Cowpea (Vigna unguiculata (L.) Walp.) is the main legume in sub-Saharan Africa produced for its dry seeds rich in proteins (≥ 25%), carbohydrates, vitamins and minerals, and supplements the diet mainly consisting of cereals in countries where cowpea is a major food crop (Gupta et al., 2016; Omoigui et al., 2018). It is an important staple in human nutrition, food security and income generation for producers, especially in the arid savanna regions of  West  Africa  (Omoigui  et  al.,  2018; Snapp et al., 2018). However, its production is hampered by several abiotic and biotic constraints including parasitic phanerogams. Parasitic plants are in fact noxious bio-aggressors of cultivated plants. Parasitic phanerogams of the genus Striga constitute a major economic problem in more than fifty countries throughout the world. For example, sub-Saharan Africa has been estimated to be over 50 million hectares allocated to cereals  and  legumes  production  are  infested by Striga (Westwood et al., 2010; Dafaallah, 2019). More than 30 species of Striga have been identified worldwide (Spallek et al., 2013); of which, 13 have been recorded in Burkina Faso (Boussim et al., 2011). Among Striga species, four are parasitic causing considerable damage to crops (Csurhes et al., 2013; Dafaallah, 2019). Striga hermonthica, Striga aspera and Striga asiatica parasitize cereals crops and Striga gesnerioides is parasite of legumes crops. The principal control method used by farmers is manual weeding (Haruna et al., 2018). However, this control method is inefficient because Striga continues to emerge after the last weeding. As a consequence, S. gesnerioides is getting more widespread in Burkina Faso. The control of this weed requires the involvement of farmers to develop participatory control strategies (Haruna et al., 2018). Therefore, it is urgent to strengthen participatory learning processes that involve farmers, whereby the importance of this study. Participatory Rural Appraisal (PRA), is an important tool in the integrated management of Striga to enhance a better understanding of the nature and magnitude of Striga issue (Batieno, 2014; Dieni, 2017). Therefore, the present study aims to determine farmers' perceptions and their indigenous knowledge on S. gesnerioides in Burkina Faso.

Study sites

The localities covered by the study were chosen to cover the four agro-climatic zones (north-Sahelian zone, south-Sahelian zone, north-Sudanian zone and south-Sudanian zone). In addition, the choice of survey sites was made taking into account ethnic groups in order to interview as many ethnic groups as possible. The survey was conducted in 15 localities in 11 administrative regions of Burkina Faso from September to October, 2018. The geographical coordinates of the various sites were recorded using a GARMIN GPS and incorporated into the ArcGis 10.0 mapping software. Figure 1 illustrates the geographical position of the localities covered.

Sampling

Two hundred and sixty-five people were interviewed in the 15 sites across the four phytogeographical sectors of Burkina Faso. The youngest respondent was 15 years old and the oldest 97 years old. In  each  location, cowpea producers of the major indigenous ethnic group were selected for questionnaire administration.

Questionnaire administration

Cowpea farmers of at least 15 years old, irrespective of their gender, were interviewed. A minimum number of 10 farmers were interviewed in each locality. SPHINX software was used to develop the questionnaire which included both closed and open-ended questions. Open-ended questions have the advantage of giving the opportunity to the respondents to provide the maximum information they have and are therefore adapted to ethnobotanical studies (Thompson and Juan, 2006). In order to meet the objectives of the study, information was collected on the ability of farmers to differentiate between Striga infesting cereal from that of legumes, local strategies to control Striga and its social usefulness.

Statistical analyses

The data were analysed with SPHINX LEXICA software for frequency. Descriptive statistics such as frequencies were determined. The Chi-square test for goodness-of-fit was also applied to determine whether there are significant differences or not between some studied parameters. EXCEL 2010 sheet was used to generate graphs.

Number of respondents by age group

Two hundred and sixty-five people were interviewed in the 15 sites across the four phytogeographical sectors of Burkina Faso. The youngest respondent was 15 years old and the oldest 97 years old. Indeed, about 40% of the respondents are between 31 and 45 years old (Figure 2). Gender analysis of the data showed that out of the 265 producers interviewed, 184 (69.4%) were males and 81 (30.6%) were females.

