Growth and carbon allocation in Pennisetum typhoides infected with the parasitic angiosperm Striga hermonthica

Abstract Growth and gas exchange measurements are used in conjunction with a carbon balance model to describe the millet (Pennisetum typhoides)–witchweed (Striga hermonthica) host—parasite association. Striga hermonthica reduces the growth of millet by 28% and radically alters the architecture of infected plants. Whilst grain yield and stem dry weight are reduced (by 80 and 53%, respectively), leaf and root growth are stimulated (by 41 and 86%, respectively). The difference in production between infected and uninfected millet plants can be accounted for by two processes: first, export of carbon to the parasite (accounting for 16% of the dry weight not gained); and second, parasite-induced reductions in host photosynthesis (accounting for 84% of the dry weight not gained). Striga hermonthica is dependent on carbon exported from the host, since the plant has low rates of photosynthesis coupled with high rates of respiration. The carbon balance model suggests that in mature S. hermonthica plants parasitic on millet, 85% of the carbon is host-derived. Carbon fluxes are also estimated using δ¹³C measurements, since S. hermonthica is a C₃ plant parasitizing a C₄ host. In conjunction with gas exchange measurements, these suggest that in root, stem and leaf of S. hermonthica, 87, 70 and 49% of carbon is hostderived, respectively.     

Tillage,crop residue,legume rotation,and green manure effects on sorghum and millet yields in the semiarid tropics of Mali

Alternative soil management practices are needed in semi-arid West Africa to sustain soil fertility and cereal production while reducing the need for extended fallow periods and chemical fertilizers. An experiment was conducted at the Cinzana Station near Segou, Mali to assess the effects of tillage, crop residue incorporation and legume rotation on the growth and yield of sorghum (Sorghum bicolor L. Moench) and pearl millet (Pennisetum glaucum L.) for a period of eight years on a loamy sand and a loam soil. The following treatments were compared under tied ridging and the traditional open ridging: continuous cereal with crop residue removed, continuous cereal with crop residue incorporated, cereal in rotation with cowpea (Vigna unguiculata (L.) Waip.), cereal in rotation with sesbania (Sesbania rostrata Bremek. & Oberm.), and cereal in rotation with dolichos (Dolichos lablab L.). Legumes in rotation were incorporated as green manures except cowpea which was removed after each harvest. Tied ridging improved cereal grain yield from 1022 kg ha⁻¹ with open ridging to 1091 kg ha⁻¹ on the loamy sand and from 1554 kg ha⁻¹ to 1697 kg ha⁻¹ on the loam, when averaged across management regimes and years of cropping. Incorporation of cereal residue at the beginning of the rainy season every other year had only small and inconsistent effects on cereal yield. Rotation with cowpea increased cereal grain and stover yields by 18 and 25%, respectively, on the loamy sand, and by 23% and 27%, respectively, on the loam compared to continuous cereal, when averaged across tillage regimes and years. Sesbania and dolichos performed similarly as green manures on both soils. Incorporation of these legumes as green manure at the end of the rainy season increased cereal grain and stover yields by 37% and 49%, respectively, on the loamy sand, and by 27% and 30%, respectively, on the loam, compared to cereal monoculture without organic amendment, when averaged across tillage regimes and years. A significant linear increase in cereal yield was observed during the eight years of the study on the loam soil when sesbania and dolichos green manures were incorporated. This revised version was published online in June 2006 with corrections to the Cover Date.     

