Effect of Striga gesnerioides on Cowpea (Vigna unguiculata L.Walp) Yield Components CURRENT STATUS: POSTED

Background: The parasitic plant Striga gesnerioides (Willd) Vatke is becoming a threat to cowpea production in the three northern regions of Ghana (Derived Savanna, Southern Guinea and Northern Guinea Savanna. The objective of this study was to determine the extent of yield loss due to Striga in F3 progenies and their parental lines. A total of twelve cowpea genotypes were evaluated to ascertain the level of damage by S. gesnerioides. Results: The results indicated varietal differences with respect to Striga susceptibility in the genotypes studied. The susceptible varieties (Asomdwee and Hewale) showed an array of emerged Striga shoots. The F3 progenies (s52, s37, s147, s272) also exhibited the same trend. The resistance genotypes (IT99K-573-1-1 and GH3684), however, were totally devoid of Striga shoots. Similarly, some of the F3 progenies (r246, r286, r282 and r69) were completely without Striga shoot. Striga height was higher on susceptible genotypes than those that are moderately resistance. The results obtained from this study revealed a percentage reduction in pods per plant range between 45.12 and 49.53 %; grain yield reduction ranged between 78.22 and 87.17% on susceptible genotypes which indicated that Striga had greater influence on yield. Fodder yield was also significantly reduced between 70.59 and 73.03% in susceptible genotypes. There was a strong negative correlation (r = -0.724) between grain yield and days to Striga emergence. A significant negative correlation (r = -0.762) was also found between Striga shoot per plant and grain yield. Conclusion: Striga infestation reduced pods per plant to 49%, grain yield loss to 87% and fodder yield loss to 73%. This study has revealed potential F3 genotypes resistant to S. gesnerioides under pot experiment. The implication is that if susceptible genotypes are grown the farmer stand the chance of loosen everything to the parasite.

among include: abiotic factors such as poor soil fertility, drought, and high temperature. In Ghana, total areas under cowpea cultivation were over 160,000 hectares which produced an estimated amount of 142,000 tonnes in 2011 [4]. An average yield of cowpea in Ghana is about 1.25 metric tonnes per hectare was observed in farmers' field [4]. Cowpea is additionally susceptible to a wide range of bacterial, fungal, viral diseases and insect pest. However, biotic limitation such as S.
gesnerioides cause a restriction to cowpea production in the three regions of northern Ghana [5].
Parasitic weed S. generioides (Willd) Vatke is an obligate root-parasitic flowering plant that influence the growth of cowpea and other legumes [6]. The genus Striga is a member of Scrophulariaceae which contains about 50 species [7]. Most members from the Scrophulariaceae are holoparasitic (without chlorophyll and absolutely reliant on the host for natural carbon, water and nitrogen), some are hemiparasitic (with chlorophyll) [8]. The geographical distribution of S. gesnerioides comprise West and Southern Africa, India, Asia, Europe and USA [9]. However, in West Africa S. gesnerioides is reported to occur in Burkina Faso, Niger, Nigeria, Benin, Togo, Ghana, Mali and Cameroon with one race designated to each country [10]. Studies have shown that Striga infestation is severe in Ghana; and affect cereals and legumes [11,12,13,14].
The parasitic weed S. gesnerioides also referred to as witch weeds in some localities and a destructive parasites as far as cowpea production is concern. The seeds of the parasites can remain dormant in the soil until a suitable host is planted. The Striga seeds are microscopic in size measuring 0.20mm to 0.35mm long, weighing 4 to 7µg [15]. Because of the miniature size of the seeds, they are effectively scattered by wind, water and animal. Under farming situations, the seeds can pollute harvested products. However, the major means of dispersal is through machinery, tools and clothing [9]. Each Striga plant can produce up to 90,000 seeds [16]. Moreover, adaptation and inactive nature of S. gesnerioides permit the seeds to remain dormant in the soil for quite a long time. This underground seed stock under suitable environmental conditions germinate and infest suitable growing hosts present in that field [10]. S. gesnerioides thrives in areas with low precipitation and poor soil fertility conditions as found in the arid and semi-arid regions. This implies thatareas with water stress and low soil fertility will continue to experience Striga invasion.
Cowpea yield loss associated with S. gesnerioides have been reported to range between 83 and 100% [10]. Often, S. gesnerioides population of over 10/plant on a genotype can result in 100% yield loss [17]. This implies that as population keep increasing and pressure on available land continuous, and soil fertility reduces, Striga will multiply at a faster rate. Striga harm happens at different parts of cowpea plants [18] influencing the physiological and biological processes of the crop. Decreased leaf area and photosynthesis, inadequate flowering and podding with reduced seed enlargement have been reported [18].
Different control measures, including cultural practices, chemical and biological controls, and host plant resistant have been suggested [15]. Nevertheless, no single strategy seem to be fully adequate in the control of this parasite. The most effective way to control the parasites is the use of host plant resistance, which is economically affordable to resource-poor farmers. The objective of this study therefore was to evaluate the impact of Striga on the off-springs of selected cowpea genotypes and also quantify the extent of damage Striga may have on yield components.