Characterization of S. gesnerioides by farmers

The results showed that 264 respondents out of 265 knew Striga before this study. The majority of them (81.90%) were able to distinguish S. gesnerioides from other Striga spp. Only 17.70% of the interviewed farmers were unable to distinguish between the two Striga spp. The c2 test revealed that there is no significant difference between men and women farmers for their ability to differentiate Striga spp. (Table 1). In contrast, respondents' knowledge about Striga spp. varied between regions (Table 2).

Morphological criteria for the identification of S. gesnerioides by farmers

The most used morphological criterion by farmers to recognize S. gesnerioides is its height (size 75.5%) (Figure 3). Farmers identified S. gesnerioides through its bushy growth habit and short height. In addition, S. gesnerioides was recognized by farmers through its very small leaves, the colour of its stem and flowers. Another criterion characterizing S. gesnerioides is the presence of haustorium attached on its host roots (Figure 4).

Farmers' perception of S. gesnerioides emergence period

The study revealed that the emergence of S. gesnerioides starts from the second weeding, which coincides with the appearance of flower buds, until pods maturity. In fact, more than 83.8% of the farmers interviewed stated that the emergence of S. gesnerioides coincides  with the period of flower bud setting (Figure 5).

However, some farmers mentioned a continuous Striga emergence throughout the life cycle of its host.

Farmers' perception of symptoms and cowpea yield losses related to S. gesnerioides

The most common symptoms of S. gesnerioides attacks from cowpea producers view are leaf discoloration (yellowish leaves) and plant stunting (Figure 6). Early defoliation and flower fall were also cited. As regard to yield losses due to Striga, the majority of respondents reported that it is generally higher than 50% (Figure 7). In very poor soils conditions characterized by high infestation density, yield losses can reach 80 to 100%. The yield loss estimates varied between regions (Table 3).

Main strategies used by farmers to control S. gesnerioides

The most widely indigenous practice used for controlling S. gesnerioides is manual uprooting before the parasite flowers (Figure 8). 86% of cowpea producers use method as the main technique of controlling Striga. In addition, cropping systems are practiced with the aim of reducing losses. In farmers’ opinion, cowpea and cereals association prevent them from entire yield loss in case of severe S. gesnerioides attack because they will at least harvest the cereal crop planted. Moreover, some farmers among the Nuni ethnic group in Léo, believe that brushing their hoe with black goat fats will prevent emergence of Striga or mitigate its damages.

Social utility of S. gesnerioides

The results of the survey revealed that S. gesnerioides and other species of the genus Striga have no culinary use for humans. Only 27% of the producers surveyed stated that Striga has some virtues for humans and animals (Figure 9). Both fresh and dry Striga plants are consumed by livestock.

The donkey is the animal which grazes more on Striga according to the farmers interviewed. In addition to the donkey, it is occasionally grazed by camels, horses, oxen and small ruminants.

On the medicinal level, S. gesnerioides is said to play a significant role in human and animal health. In terms of human  health,  it   would  be  used  to  repel  mosquitoes through its mosquito repellent properties. It would also be used in wound dressings and wound healing. In addition, it would be used in children as a decoction to increase their appetite and to lower fever by purging them. Among other things, it would treat heartaches, earaches, bellyaches in children, diabetes, yellow fever, snake bites, etc. In terms of animal health, Striga is used to treat belly bloating and urinary problems in animals. Indeed, an infusion of Striga would facilitate and/or accelerate delivery after parturition in animals when the placenta is slow  to   be   ejected.   Finally,   S.   gesnerioides   would intervene on the mystical level, in the preparation of certain magic potions to counter bad tongues (curses).

This investigation revealed that S. gesnerioides is well known by farmers since their identification criteria such as size, bushy habit, presence of large haustorium are specific to S. gesnerioides (Spallek et al., 2013). This large  haustorium comparable to the tubers of broomrape tubers, ensures its fixation on the host roots and serves for nutrients uptake (Dafaallah, 2019). Farmers’ awareness of S. gesnerioides was also reflected through its local names which all refer to the symptoms and damages. In fact, farmers knew very well that Striga attack leads to cowpea plants stunting, leaf discoloration and drop off and plants death in heavily infested fields. Similar symptoms have been reported by Haruna et al. (2018). Tignegré (2010)and Dieni et al. (2019)reported that farmers in Burkina Faso are well aware of Striga and ranged it as one of the major constraints to cowpea production.