Decomposition of cowpea and millet amendments to a sandy Alfisol in Niger

Current inputs of organic materials to cropped lands on sandy Alfisols and Entisols in Sahelian West Africa are insufficient to arrest soil organic matter (SOM) decline. Crop residues and green manures require proper management in order to maximize their contribution to nutrient supply and SOM maintenance. The objectives of this study were to quantify the rates of C and N mineralization from cowpea (Vigna unguiculata (L.) Walp.) green manure, cowpea residue, and millet (Pennisetum glaucum (L.) R.Br.) residue under field conditions in Niger and to determine the effect of these organic amendments on pearl millet yield. Millet was grown (1) as sole crop, (2) as intercrop with cowpea, (3) as intercrop with cowpea that was incorporated as green manure during the second half of the growing season, (4) with incorporated cowpea residue (2000 kg ha^–1), (5) with millet residue mulch (3000 kg ha^–1), and (6) with N fertilizer. Carbon loss as CO₂ from soil with and without organic amendment was measured three times per week during the growing season. Nitrogen fertilizer increased millet yield only in a year with a favorable rainfall distribution. Cowpea grown in intercrop with millet during the first part of the growing season and subsequently incorporated as green manure between millet rows increased millet grain yield in a year with sufficient early rainfall, which could be attributed to the rapid rate of decomposition and nutrient release during the first 3 weeks after incorporation. In a year with limited early rainfall, however, densely planted green manure cowpeas competed for water and nutrients with the growing millet crop. Incorporated cowpea residue and millet residue mulch increased millet yield. Surface applied millet residue had high rates of decomposition only during the first 3 days after a rainfall event, with 34% of the millet residue C lost as CO₂ in one rainy season. Recovery of undecomposed millet residue at the end of the rainy season was related to presence or absence of termites, but not to seasonal C loss. Millet residue mulch increased soil organic C content of this sandy Alfisol in Niger. Cowpea and millet residues had a greater effect on SOM and millet yield than cowpea green manure due to their greater rate of application and slower rate of decomposition.     

Plant growth-promoting rhizobacteria do not pose any deleterious effect on cowpea and detectable amounts of ethylene are produced

Summary The use of trap crops such as cowpea could reduce the effects of the root parasitic weed, Striga hermonthica and its subsequent constraints on the growth of cereals. Certain bacteria could augment the trap crop stimulatory effect. We studied the effect of three bacteria introduced to the rhizosphere of three cowpea varieties at planting. Number of days to cowpea flowering was noted and at harvest, data were collected on pod characteristics and biomass. Means of data subjected to ANOVA were compared using Tukey’s Studentized Range Test. We analysed bacterial headspace volatiles for ethylene by gas chromatography and gas chromatography–mass spectrometry. Bacterial type significantly influenced the cowpea varieties with better performance over the non-inoculated control. Average pod weight (g) with bacterial treatment was 37.97 for Enterobacter sakazakii 8MR5, 34.38 for Pseudomonas 44MS8 and 27.46 for Pseudomonas 10M3. Non-inoculated control had an average weight of 20.98 g. Bacteria promoted a significant increase in pod weight (≥30.89%), fresh biomass (≥24.22%), and improved pod number (≥20.54%) and pod wall thickness (≥7.33%) with no deleterious effect on plant health. Ethylene released by the bacteria ranged from trace concentrations in Pseudomonas sp. to 210 nmoles/10⁸ c. f. u./ml in Ent. sakazakii 8MR5.     

Striga seed-germination activity of root exudates and compounds present in stems of Striga host and nonhost (trap crop) plants is reduced due to root colonization by arbuscular mycorrhizal fungi

Root colonization by arbuscular mycorrhizal (AM) fungi reduces stimulation of seed germination of the plant parasite Striga (Orobanchaceae). This reduction can affect not only host plants for Striga, resulting in a lower parasite incidence, but also false hosts or trap crops, which induce suicidal Striga seed germination, thereby diminishing their effectiveness. In order to better understand these AM-induced effects, we tested the influence of root colonization by different AM fungi on the seed-germination activity of root exudates of the Striga hermonthica nonhost plants cowpea and cotton on S. hermonthica. We also tested the effect of AM fungi on the seed-germination activity of the Striga gesnerioides host plant cowpea on S. gesnerioides. Moreover, we studied whether mycorrhization affects the transport of seed-germination activity to above-ground plant parts. Mycorrhization not only resulted in a lower seed germination of S. gesnerioides in the presence of root exudates of the S. gesnerioides host cowpea but also seed germination of S. hermonthica was also lower in the presence of root exudates of the S. hermonthica nonhosts cowpea and cotton. Downregulation of the Striga seed-germination activity occurs not only in root exudates upon root colonization by different AM fungi but also in the compounds produced by stems. The lowered seed-germination activity does not appear to depend on the presence of seed germination inhibitors in the root exudates of mycorrhizal plants. The implication for Striga control in the field is discussed.     

