MATERIALS AND METHODS

Experimental treatments design and lay out: Field experiments were carried over two growing seasons at the Faculty of Agriculture field station, University of Nairobi, between July 2008 and May 2009. The site is a hot spot for downy mildew and purple blotch of onion. The experimental design was randomized complete block design with split plot lay out. The three onion varieties (Bombay Red, Red creole and Orient F1) comprised the main plots of 4.5x6 m while the vegetable intercrops (carrot, spinder plant and French bean) comprised the subplots of 4.5x2 m. Two and a half month old onion seedlings were separately transplanted in the main plots at a spacing of 30x10 cm. Carrot (var. Nantes), spider plant and French bean (var. paulister) were separately intercropped with onion within the subplots such that one row of vegetable alternated four rows of onion. The intra-row spacing was 20 cm for French bean, 10 cm for carrot and 15 cm for spider plant. Control plots consisted of pure stand onion variety with and without fungicide Tata Master™. (metalaxyl 8%+ mancozeb 64%) applied at the rate of 3 kg ha^-1 weekly. The vegetable intercrops were established before transplanting the onion. Guard rows of susceptible onion variety and volunteer plants infected with both downy mildew and purple blotch were allowed to grow around the experimental plots to provide inoculum. The plots were fertilized with di-ammonium phosphate at the rate of 80 kg N ha^-1 at planting and top dressed with calcium ammonium nitrate (26% N) at the rate of 40 kg N ha^-1, 3 and 6 weeks after transplanting. Natural infection of onions by downy mildew and purple blotch was allowed to occur. Data collected included disease incidence and severity and yield of both onion and vegetables.

Downy mildew and purple blotch assessment: Recording of disease incidence and severity commenced at the onset of symptoms and was done weekly until physiological maturity of the crop. Downy mildew and purple blotch severity were assessed visually as the percentage leaf area diseased following assessment scales described by Ahmad and Khan (2002) and Kingiri (1993), respectively. Disease severity was evaluated on five alternate leaves of each of the ten plants picked at random from the inner rows of each plot making a total of 200 leaves from 40 plants per treatment. Disease incidence was determined by counting the number of diseased plants out of the total number of plants in each plot. The incidence data and severity was used to calculate the area under disease progress curve (AUDPC) according to Sparks et al. (2008).

where, Y is disease incidence at time T and i are the time of the assessment.

Onion and vegetable yield assessment: Bending the necks cured the onion bulbs and the bulbs harvested when the foliage was dry. Bulbs from each plot were graded into marketable and non-marketable yield according to HCDA (1991) and each grade weighed separately. French bean pods and mature spider plant leaves were harvested weekly starting from 37 days and fourth week, respectively, after sowing. Carrot was harvested from 120 days after sowing and the total yield for each crop was extrapolated into kilograms per hectare. Gross return per hectare for each onion-vegetable intercrop or pure stand crop was determined by multiplying the average prevailing market price per kilogram and the respective yield per hectare as follows:

Where:

Data analysis: All data was subjected to analysis of variance (ANOVA) using Genstat computer software package (Lawes Agricultural Trust Rothamsted Experimental Station 2006, version 9). Means when significantly different were separated using the Fisher’s protected LSD test at 5% probability level (Steel and Torrie, 1981).

RESULTS

Effect of vegetable intercrops on downy mildew: Intercropping onion with vegetables reduced downy mildew incidence and severity on all the three onion varieties (Table 1). However, only spider plant showed consistent and significant (p≤0.05) reduction of the disease incidence, severity and the area under disease progress curve (AUDPC) by up to 24.5% on all onion varieties (Table 2). Carrot and French bean significantly reduced downy mildew severity on Red creole and Orient F1.

Table 1:	Mean downy mildew incidence, severity and area under disease progress curve on three bulb onion varieties intercropped with different vegetables
LSD: Least significant difference, CV: Coefficient of variation

Table 2:	Mean percentage reduction in downy mildew and purple blotch on onion sprayed with fungicide or intercropped with vegetables
AUDPC: Area under disease progress curve

Table 3:	Mean percentage incidence, severity and area under disease progress curve (AUDPC) of downy mildew and purple blotch on three onion varieties
AUDPC: Area under disease progress curve

The AUDPC was significantly reduced on Orient F1 by all the vegetable intercrops but only French bean and spider plant had a significant effect on the AUDPC on variety Red creole (Table 1). The three onion varieties significantly differed in susceptibility to downy mildew, with Orient F1 showing the highest disease incidence, severity and the AUDPC (Table 1, 3). Variety Red creole was the least susceptible with up to 17% less disease compared to the Orient F1.

Table 4:	Mean purple blotch incidence, severity and area under disease progress curve (AUDPC) on three bulb onion varieties intercropped with different vegetables
LSD: Least significant difference, CV: Coefficient of variation

Effect of vegetable intercrops on purple blotch: Only fungicide application and spider plant reduced purple blotch incidence compared to the untreated controls (Table 4). Among the three varieties, Bombay red and orient F1 had higher disease incidence compared to red Creole. Intercropping onion with vegetables significantly (p≤0.05) reduced purple blotch severity and AUDPC with spider plant having the highest reduction of up to 18% compared to the untreated controls (Table 3, 4). The three onion varieties differed in susceptibility to purple blotch, with Orient F1 showing significantly higher infection levels compared to Bombay red and Red creole.

