MATERIALS AND METHODS

The study was conducted at the University of Ilorin, Teaching and Research Farm during the 2008 and 2009 cropping seasons. The Research farm is approximately 307 m above sea level and is located within the Southern Guinea savannah ecological zone (9°29' N, 4° 35' E) of Nigeria. The annual rainfall is between 1250-1500 mm which peaks between June and September with a characteristic dry spell in mid July/early August. The mean temperature range for the area is between 20^-35°C. The soil type is well drained sandy-loam.

Experimental design and field layout: A split-plot treatment fitted into randomised complete block experiment with three replications was established with three cowpea cultivars (IT89KD-288, IAR-48 and Ife brown), in the main plots and three planting densities (25x75, 50x75 and 75x75 cm) in the sub-plots. Each experimental plot consisted of nine (9) ridges, each 25 m long. Land preparation followed the conventional method of ploughing, harrowing and ridging. The cowpea cultivars were obtained from the National Seed Service Centre Ilorin, Kwara State Nigeria. The seeds were planted at the rate of three seeds per hole and thinning to two plants per stand was done some fourteen days later. Cypermethrin dimethrate at the rate of 50 g a.i ha^-1 was sprayed at 4 weeks after seedling emergence and twice after flowering to control insects. Weeding was done manually as and when due.

Data collection: Data collected were on number of plants per plot, number of infected plants per plot, plant height, number of pods using 10 plants from each sub-plot, pod weight and grain weight.

Percentage disease incidence was calculated from collected primary data as follows:

Harvesting: The cowpea pods harvested at maturity from each plot at 75-80 days after planting were manually threshed and weight of pods and grains appropriately measured using electronic weighing balance (Model Kerro No. Ka-3002 c).

Statistical analysis: All collected data were subjected to analysis of variance using the statistical package for the social sciences SPSS version 15.0. Treatment means, where significant, were separated using the New Duncan’s multiple range test at 5% level of probability.

RESULTS
Percentage disease incidence: Table 1a and b are the results of the analysis of variance on percentage disease incidence for the 2008 and 2009 seasons. The results indicated that variety, density and a combination of these two factors had significant effect on incidence of viral disease. An assessment of the results for 2008 season on varietal basis (Table 1a) showed that by the 8th week after planting, variety IT89KD-288 had the lowest viral incidence (10.3%) while variety IAR-48 had the highest viral incidence of 28.6%. Analysis of results on the effect of planting density showed that planting density at 25x75 cm consistently produced the lowest viral disease incidence.

Table 1a:	Main and interactive effects of variety and planting density on percentage viral disease incidence in cowpea during the 2008 cropping season
Means with the same letter. (s) within each segment of a column are not significantly different at p = 0.05 level using Duncan’s multiple range test

Table 1b:	Main and interactive effects of variety and planting density on percentage viral disease incidence in cowpea during the 2009 cropping season
Means with the same letter. (s) within each segment of a column are not significantly different at p = 0.05 level using Duncan’s multiple range test

The percentage disease incidence at the 8th week was 11.3%. The lowest plant population density (i.e., spacing at 75x75 cm) led to the highest viral disease incidence (28.4%). Analysis of the effect of variety and density on viral incidence at flowering showed that variety IT89KD-288 at planting density 50x75 cm had the lowest incidence of viral disease (19.8%). The highest incidence (43.7%) was attained when variety IAR-48 was planted at a density of 75x75 cm.

Infection levels were generally higher for the 2009 season (Table 1b). However, the results obtained followed the same trend that was observed in the 2008 season. An analysis of varietal effect per sec showed that variety IAR-48 cumulatively manifested the highest infection levels from the 4th to the 8th week. It attained an incidence rate of 52.3% which was significantly higher compared with the variety IT89KD-288 which had the lowest value of 34.5%. Consideration of planting density as a main effect showed that at 25x75 cm, significantly lower viral infection was recorded compared to other density regimes. However, the combination of variety IT89KD-288 at the planting density of 50x75 cm produced the significantly lowest viral disease incidence (17.3%).

