Site description
The study was carried out in the Central Wooded Savanna ecological zone of Uganda, in Nakaseke (0°43’29” N, 32° 54’04” E) and Nakasongola (1°18’32” N, 32° 27’23”E) districts. In Nakasongola, the annual daily temperatures range from 18oC to 35oC, with mean daily maximum of 30°C. Rainfall ranges between 500 to 1000 mm per annum and there are two rain seasons. The vegetation in the study site mainly comprises three vegetation cover types depending on the extent of anthropogenic activities/disturbance on specific ranch sites. The three vegetation cover types include dense vegetation cover (>50% basal cover), sparse vegetation cover (25 to 50% basal cover) and bare ground. The area is characterized by prolonged droughts and floods due to shifting rainfall pattern (Nimusiima et al. 2013). Hitherto dominated by livestock grazing, the area is increasingly changing in land use, with crop farming especially for maize production becoming common. The altitudinal range is 600-1160msl.
Nakaseke site has traditionally been described as the coffee–banana farming system. This area falls within an altitudinal range of 1086–1280 masl, with mean annual rainfall of up to 1100 mm. The annual daily temperatures range from 16 ◦C to 30 ◦C.
Bi-modal rainfall distribution characterizes the two districts with the first rainy season extending from March to June, while the second rainy season starts in late August or early September to November-December (Ogwang et al.2016). The main rain season occurs from March-April to June July while the second rain season follows from August to October November. A long dry season occurs from December to February while a short spell comes around July-August.
Trial gardens were established in two districts of Nakasongola and Nakaseke, each district taken as a site. In each site replication was done in four villages. In Nakaseke trial gardens were established in the villages of Kiziba, Namirali, Kalagala and Kyamutakasa while in Nakasongola they were established in Naitondo, Kasambya, Kalobokwe and Kiralamba (Figure 1 & Figure 2)
Field layout
Twenty-three groundnut varieties were planted in each site. The varieties included improved and farmer local varieties. The seeds for the improved varieties were sourced from the National Agricultural Semi-Arid Resources Research institute (NASARRI) while the farmer local varieties were from farmers in Amuria district. Each variety was planted to a single plot of 3 m x 4 m and at a plot-plot spacing of 1m with a plant population of 300 plants. A row to row spacing of 40 cm, and plant to plant spacing of 10 cm was used.
Data collection
Data was taken on common groundnut diseases and pests. The diseases included; Groundnut Rosette, Leaf Spot, Root-rot, and Groundnut Mosaic while the pests included aphids and leaf minor. Disease and pest incidences were assessed (at 1, 2 and 3 months after planting by counting the number of plants affected per variety (NaSARRI, unpublished). In addition, plant growth performance was assessed by measuring plant height while germination uniformity was scored 2 weeks from germination (Wood and Roper, 2000). Using a scale of 1 to 4, where 1= poor and 4 = very good performance. Stay green as a measure of drought tolerance was also evaluated using a scale of 1 to 4 which were subsequently converted to percentages. Yield was assessed in terms of number of pods per plant measured on twenty selected plants from five middle rows per plot for each variety
Disease scores
Leaf spot (Early and late leaf spot disease)
Each leaf spot disease was scored separately using 1-9 scale on periodic basis after first appearance of the disease and subsequent scoring was done at an interval of 15 days and stopped at 20 days before harvest. Leaf spot was recorded using 9-point scale as described below (Subrahmanyam, P. et al. 1995). The severity scores were used to compute the area under disease progress curve AUDPC values (Van Der Plank 1965).
Groundnut Rosette Virus Disease (GRVD) severity rating
Visual assessments and scoring of GRVD severity were carried out thrice; at 30, 60 and 90 days after planting (DAP). The disease severity on each individual plant was rated using a quantitative scale adapted from Waliyar et al. (2007) as follows: no visible symptoms on leaf (highly resistant), rosette symptoms covering 1- 20% of leaf area but no obvious stunting (resistant), rosette symptoms on 21-50% area of leaf with stunting (moderately resistant), severe rosette symptoms on 51-70% area of leaf with stunting (susceptible), and severe symptoms on 71-100% area of leaf with stunting (highly susceptible). Area under the disease progress curve (AUDPC) values were calculated.
Root rot
This disease was scored after first appearance of plants showing rot root symptoms and subsequent scoring was done at an interval of 15 days up to 30 days after planting. Scoring was done based on visual rating scale of 0 to 3 where: 0 = clean, 1 = slight, 2 = moderate and 3 = severe (Ledingham et al. 1973). Area under the disease progress curve (AUDPC) values were calculated.
Groundnut Mosaic (CABMV)
Disease severity was scored using a scale of 1-5 (Adamu et al.2015) as follows: 1 = no symptoms (apparently healthy plant); 2 = mild mosaic (10-30 % infection); 3 = moderate mosaic (31-50 % infection); 4 = severe mosaic, chlorosis and stunting; (51-70 % infection); 5 = very severe mosaic, chlorosis, stunting; and plant dead (>70 % infection). Area under the disease progress curve (AUDPC) values were calculated.
Leaf miner and aphids
Observations on leaf miner incidence and defoliator damage were recorded regularly at 15-days interval. Observations were made on top five leaves of five randomly selected plants in each replication for number of leaflets damaged by leaf miner and extent of defoliation by defoliators. From these observations, per cent incidence of leaf miner and per cent defoliation were calculated.
Agronomic data
Data on selected agronomic traits were collected on plot basis. Yield data were recorded from five middle rows, excluding plants at the end of rows for each of the variety, based on number of pods per plot. All plants were clipped at the soil surface, the pods were dug up and pods were then detached, bulked together and counted.
Stay green trait
Starting from pod initiation to physiological maturity, visual scoring for stay-green were carried out at two-weeks intervals. The stay-green characteristic of the genotypes was scored on a scale of 1 to 5 based on the proportion of the total leaf area that had senesced with 1 being no leaf senescence and 5 completely senesced plant (Xu et. 2000). The stay green scores were used to compute the leaf area under greenness (LAUG) values (Joshi et al. 2017).
Data analysis
Data for each variety was summarised using descriptive statistics with means presented with respective standard error of the means. All variables were tested for normality using Shapiro-Wilk test and the strongly skewed variables were transformed prior to analyses of variance where necessary, to meet the assumption of normality and homogeneity of variances. Variables expressed as percentages (%) were arcsine-square-root (+0.5) transformed, while counts of individuals were log (log (x +1)) transformed. Where transformation was not sufficient to improve data shape, an appropriate non-parametric test was applied. The differences among varieties in yield performance was compared using analysis of variance (ANOVA), with post-hoc means separation tested using Tukey (HSD) at 5% probability level. Differences in medians of germination rate (%), growth (%), pest and disease incidence, and drought tolerance among varieties were compared using Kruskal-Wallis test, with Mann-Whitney post-hoc medians separation at 5% probability level. Site and variety interactions in growth performance, pest, and disease and drought tolerance were tested with General Linear Model (GLM) two-way analysis of variance (ANOVA). Where the GLM test indicated significant differences, post-hoc Tukey (HSD) test was used. To assess similarity among varieties, hierarchical cluster analysis using Bray-Curtis distance measure was used to depict variety performance similarity with dendrogram. Correspondence analysis ordination with symmetric scaling was used to assess associations between pest and diseases, and the various varieties. All the tests were done using PAST software (Oyvind 2002).