Effect of vegetable intercrops on bulb onion yield: Bulb yield for all the three onion varieties was significantly (p≤0.05) reduced by intercropping with all the three vegetables (Table 5). Carrot showed the highest yield reduction of up to 60% followed by spider plant (47%) and French bean (29%) when compared to pure stand onion with fungicide application. Pure stand onion without fungicide application showed a reduction in bulb yield of up to 17%. A higher reduction in yield was observed in marketable bulbs compared to the total bulb yield. The reduction in bulb yield due to intercropping of onion with other vegetables was compensated by the additional yield from the vegetables resulting in higher gross return per hectare (Table 6).

Table 5:	Total and marketable bulb yield (tons/ha) of three onion varieties intercropped with carrot, French bean and spinder plant as compared to pure stand and fungicide spray
LSD: Least significant difference, CV: Coefficient of variation

Table 6:	Average return per hectare (KES) of bulb onion in pure stand or intercropped with other vegetables
KES: Kenya Shillings, LSD: Least significant difference, CV: Coefficient of variation

DISCUSSION

This study has demonstrated that vegetable intercrops were effective in reducing downy mildew and purple blotch on onion. The results of the study agree with reports by Sahile et al. (2008) who demonstrated the suppression of chocolate spot (Botrytis fabae) on faba bean when intercropped with field pea, barley or maize. The reduction of downy mildew and purple blotch diseases on the onion crop may be attributed to reduction of the probability of fungal spores to encounter onion plants amongst the vegetable intercrops as indicated by Garrett and Mundt (1999). Studies by Ramert and Lennartsson (2002) have indicated that intercropping reduces the proportion of susceptible host tissues and, therefore, impacting on the production, amount and efficiency of the disease inoculum and, therefore, limiting spread and development of the disease. In addition, intercropping increases the space between the rows of onion crop, resulting in greater distance to be travelled by the disease inoculums. Further, the vegetable intercrops may have created a physical barrier that intercepted or filtered the fungal propagules, thus effectively limiting dissemination of the pathogen.

Among the intercrop species, spider plant showed the greatest suppression of both the downy mildew and purple blotch diseases on the onion crop. This is in agreement with Makelo (2004) who showed that spider plant possesses antifungal properties and its extracts inhibited the growth of Alternaria porri. The suppression of the diseases could also have been due to inherent allopathic effects in the spider plant (Maniania et al., 2002). Other factors that could have contributed to disease reduction include creation of unfavorable micro-climate that inhibited the foliar diseases on onion (Ramert and Lennartsson, 2002) and fungicidal properties in the spider plant (Makelo, 2004).

Onion variety Red creole was the least susceptible to both downy mildew and purple blotch diseases compared to varieties Bombay red and Orient F1. Similar genotypic variation between onion varieties has been reported by Abubakar and Ado (2008a, b), Malik et al. (2003) and Soumah and Keita (2004). The observed phenotypic variation among the onion varieties could be due to both genotypic and environment factors . Varietal differences have been shown to be due to inherent biochemical characteristics, such as quercetin levels, resulting to differences in bolting, marketable yields and disease susceptibility. Studies done in Uganda by Abubakar and Bakare (2008) showed genotypic variations among onion populations and indicated that Red creole was resistant to purple blotch.

Intercropping onion with the vegetables reduced both the total and marketable bulb yield. This is in agreement with findings by Trdan et al. (2006), Kabura et al. (2008) and Kucharczyk and Legutowska (2003), who reported that intercropping leek with carrot reduced yields. The reduction in yield could have been caused by the reduction in onion population and intercrop competition for nutrients, water and light (Ansari and Mamghani, 2008; Ofosu-Anim and Limbani, 2007). The reduction in bulb yield may be compensated by the yield from the vegetable intercrops and, therefore, no net loss to the farmer. In addition, the intercrop may act as buffer against total crop loss and diversifies the production. Therefore, the onion-vegetable intercrop would be suitable to small-scale farmers who do not have adequate resources for purchase of chemical fungicides. The system would also be very ideal for organic vegetable production, in which chemical application is not desirable. Apart from increasing total farm productivity, mixed species cropping can bring many important benefits such as improvement of soil fertility management and suppression of pests and/or diseases (Ramert and Lennartsson, 2002). Other benefits of intercrop system include erosion control, reduce leaching of nutrients, balanced distribution of labour and higher economic returns than sole cropping (Odo and Futuless, 2002; Blaser et al., 2007). In addition, the system would contribute to food and nutrition security for the small farmers by offering a continuous supply of vegetables during the growing season.

CONCLUSION

The study showed that intercropping bulb onion with spider plant would be beneficial in the management of downy mildew and purple blotch in small holder onion production systems. Although the intercrop would reduce onion bulb yield, the combined gross return per unit area would be comparable to that obtained in onion pure stand.

ACKNOWLEDGMENT

This research was carried out with the financial support from the Deans Committee Research Grant of the University of Nairobi, Kenya.