Destructive and non-destructive measurements of residual crop residue and phosphorus effects on growth and composition of herbaceous fallow species in the Sahel

Little is known about the residual effects of crop residue (CR) and phosphorus (P) application on the fallow vegetation following repeated cultivation of pearl millet [Pennisetum glaucum (L.) R. Br.] in the Sahel. The objective of this study, therefore, was (i) to measure residual effects of CR, mulched at annual rates of 0, 500, 1000 and 2000 kg CR ha^–1, broadcast P at 0 and 13 kg P ha^–1 and P placement at 0, 1, 3, 5 and 7 kg P ha^–1 on the herbaceous dry matter (HDM) 2 years after the end of the experiment and (ii) to test a remote sensing method for the quantitative estimation of HDM. Compared with unmulched plots, a doubling of HDM was measured in plots that had received at least 500 kg CR ha^–1. Previous broadcast P application led to HDM increases of 14% compared with unfertilised control plots, whereas no residual effects of P placement were detected. Crop residue and P treatments caused significant shifts in flora composition. Digital analysis of colour photographs taken of the fallow vegetation and the bare soil revealed that the number of normalised green band pixels averaged per plot was highly correlated with HDM (r = 0.86) and that red band pixels were related to differences in soil surface crusting. Given the traditional use of fallow vegetation as fodder, the results strongly suggest that for the integrated farming systems of the West African Sahel, residual effects of soil amendments on the fallow vegetation should be included in any comprehensive analysis of treatment effects on the agro-pastoral system.     

Short communication. Identification of developmentally-specific markers in germinating and haustorial stages of Striga hermonthica (Del.) Benth. seedlings

Developmentally-specific markers have been identified in the germinating and haustorial stages of Striga hermonthica seedlings. Four water-soluble proteins, preferentially expressed in germinated seedlings, were microsequenced. An haustorial-specific cDNA clone was isolated by differential screening. Tissue specificity for this clone was assessed by Northern blot hybridization analysis.Key words: Striga hermonthica (Del.) Benth, microsequencing, lipid transfer protein, superoxide dismutase.      

Effect of crop rotation on native vesicular arbuscular mycorrhizal propagules in soil

The effect of crop rotation of native vesicular arbuscular mycorrhizal fungi was studied. Finger millet was grown as the first season crop in 15 plots. In the second season a mycorhizal host (cowpea) and a non-mycorrhizal host (mustard) were grown in 5 plots each, and the remaining 5 plots were left fallow. In the third season cowpea was grown in all the plots. Leaving the land fallow reduced the mycorrhizal propagules by 40% while growing a non-mycorrhizal host reduced it by 13%. Cowpea grown in the third season coincided with a slow build up of mycorrhizal propagules in soil. There was a slow build up of mycorrhizal propagules late in the season irrespective of the treatment in the preceding season.     

Effect of preceding grain legumes on growth and nitrogen uptake of dryland pearl millet

Summary In a dryland cropping systems study preceding crops either of groundnuts, cowpea or pigeon pea were found to increase the early seedling vigour, rate of plant growth and grain production of the subsequent pearl millet. No such benefit was noted from a previous crop of mung. Grown after groundnuts and cowpea the unfertilized pearl millet removed from the soil 39.9 and 32.5 kg N compared to 18.9 kg N per hectare following a pearl millet crop. At harvest the number of viable nodules was the highest in groundnuts and cowpea. Especially in groundnuts the number of viable nodules increased after flowering stage.     