Experimental Site
The experiment was conducted in the screen house at CSIR-Savanna Agricultural Research Institute at Manga-Bawku in pots during 2015 and repeated in 2016 growing seasons. The study area lies latitude 11°02'60.00" N and longitude 0°13'60.00" E.and with 229 meters elevation above the sea level. It shares boundaries with Burkina Faso to the north, Republic of Togo to the east. The topography is slightly undulating with heights of 120-150 metres above sea level. Soils are generally of the savanna ochrosol type which are shallow and low in fertility. Soils in the area are mostly sandy loam, clay loam to silty loam. The area is characterized by one rainy season from May to June or September to October. There is a pronounced long dry season from November to mid-February and characterized by cold, dry dusty harmattan winds. The annual rainfall of the area is between 800-850mm and average temperatures is 32 0 c -37 0 c. The vegetation of the area is Sahel Savanna woodland type characterized by short scattered drought resistance tress and grasses.

Experimental material and details
The two resistant varieties used were IT99K-573-1-1 and GH3684. IT99K-573-1-1 was obtained from International Institute of Tropical Agriculture, Nigeria, Ibadan while GH3684 was obtained from CSIR-Plant Genetic Research Institute, Ghana, Bunso. Two susceptible varieties (Asomdwee and Hewale) were obtained from CSIR-Crops Research Institute, Ghana, Fumesua. Direct crosses were made between the resistant genotypes and the susceptible varieties to obtain two (2) sets of F 1 progenies (IT99K-573-1-1 x Hewale and GH3684 x Asomdwee). The F 1 progenies were selfed to obtain F 2 seeds and further selfed to obtain F 3 seeds. In all eight (8)

Data collection and analysis
Data were collected on week after planting with emergence of Striga (WAP) and days to 50% flowering. Striga height (cm) was measured on 15 randomly selected plants as the distance from the soil surface to the tip of the shoot and the number of branches were counted. The Striga height was measured two weeks after emergence and it was subsequently measured weekly for five (5) weeks.
The cowpea plants matured eight (8) weeks after planting.
The cowpea grain yield, number of pods per plant were counted on both infested and non-infested pots. Pod and seed weights were also determined. After the pods were separated, the leaves and stem were oven dried for 48 hour at 180 o C to estimate fodder yield.
All field data recorded were subjected to analysis of variance (ANOVA) using GenStat Version 12.
Means were compared using Least Significant Difference (LSD) at 5% level of probability. Pearson's correlation coefficient was used to compute correlation between emerged Striga, number of pods per plant, grain, fodder yields and other components.
The results obtained on these variables from Striga infested cowpea plants were compared with that from non-infested cowpea plants, as shown below.
[The formula could not be inserted here. It can be found in the supplemental files]