From farmers’ opinion, cowpea yield losses due to S. gesnerioides ranged from 20 to 100% depending on the degree of infestation. Similar results have been reported in previous researches (Ibrahim et al., 2017; Runo and Kuria, 2018). The entire yield loss often leads farmers to abandon their fields and sometimes cowpea production as well (Haruna et al., 2018). However, farmers’ awareness of S. gesnerioides varied from region to another. The weed was very well known in both Sahelian and north-Sudanian zones whilst in the south-Sudanian zone farmers were less aware of its effects. This can be explained by the low degree of S. gesnerioides infestation in this area as a result of relatively recent occurrence of the weed in this part of country. For example, the study revealed the presence of S. gesnerioides in cowpea fields in Niangoloko, which was free of Striga infestation, but the extent of the infestation is negligible yet. Several consecutive years cowpea production on less fertile soils, might have favoured this infestation since Striga preferable invades poor soils. These observations corroborate those of Sibhatu (2016).

As regard to management strategies, the surveys revealed that manual uprooting is the most widely used local method for controlling Striga across the country. This method appears to be the easiest and most accessible to farmers. As such, it has been reported as the main local strategies to control Striga in Ghana (Haruna et al., 2018).

Another cultural practice used to control Striga is fertilizer application. Both farmers and researches are aware of the importance of soil fertility as a factor limiting S. gesnerioides virulence on cowpea and it impact of yield (Omoigui et al., 2017). Knowing that the degree of Striga spp. infestation increases in low soil fertility conditions and vice versa (Larweh, 2016; Gebreslasie et al., 2018). In addition to these methods, farmers asserted that they leave highly infested fields to fallow for some years or practice crop rotation in order to reduce the intensity of the infestation. However, the success of these methods is conditioned by the availability of arable lands. The efficiency of fallow is limited by two factors: the ability of Striga seeds to remain viable in the soil for more than ten years (Sibhatu, 2016; Runo and Kuria, 2018)and the existence of its alternative hosts that contributes to maintaining the parasite in  infested areas  (Sawadogo  et al., 2020). The use of resistant varieties was not very successful according to some farmers. However, this method is the most efficient and the most protective of the environment (Haruna et al., 2018). The issues encountered by farmers with this method can be explained by host-parasite specific resistance on one hand and on the other hand by the phenomenon dealing with breakdown of resistance. The over-use of generations of certified seeds of the resistant variety leading to decrease in varietal purity may also be another reason. It is therefore necessary to strengthen farmers’ technical capacities for a proper use of this method which will facilitate the control of the weed.

Although S. gesnerioides is drastically affecting cowpea production, farmers recognized that it has some useful virtues. Medical use like cancer treatment of other parasitic weeds have been reported (Strüh et al., 2012). A biological insecticide property of S. hermonthica against storage insects of cowpea (Callosobruchus maculatus (Fab)) has been mentioned by Kiendrebeogo et al. (2006). Jansen (2005)has reported on the social utility of the weed. However, parasitic weeds like S. gesnerioides remain by far more noxious to human being than they can serve him. Therefore, appropriate control strategies should be developed for minimizing its damages on cowpea production.

Farmers in the different regions identified S. gesnerioides by its short height, bushy growth habit and haustorium. Their estimations of yield losses caused by Striga ranged between 20 and 100%. The widely indigenous control method used by farmers is hand weeding. In spite of its harmfulness to cowpeas, S. gesnerioides is still of considerable importance in the field of human and animal health. Thus, the vegetative and reproductive parts of the parasite are used in the treatment of several human and animal diseases. However, they show a feeling of helplessness towards this parasitic plant which is the cause of significant damage. To better counter the harmful effects of Striga on crops, the development of varieties with stable resistance seems to be the best solution adapted and accessible to the context of sub-Saharan Africa, which is characterized by family and subsistence agriculture.

The authors have not declared any conflict of interests.

The authors thank the CREAF/Kamboinsin Cowpea team for their technical assistance and support during the realization of the study as well as to all the producers of the villages visited during the prospection for the fraternal welcome. They also acknowledge the Kirkhouse Trust-SCIO foundation for the financial support.