Nitrogen contribution of cowpea green manure and residue to upland rice

Cowpea, Vigna unguiculata (L.) Walp., is well adapted to acid upland soil and can be grown for seed, green manure, and fodder production. A 2-yr field experiment was conducted on an Aeric Tropaqualf in the Philippines to determine the effect of cowpea management practice on the response of a subsequent upland rice crop to applied urea. Cowpea was grown to flowering and incorporated as a green manure or grown to maturity with either grain and pods removed or all aboveground vegetation removed before sowing rice. Cowpea green manure accumulated on average 68 kg N ha⁻¹, and aboveground residue after harvest of dry pods contained on average 46 kg N ha⁻¹. Compared with a pre-rice fallow, cowpea green manure and residue increased grain yield of upland rice by 0.7 Mg ha⁻¹ when no urea was applied to rice. Green manure and residue substituted for 66 and 70 kg urea-N ha⁻¹ on upland rice, respectively. In the absence of urea, green manure and residue increased total aboveground N in mature rice by 12 and 14 kg N ha⁻¹, respectively. These increases corresponded to plant recoveries of 13% for applied green manure N and 24% for applied residue N. At 15 d after sowing rice (DAS), 33% of the added green manure N and 16% of the added residue N was recovered as soil (nitrate + ammonium)-N. At 30 DAS, the corresponding recoveries were 20 and 37% for green manure N and residue N, respectively. Cowpea cropping with removal of all aboveground cowpea vegetation slightly increased (p<0.05) soil (nitrate + ammonium)-N at 15 DAS as compared with the pre-rice fallow, but it did not increase rice yield. Cowpea residue remaining after harvest of dry pods can be an effective N source for a subsequent upland rice crop.     

A fungal pathogen with potential for biocontrol ofStriga hermonthica (Scrophulariaceae)

Fusarium equiseti (Corda) Saccardo (IMI Number 311922b) was isolated from diseasedStriga hermonthica (Del.) Bentham parasitic on sorghum in the Nuba Mountains Rural Development Project, Kordofan, Sudan in November 1986. Symptoms of the disease consisted of progressive blackening and drying of the leaves and flowers. The disease caused 50% mortality ofS. hermonthica populations in two areas.     

Effect of cultural methods on Striga (Striga hermonthica (Del.)) Benth management and yield of cereals in the Savanna Zone of Nigeria: a review

Data were obtained from the research done in the Guinea Savanna (Zaria 11°11′N; 070 38′E) and Sudan Savanna (Maiduguri ?11°51′N; 13°15′E) regions of Nigeria, respectively on different cultural methods of Striga control and management. In the Guinea Savanna, trials on the effects of nitrogen on the response of resistant and susceptible upland rice varieties to Striga hermonthica infestation and the effect of resistant and susceptible varieties of maize and crop rotation on Striga infestation was carried out, while the effect of inter-cropping trap crop (Bambaranut) with resistant sorghum varieties on S. hermonthica was studied in the Sudan Savanna Zone of Nigeria. In the Guinea Savanna, it was observed that a combination of upland rice variety, Faro 40 and an application of 90 kg N/ha in the wet season and WAB 56-50 upland rice variety and 120 kg N/ha in the dry season, respectively reduced Striga infestation and produced maximum grain yield. Also, the growing of resistant variety of maize (Across 97ITZ comp. I-W) after 1 or 2 years' rotation with cowpea or soybean was observed not only to be effective in Striga control, but resulted in higher grain yield of maize. In the Sudan Savanna, the use of resistant varieties of sorghum, 1CSV1002 and 1CSV1007 intercropped with bambaranut significantly reduced Striga infestation, but the grain yield of the resistant varieties was low. From these studies, Faro 40 with 90 kg N/ha application rate and WAB56–50 with 120 kg N/ha were suitable for the management of Striga and for higher grain yield of upland rice in both wet and dry seasons, respectively, while Across 97ITZ comp. I-W, resistant maize variety and 1 or 2 years rotation with cowpea or soybean were also the best for the management of S. hermonthica and for higher maize yield in the Guinea Savanna zone. Further research needs to be carried out in the Sudan Savanna to select a high yielding resistant variety of sorghum which when intercropped with bambaranut will not only control Striga infestation but will also give high grain yield.     