Results And Discussion Decline in Cowpea Characters
The analysis of variance revealed significant differences among the genotypes (P<0.05) ( Table 1).
Striga infestation significantly reduced the yields of all the susceptible genotypes. The effect of Striga emergence and level of invasion affected most of the traits measured (Table 1). Mean days to Striga emergence ranged from 35 to 48 days. The results showed that two susceptible varieties (Asomdwee and Hewale) and four of the F 3 progenies (s52, s37, s147 and s272) showed Striga susceptibility traits with yellowing of leaves, chlorosis, necrotic and stunted growth. The two resistant genotypes (IT99K-573-1-1 and GH3684) and four F 3 progenies (r246, r286, r282, r69) were absolutely devoid of Striga shoots emphasizing that these lines were resistant. Godwa et al. [19], pointed out that the response to Striga varied among genotypes signifying that differences exist in the ability of these plants to identify the pest and to trigger defence response mechanisms. Botanga and Timko [7], reported that inconsistency appeared to be the result of the collapse of the parasite to establish appropriate vascular connection with the host. This means that the linkage between the host and parasite must be strong for proper establishment. But if the connection fail the parasite stand the chance of been deprived of food and eventually die. In actual sense, the resistant genotypes were unsuccessful host for the parasites. This implied that resistant parasite has a defence mechanisms that fight against the parasite. The resistant genotypes have fairly good growth and development, where less assimilates are channelled to the parasite as reported by Hibbered et al. [20], and Alonge et al. [21].This indicated that the nutrients and water were readily available for the host rather than the parasite. According to Hibbered et al. [20], final biomass accumulation by cowpea infested with S. gesnerioides was appreciably lesser than that of uninfected plants.
In terms of Striga shoots per plant the susceptible genotypes had on average 27 (emerged seedlings) while F 3 progenies had Striga shoots ranging from 2 to 15 per plant, indicating that some of the F 3 progenies were moderately resistant. This is not surprising because a cross between susceptible and resistant genotypes would yield an array of progenies with dominant and recessive traits. Striga height was significantly (P>0.05) different among genotypes (Table 1). Striga height was higher on genotypes that had higher number of Striga compared to those that had fewer Striga emergence.
Hence the productive efficiency of the genotype was very much reduced. It is reported that a vascular connection is established when Striga infest its host, allowing the weed to imbibe water and nutrients that are essential for its growth and development [15].

Reduction in harvestable yields
The study revealed a reduction in cowpea character traits (pods per plant, grain yield and fodder yield) measured on susceptible genotype. Significant differences were observed among the genotypes for number of pods per plant, grain yield (kg/ha) and fodder yield (kg/ha) ( Table 2). The susceptible genotypes recorded fewer pods per plant compared with the resistant or F 3 progenies.
The percentage yield reduction for the number of pods per plant for the susceptible varieties range from 45.12 to 49.53%, while F 3 progenies that were also susceptible had a yield loss ranging from 16.17 to 38.84%. The result revealed that the progenies, though susceptible had moderate susceptibility compare to the parental lines. This implies that the parasite activities on the roots reduced the efficiency of the cowpea plants. For this reason the affected plants were less able to produce adequate dry matter per pod. Press [22], pointed out that affected plants produce lower biomass due to parasite-host competition for water and carbon. The affected plants have hindered development, with reduced leaves, (leaf size) poor flowering and no pod formation. The grain yield also demonstrate a reduction in the susceptible genotype with its yield ranging between 100 and 212kg/ha, while resistant genotype had yield between 1.045 and 1.062Mt/ha and F 3 progenies moderately resistant ranged from 316 to 1050kg/ha. Yield losses were statistically significant among the susceptible, resistant and F 3 progenies ( Table 2).
The grain yield loss from susceptible varieties ranged from 78.22 to 87.17%. This implies that if a farmer cultivates the susceptible variety the chances are that yield losses would be 87% to the parasite. Also under severe situations the farmer loses the entire farm to parasite infestation. Alonge et al. [21], pointed out that on susceptible cowpea, Striga infestation induced grain yield losses by 78.9 to 86.2%.
Fodder yield also showed significant reduction among the genotypes. The susceptible varieties suffered fodder yield loss of between 70.59 and 73.03%; indicating there will be shortage of cowpea haulm during dry seasons. Reduction in the haulm thus will lead to scarcity of fodder for animals.
Hence, farmers who engage in fodder production may also experience lower income.
The pod length, 100-seed weight and number of seeds per pod had significant differences among the genotypes ( Table 3).The susceptible genotypes had reduced pod length ranging from 7.40 to 8.40cm while resistant genotypes had 13 to 14cm and F 3 progenies had 10 to 13cm. Striga effect resulted in the reduction of pod length (ranging between 31.88 and 37.17%), 100-seed weight between 31.39 and 36.30% and number of seeds per pod between 32.29 and 37.15%. In this study, those that upheld higher number of Striga additionally recorded lower number of pod per plant. This suggests that the photosynthetic capacity of the cowpea plant was reduced [23]. The Striga had influence on the physiology and growth of cowpea thereby making it stunted in growth. The decrease in grain yield could also be due to the reduction in root nodulation and root growth and competition between the host and parasite. Therefore, Striga drastically reduced a number of yield components.  Table 3. Mean Pod length, 100 Seed weight and Number of seeds per pod of twelve Cowpea Genotypes There were significant differences among the genotypes for days to flowering, days to maturity and plant height (Table 4)  The T-test comparison of infected and non-infected showed significant differences among the various parameters measured on cowpea plant ( Table 5). The results of the T-value showed significant difference for grain yield, fodder yield, number of pods/plant and number of seeds/pod between infected and non-infected cowpea genotypes. In contract non-significant difference of pod length and 100 seed weight were recorded between infected and non-infected genotypes. Nevertheless, the means of the non-infected genotypes for pod length and 100 seed weight were higher than the infected genotypes. Table 5.

