


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


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


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.


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).


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



































c.v. (%)





aAverage 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































c.v. (%)





aRates 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





















aSeed 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%


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.


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Copyright 1999, African Crop Science Society

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