Evaluation of Released and Local Groundnut Varieties against Groundnut Rust (Puccinia Arachidis) at Babile, Eastern Ethiopia

Description of the Study Site

The field experiment was conducted at Babile Sub Research Station of Haramaya University in East Hararghae Zone during 2010 main cropping season. The research station is located at longitude of 9°08’40’’North, latitude 42°21’ 30’’East at an altitude of 1650 m a.s.l. The area is characterized by bimodal rainfall pattern occurring mainly during March to May and July to October. The average rainfall is 671 mm per annum with a mean temperature of 22°C. The averaged annual temperature data for Babile show a maximum and a minimum temperature of 28.05°C and 15.52°C, respectively [8]. The type of soil at Babile Research Station is a well drained sandy-loam with pH 7.0, organic matter 1.9% and available phosphorus 3.2 ppm.

Evaluation of Groundnut Varieties for Resistance to Rust

Treatments, procedures and experimental design: A total of nine varieties (seven improved groundnut varieties, namely Werer 961, Sedi, Werer 962, Nc-4x, Shulamith, Roba and Nc-343, and two local varieties, namely Sartu and Oldhale varieties were evaluated (Table 1). The treatments were arranged in randomized complete block design (RCBD) with three replications. The plot size was 5 m x 2 m (10 m2) and there were five rows per plot and five plants per row were taken from the central three rows and 15 plants per plot were tagged for data recording, Distance between plots was 60 cm, while the distance between blocks was 1 m and row-to-row distance was 35 cm and plant-to-plant distance was 25 cm. Planting was done on 30 April 2010 by dropping two seeds per hole and the less vigorous seedlings were thinned out afterwards. All recommended agronomic practices were done as required. Rust Assessment: Disease incidence was recorded by counting the number of infected plants from the central three rows in proportion of the total plants and then, converted into percentage, while disease severity was recorded by estimating the percentage of leaf area diseased using the 1-9 disease scoring scale [9] on the 15 tagged plants in the middle three rows of each plot every seven days starting from the first appearance or onset of the disease symptoms. Disease severity scores were then converted into percentage severity index (PSI) for the analysis using the formula stated below.

𝐏𝐒𝐈 = 𝐒𝐍𝐑 𝐱𝟏𝟎𝟎

𝐍𝐨. 𝐏𝐒 𝐱𝐌𝐒𝐒

Where SNR =Sum of numerical ratings, No.PS= number of plants scored and MSS= maximum score on scale.3.4. Data collected in the field.

Disease incidence: This was assessed by counting the number of plants showing rust symptoms on the central three rows of every plot in proportion to the total plant in the row and then converted into percentage. Disease severity: Percent tissue area affected by the disease was assessed on 15 tagged plants in the central three rows of every plot. Data were recorded using 1-9 disease scale [9]. Severity was expressed as PSI computed based on 1-9 disease scale. Stand count at harvest: Number of plants in the central three rows at harvest was counted. Number of pods per plant: Number of pods per plant was recorded from 15 tagged plants at harvest. Number of seeds per pod: number of seeds per pod was recorded from 15 tagged plants from each plant 5 pods shelled to find out number of seeds. Hundred seed weight measures: 100 seed weight were recorded by taking 100 seeds from each 15 tagged plants and expressed by gram. Shelling percentage: This was calculated from 15 tagged plants by dividing the weight of seeds to the weight of pods and multiplying it by 100.ss Yield kg/ha: This was recorded from the harvested plots and converted into kg/ha. Data Analysis: The percentage data on disease incidence and severity were subjected to ANOVA to determine the treatment effects. Mean disease severity from each plot was used in data analysis. Disease progress rate was obtained from the regression of PSI data fit to logistic model loge (y/1-y) [10] with dates of assessments. The area under disease progress curve (AUDPC) was calculated for each plot using the formula of [11] as follows.

AUDPC = Σ [(Xi + Xi+1) / 2] [ti+1 - ti] i=1 Where xi is the cumulative disease severity expressed as a proportion at the ith observation (percentage of disease severity) ti is time of the ith assessment in days from the first assessment date and n is the total number of assessments made. Correlation analysis was performed to determine the relationship between yield and AUDPC across the treatments. The relationships of AUDPC, severity, disease progress rate and yield were examined using correlation analysis. Least significant difference (LSD) value was used to separate the treatment means. Data were analyzed by using statistical analysis software version 9.0.