Growth attributes
Plant height: The results showed that variety and planting density interaction significantly influenced plant heights. At flowering (8th week) in the 2008 season as shown in Table 2a, variety Ife-brown at a density of 25x75 cm produced the significantly tallest plants (44.7 cm) while the combination of variety IAR-48 at density 75x75 cm had the shortest plants (29.9 cm). In the 2009 season (Table 2b) the effect of the combinations showed that there was a higher growth rate for the year compared to 2008. An analysis of the results, however, showed a pattern that was not different from the 2008 season.

Table 2a:	Combination effect of variety and planting density on plant height of Cowpea during the 2008 cropping Season
Means with the same letter (s) within each column are not significantly different at p = 0.05 level using Duncan’s multiple range test

Table 2b:	Combination effect of variety and planting density on plant height during the 2009 cropping season
Means with the same letter (s) within each column are not significantly different at p = 0.05 level using Duncan’s Multiple Range Test

Table 3:	Combination effect of variety and planting density on yield attributes
Means with the same letter (s) within each column are not significantly different at p = 0.05 level using Duncan’s multiple range test

The results shown in the Table 2b, indicate that at the flowering stage (8th week after planting), the combination of variety Ife-brown at density 25x75 cm produced the tallest plants (58.1 cm) while the combination of variety IAR-48 planted at a density of 75x75 cm had the shortest plants (46.3 cm).

Yield attributes: Table 3 shows the results of analysis of the combination of variety and planting density on some yield attributes for the two years of the experiment. In both seasons of the study, cowpea variety IT89KD-288 at density 50x75 cm produced the significantly highest yields per plant. In the first year (2008) for example, the number of pods, weight of dry pods and weight of dry seeds averaged 32.8, 59.0 and 51.6 g per plant, respectively while variety IAR-48 at density 50x75 cm had the lowest yields. In the second year of the experiment (2009), though yields were slightly higher than in the preceding year, the observations were similar to that of the first year, with variety IT89KD-288 at density 50x75 cm also having the highest yield attribute values while variety IAR-48 at density 50x75 cm the lowest.

DISCUSSION

The identification of tolerant cowpea varieties is a crucial step towards achieving the objective of economic control of viral diseases. Tolerance in this case can be defined as the ability of a genotype to produce a good crop even when it is infected with a pathogen (Agrios, 2005). Nevertheless, different sources of resistance is recommended for enhanced durability of resistance (Ochola and Tusiime, 2011). The present study showed that the incidence of virus diseases was highest in variety IAR-48 and lowest in variety IT89KD-288. The ability of variety ITKD-288 to have less virus incidence could be attributed to the genetic inherencies in the variety. This assertion is in agreement with Goenaga et al. (2008) who reported that the different yield potential of cowpea genotypes grown under virus pressure was due to the genetic diversity. Alsemaan et al. (2011), likewise reported the existence of genetic diversity within Rosa damascene accessions used to broaden the production of rose oil.

Virus disease incidence was observed to be highest at a plant population density of 75x75 cm, and lowest at a population density of 25x75 cm, this implied that low virus incidence existed under high plant population. It has long been known (Matthews, 1991) that half of all known plant viruses are transmitted by aphids. Allarangaye et al. (2007) reported that the transmission of Rice Yellow Mosaic Virus (RYMV) was by coleopterous insects. One factor which governs the rate of virus epidemics, is the interaction of the aphid vector with the plant environment especially plant density (INHS, 1995). It can therefore be inferred that the low virus incidence recorded at high plant population is attributable to reduced aphid infestations which hitherto would have served as vectors of the virus pathogens.

This study also showed that cowpea variety IT89KD-288 planted at a density of 50x50 cm produced the highest yield parameters. This suggests that although planting density could be effective in viral disease control, it is better determined on varietal basis. Findings by Payne (2000) corroborates the point that competition for space can become very intense when plant population is high which could then predispose plants to mechanical inoculation of virus pathogens occasioned by leaf contact.

CONCLUSION

This study concludes that moderately high plant population could be an effective tool in the control of viral diseases of cowpea especially in combination with the right variety. To this end variety IT89KD-288 at a planting density of 50x75 cm which performed best in this study is recommended to help control yield losses due to virus attack in this ecological zone.