Germination stimulants produced by Vigna unguiculata Walp cv Saunders Upright

A germination stimulant, alectrol, for the seeds of the angiospermous root parasites Alectra vogelii Benth. and Striga gesnerioides (Willd.) Vatke has been isolated from root exudates of its genuine host plant Vigna unguiculata Walp cv Saunders Upright. Its spectroscopic data lead to a chemical structure closely related to (+)-strigol. The compound is more active than strigol in stimulating seeds of the respective parasites, Alectra vogelii and Striga gesnerioides.Abbreviations AV Alectra vogelii - OA Orobanche aegyptiaca - SA Striga asiatica - SG Striga gesnerioides - SH Striga hermonthica - TFA trifluoroacetic acid     

Effect of Jasmonates and Related Compounds on Seed Germination of Orobanche minor Smith and Striga hermonthica (Del.) Benth

Jasmonates and related compounds were found to elicit the seed germination of the important root parasites, clover broomrape (Orobanche minor Smith) and witchweed [Striga hermonthica (Del.) Benth]. The stimulation of seed germination by the esters was more effective than by the corresponding free acids, and methyl jasmonate (MJA) was the most active stimulant among the compounds tested.     

Nitrogen-15 balances for urea and neem coated urea applied to lowland rice following two cowpea cropping systems

Little is known about whether the high N losses from inorganic N fertilizers applied to lowland rice (Oryza sativa L.) are affected by the combined use of either legume green manure or residue with N fertilizers. Field experiments were conducted in 1986 and 1987 on an Andaqueptic Haplaquoll in the Philippines to determine the effect of cowpea [Vigna unguiculata (L.) Walp.] cropping systems before rice on the fate and use efficiency of¹⁵N-labeled, urea and neem cake (Azadirachta indica Juss.) coated urea (NCU) applied to the subsequent transplanted lowland rice crop. The pre-rice cropping systems were fallow, cowpea incorporated at the flowering stage as a green manure, and cowpea grown to maturity with subsequent incorporation of residue remaining after grain and pod removal. The incorporated green manure contained 70 and 67 kg N ha⁻¹ in 1986 and 1987, respectively. The incorporated residue contained 54 and 49 kg N ha⁻¹ in 1986 and 1987, respectively. The unrecovered¹⁵N in the¹⁵N balances for 58 kg N ha⁻¹ applied as urea or NCU ranged from 23 to 34% but was not affected by pre-rice cropping system. The partial pressure of ammoniapNH₃, and floodwater (nitrate + nitrite)-N following application of 29 kg N ha⁻¹ as urea or NCU to 0.05-m-deep floodwater at 14 days after transplanting was not affected by pre-rice cropping system. In plots not fertilized with urea or NCU, green manure contributed an extra 12 and 26 kg N ha⁻¹, to mature rice plants in 1986 and 1987, respectively. The corresponding contributions from residue were 19 and 23 kg N ha⁻¹, respectively. Coating urea with 0.2g neem cake per g urea had no effect on loss of urea-N in either year; however, it significantly increased grain yield (0.4 Mg ha⁻¹) and total plant N (11 kg ha⁻¹) in 1987 but not in 1986.     

Cowpea N rhizodeposition and its below-ground transfer to a co-existing and to a subsequent millet crop on a sandy soil of the Sudano-Sahelian eco-zone

Nitrogen (N) rhizodeposition by cowpea (Vigna unguiculata (L.) Walp) is potentially a large N source in cropping systems of Sub-Saharan Africa. A field experiment was conducted to measure cowpea N rhizodeposition under the conditions of the Sudano-Sahelian zone using direct ¹⁵N labelling techniques to trace the amount of deposition and its transfer to associated and subsequent crops. Half of the total cowpea crop N was located below-ground at plant maturity, which exceeded 20 kg?N ha^?1 when intercropped with millet. Only 15% of the below-ground cowpea N was recovered in roots, while 85% was found in the rhizodeposited pools. The experiment demonstrated that direct below-ground N transfer occurred from cowpea to millet in intercrop at a rate of 2 kg?N ha^?1 over the growing season. Forty percent of the 25 kg below-ground N that the cowpea crop left at harvest were identifiable in the top 0.30 m soil in the beginning of the next planting season 7 months later; a pool still present at the end of that second season. Thus, the subsequent crop of millet (Pennisetum glaucum (L.) R. Br.) only recovered 2.5 kg?N ha^?1 from the below-ground cowpea pre-crop N during this growth season. The role and potential of cowpea as N provider has been underestimated in the past by ignoring the large proportion of N contained in its rhizodeposits. However, information is needed to determine how losses of the rhizodeposited N can be minimized to fully harness the potential of cowpea as N provider in agro-ecosystems of the region.     