Reaction of Groundnut Varieties to Rust

Disease Incidence: Incidence of groundnut rust during initial assessment at flowering stage varied significantly (p ≤ 0.001) (Appendix Table 1) among the varieties. The highest initial incidence (79.41%) was recorded on the variety Shulamith while it was the lowest (54.85%) on the variety Werer 962 (Table 4). There were no significant (p > 0.05) difference between Shulamith and Roba in terms of initial incidence. Shulamith had significant (p ≤ 0.05) difference from seven of the varieties in initial disease incidence with the exception of Roba. On the other hand, varieties Sedi and Oldhale had similar initial disease incidence (Table 1). Rust incidence at final assessment (i.e. at the second scoring time) was also significantly (p < 0.001) different among the varieties (Appendix Table 1). Relatively low final incidence was recorded on the varieties Werer 962 (61.80%), Sartu (64.89%) and Oldhale (65.36%) (Table 2). The levels of final disease incidence on Shulamith and Roba were also significantly different from that of the other varieties tested these varieties showed consistent reaction at their initial and final disease incidence, in that both had significantly higher initial and final disease incidence. Groundnut rust appeared in all the varieties during the third assessment (92 DAP). To this effect, the data were not included in the statistical analysis since the incidence was 100% in all of the varieties. Differences in disease incidence among the varieties might result from partial resistance, which would reduce infection efficiency, uredium size and sporulation during each cycle of infection [12]. Rust development on groundnut varieties: Groundnut rust development was evaluated using disease severity, AUDPC, disease progress rate and rust scale. These parameters were finally used to classify reaction groups among the groundnut varieties evaluated. During the growing season differences between partially resistant and susceptible varieties might become greater, even though all varieties developed some resistance with increase in age (Table 2).

Disease Severity: The mean disease severity score of the test varieties for rust disease ranged from 2.68 (resistant) to 7.88 (susceptible) (Table 2). The severity of rust on the groundnut varieties evaluated under natural infections using 1-9 disease scale [9] showed different levels of disease reaction (Figure 1). Very highly significant variations (p < 0.001) were observed in severity of rust among the varieties (Appendix Table 2). Different levels of rust severities (as percent severity index) were recorded on the different varieties tested under natural infection. The varieties more or less showed differential responses to the rust. The mean percent severity index (PSI) ranged from 61.27% for Shulamith (susceptible) to 36.41% for Werer 962 under natural infection (Figure 1, Table 2). Compared to the susceptible variety the second highest mean percent severity (58.07%) was recorded on the variety Roba (Figure 1, Table 2). This variety had a marked difference in severity from all the other varieties. On the other hand, levels of disease severity were notably low on the varieties Werer 962, Sartu and Old hale, which had 36.41%, 45.95% and 48.07%, respectively. However, none of the varieties exhibited an immune reaction to the disease. Varieties categorized under the same group were comparable in their responses to the disease parameters and rust reaction groups. The variety Werer 962 showed the lowest disease severity, relative to the other varieties, the level of severity on this variety was significantly different (p ≤0.0001) from that of all the other varieties (Appendix Table 2), whereas the varieties Werer 961 and Sedi were not significantly different in their mean disease severity, but the variety Nc-4x and Nc-343 had similar levels of disease reaction which were estimated at (54.56%) and (55.52%) mean disease severity, respectively (Table 1).

Means with the same letter in the same column are not significantly different, DAP= days after planting. The disease severities in the susceptible varieties were significantly higher than those on the other varieties in all assessment dates (Figure 1).

