The witchweed Striga gesnerioides and the cultivated cowpea: A geographical and historical analysis of their West African distribution points to the prevalence of agro-ecological factors and the parasite's multilocal evolution potential.

Abou-Soufianou Sadda,Abdoulaye Diouf,Hassane Bil-Assanou Issoufou,Abdoul-Aziz Saidou,Oumarou Malam-Issa,Geo Coppens D’Eeckenbrugge,Nouhou Salifou Jangorzo

doi:10.1371/journal.pone.0254803

Abstract

The increasing severity of Striga gesnerioides attacks on cowpea across West Africa has been related to its prolificity, seed mobility and longevity, and adaptation to aridity, in a context of agricultural intensification. To understand this fast extension, we analyzed (1) the distributions of the crop and the witchweed with ecological niche modeling and multivariate climate analysis, and (2) the chronological information available from collections and the literature. The ecoclimatic envelope of S. gesnerioides attacks on cowpea is the same as on wild hosts. Consistently, the modeled distribution of cowpea infestations is closely similar to the simple superposition of the parasite model (involving all hosts) and the crop model. Striga gesnerioides infestations are restricted to the driest component of the cultivated cowpea ecoclimatic niche, corresponding to the Sahelian and Sudano-Sahelian belts and the Dahomey gap. Thus, the parasite distribution, determined by its own requirements, does not constrain cowpea cultivation under Guinean climates. The spatial and temporal distributions of S. gesnerioides field infestations are consistent with an earlier impact on cowpea production in eastern West Africa, related itself to a similar trend in cowpea cultivation intensification from Niger, Nigeria and Benin to Burkina Faso and Ghana. Mali and Senegal were affected later, and literature reports of Senegalese strains of S. gesnerioides from the wild developing virulence on cowpea offer a model for the diffusion of witchweed parasitism by multilocal evolution, through host-driven selection, instead of epidemic diffusion. A contrario, in Côte d’Ivoire, cowpea is much less widespread, so the parasite has remained confined to the wild compartment. Thus, both historical and ecogeographic analyses refute the vision of S. gesnerioides as an invader. Instead, they point to the increasing importance and intensification of the crop, and the consequent loss of biodiversity, as the main drivers of the extension and diversification of its crop-specific strains.

Highlights

Cowpea [Vigna unguiculata (L.) Walp.] is one of the most widely grown legume crops throughout the tropics and subtropics of Africa, East and Southeast Asia, Latin America, parts of southern Europe, as well as in the southern United States and in Oceania, with a 2017 world production above 7.4 million tons [1]
Given the very high social and economic importance of cowpea and the increasing severity of S. gesnerioides infestation in West Africa, the present study focuses on the ecological drivers of the interaction between the parasite and its main host in this region
In the eastern half of Senegal and the western half of southern Mali, the uniform and relatively low density is linked to the source of data, mostly the CoEx project, which followed a particular sampling strategy. This relative eastwest contrast probably reflects the economic importance of the crop among West African countries, with the relative exception of Nigeria, the main producer country, where the datapoint density is lower than for the second and third producers, Niger and Burkina Faso

Summary

Introduction

Cowpea [Vigna unguiculata (L.) Walp.] is one of the most widely grown legume crops throughout the tropics and subtropics of Africa, East and Southeast Asia, Latin America, parts of southern Europe, as well as in the southern United States and in Oceania, with a 2017 world production above 7.4 million tons [1]. Its production is by far most important in Africa (7.1 Mt), in West Africa, with Nigeria (3.4 Mt) and Niger (1.96 Mt) as the main producing countries. It has more than tripled since the mid-1980s in Cameroon, Nigeria, Niger, Burkina Faso, Mali, and Senegal, mostly based on an extension of cultivated areas [2]. Cowpea contribution to soil fertility, through soil covering and the fixation of atmospheric nitrogen, is important in smallholder farming systems where limited or no fertilizers are used [7]

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Conclusion