T-test Comparison of Yield Components of Infected and Non-infected Cowpea Varieties
The correlation coefficients among most of the parameters were negative and highly significant (Table 6). There were significant correlations found among percentage yield reduction and percentage reduction in various yield components. There was a strong negative correlation (r = -0.625) between number of pods per plant and days to Striga emergence.
The number of pods per plant, 100 seed weight, grains and fodder yields were highly negatively correlated with number of emerged Striga shoot indicating that as the number of emerge Striga shoot increases the other components keep decreasing. Moreover, the drastic reduction of these parameters may be due to inadequate nitrogen and nutrient absorption for vegetative growth. The results further revealed that Striga was solely responsible for overall yield reduction in the various components. The result showed that the early emergence of Striga attached to the host root drastically affects the physiological and morphological growth of the host plant thereby reducing its efficiency to produce. According to Singh and Emebeche [24], the extent of yield reduction depends upon the time and level of infection.

Conclusion
The study has revealed significant impact on Striga gesnerioides infestation on cowpea yield and yield components. Breeding of Striga resistant varieties for farmers could be the only solution for improving farmers' livelihoods. This gives a clue to famers on the need to cultivate Striga resistant cowpea varieties. The study had revealed some F 3 genotypes that are resistant to Striga gesnerioides under artificial infestation in pot. Thesegenotypes had high genotypic variation for resistance to the Striga strains and therefore such genotypes provide opportunity for genetic improvement through selection.

Declarations
Ethics approval and consent to participate 'Not applicable'

Authors Contributions
The corresponding author RA was the main supervisor reviewed the write-up for technical content; SA reviewed literature and also helped with data analysis; JYA search literature, experimental design and data analysis; FK helped in the field establishment and data collection and RP helped with data entry and analysis.

ACKNOWLEGEMENTS
We wish to thank the members of the CSIR-SARI at Bwaku who helped either directly or indirectly in carrying out the work. This work was supported by West Africa Agricultural Productivity Programme (WAAPP). Means followed by the same letter in each column are not significantly different (P> 0.05). Means followed by the same letter in each column are not significant different (P>0.05) using LSD Means followed by the same letter in each column are not significantly different (P>0.05) using LSD Means followed by the same letter in each column are not significantly different (P>0.05) using LSD  Formula.JPG