Click to enlarge

Figure 1: Disease severity of rust under natural infection on groundnut varieties evaluated for their reaction on different assessment days at Babile in 2010 main cropping season. Consistent findings were reported by Bromfield and Cevario (1970). Differences in mean disease severity could be due to variation in the pathogen,could be responsible where interaction between host pathogen and environment or variation within the host species. Thus this work was consistent with the previous findings. In Ethiopia, there were no promising varieties screened against rust infection. This current investigation implies the need for further studies. Area under disease progress curve (AUDPC): The area under disease progress curve (AUDPC) is a very convenient summary of plant disease epidemics that incorporates the initial intensity, the rate parameter and the duration of the epidemic which determines the final disease intensity [13]. Hence the effects of disease resistance on disease progress on crops can be evaluated using AUDPC Boiteux (1995) [14] also used AUDPC as a criterion for resistance by comparing AUDPC values of many genotypes with that of a resistant variety. Thus this study was used the following parameters for classifying the varieties into susceptible or resistant categories. The

criteria tried were low AUDPC, low infection rate, low severity, and low rust scale value according to proposed scale of reaction types for the groundnut-rust pathosystems. The reaction classes are derived from for cereal rusts and are defined by characteristic lesion types. The host responses resistant (rust scale 2-3), moderately resistant (rust scale 3-5), moderately susceptible (rust scale 5-7), and susceptible (rust scale 7-8) were detected in the present study [15]. Accordingly, the varieties were classified into either of S (susceptible), MS (Moderately susceptible), MR (moderately resistant) or resistant (R) reaction types. Among the nine groundnut varieties screened, Werer 962 was found to be resistant (R) because it had least severity, AUDPC, infection rate and rust scale. The four varieties namely Sartu, Oldhale, Sedi and Werer 961 appeared to cluster together in their severity level, AUDPC, infection rate and rust scale even if the former two varieties and the latter two varieties had different values of disease parameters, which might be described as moderately resistant (MR). Likewise, Nc-4x and Nc-343 had similar severity levels AUDPC, rust scale and different infection rates and thus were grouped as moderately susceptible (MS). The disease parameter on Shulamith and Roba was markedly higher than the rest test varieties that were identified as susceptible varieties (Table 2).

Means with the same letter on the same column are not significantly different, R= resistance, MR = moderately resistance, MS = moderately susceptible, S = Susceptible PSI= Percent Severity Index, AUDPC= Area under Disease Progress Curve, DPR= Disease Progress Rate. Values for the area under disease progress curve also varied significantly among the varieties (Table 3).

Effect of rust on groundnut varieties, yield and yield components: Groundnut varieties showed significant (p ≤ 0.001) difference in their seed yield (Appendix Table 4).The highest seed yield (1279.31 kg/ha) was obtained from the variety Werer 962, whereas the lowest (740.38 kg/ha) was obtained from Sedi. However, the yield obtained from the variety Werer 962 was not significantly different from that of Sartu and Oldhale. Similarly, yield of Werer 961 was not significantly different from that of Nc- 4x, Roba and Nc-343 (Table 4).

Oldhale (12.46) and Sedi (12.57) gave the lowest pod number per plant (Table 6). There were significant difference (p < 0.001) (Appendix Table 4) among varieties in response to stand counts at harvest. This might be due to reaction level of varieties to the rust disease. Generally, the reason why relatively highest seed yield was obtained from a given variety could not be only due to the resistance of the disease, but also due to some important genotypic characteristics of the particular variety. Association of yield and disease parameters: The association of yield and disease parameters that included percent severity index (PSI), AUDPC and disease progress rate was evaluated using correlation analysis. The results revealed that yield and percent severity index of groundnut rust were significantly and negatively correlated (r = -0.83) (Table 5). This correlation indicates that the impact of rust severity on the yield of the groundnut varieties was high. Yield and AUDPC were also significantly and negatively correlated at a slightly higher level (r = -0.85) than that of yield and percent severity index indicating that lower AUDPC implies higher yield. There was also slightly lower negative (r= -0.68) correlation between yield and disease progress rate (Table 4). A very strong significantly positive(r = 0.94) correlation was observed between percent severity index of groundnut rust on groundnut varieties and AUDPC which indicated that varieties which were severely infected showed high AUDPC values. Similarly high positive correlation was observed between percent severity index and disease progress rate (r = 0.84). There was also a positive (r = 0.85) correlation between AUDPC and disease progress rate (Table 5). This is good evidence that the use of AUDPC as a criterion to classify the varieties into reaction groups is justifiable.