Structural requirements of strigolactones for germination induction and inhibition of Striga gesnerioides seeds

Key message

Structure–activity relationship studies of strigolactones and Striga gesnerioides seed germination revealed strict structural requirements for germination induction and a new function of the plant hormones as germination inhibitors.

Abstract

Stereoisomers of the naturally occurring strigolactones, strigol, sorgolactone, orobanchol, sorgomol and 5-deoxystrigol, 36 in total, were prepared and screened for the ability to induce and/or inhibit the germination of Striga hermonthica and Striga gesnerioides seeds collected from mature plants that parasitized on sorghum and cowpea, respectively. All of the compounds induced S. hermonthica seed germination, albeit displayed differential activities. On the other hand, only a limited number of the compounds induced significant germination in S. gesnerioides, thus indicating strict structural requirements. Strigolactones inducing high germination in S. gesnerioides induced low germination in S. hermonthica. Strigolactones with the same configuration at C3a, C8b and C2′ as that in 5-deoxystrigol (9a) induced high germination of S. hermonthica seeds, but most of them inhibited the germination of S. gesnerioides. The differential response of S. gesnerioides to strigolactones may play an important role in the survival of the species. However, the compounds could be used as means of control if mixed cropping of cowpea and sorghum is adopted.     

Physiology of the parasitic association between maize and witchweed (Striga hermonthica): is ABA involved?

Striga hermonthica (Del.) Benth. is an obligate hemiparasiticangiosperm which can cause severe losses of yield in cerealcrops in the semi-arid tropics. The effects of this parasiteon the growth and stomatal conductance of three varieties ofmaize (Zea mays L.) during the first 6 weeks of the associationhave been studied. From 24 d after planting (DAP), infectedplants were significantly shorter than uninfected controls.When the plants were harvested 45 DAP, infected plants had fewerfully expanded leaves, less leaf biomass and less pseudo-stembiomass than uninfected controls. However, the parasitized plantshad more root biomass and hence a higher root:shoot ratio thanuninfected controls. The stomatal conductance of infected hostswas severely inhibited by comparison with that in uninfectedplants. The possibility that abscisic acid (ABA) may be involved inthe regulation of the parasitic association was investigated.ABA concentrations in leaf tissue of maize (cv. Cargimontana)and S. hermonthica were determined by radioimmunoassay. Whilethere was a difference between cultivars in the extent of theresponse, the concentrations of ABA were significantly higherin infected maize plants than in the uninfected controls. InS. hermonthica, leaf tissue ABA concentration was found to bean order of magnitude higher than in the host leaf tissue. Detachedleaves of S. hermonthica which were dehydrated at room temperatureuntil they had lost 10–20% of their fresh weight containedthree times the ABA concentration of control leaves. This suggeststhat leaves of S. hermonthica can synthesize or re-mobilizeABA in response to water deficit. It is not yet known whetherthis contributes to the higher concentration in infected hosts,but the results suggest that ABA has a role in this parasiticassociation. Key words: Striga hermonthica, abscisic acid, growth, parasitic angiosperm, stomatal conductance     

Alleviation of water stress effects in cowpea by Bradyrhizobium spp. inoculation

Experiments were carried out to investigate the effects of different degrees of water stress on cowpea in the presence and absence of Bradyrhizobium spp. inoculation and to evaluate physiological responses to stress. The soil used was Yellow Latosol, pH 6.3 and the crop used was cowpea (Vigna unguiculata (L.) Walp.) cv. ‘IPA 204’. Stress was applied continuously by the control of matric potential (ψ _m ) through a porous cup. The lowered soil ψ _m had a direct effect on the N2 fixation, but the strains Bradyrhizobium introduced by inoculation in the cowpea plants were superior to the indigenous strain demonstrating the importance of inoculation in the stressed plants. At the more negative ψ _m plants inoculated with the strains EI 6 formed associations of greater symbiotic efficiency which helped the cowpea plants to withstand drought stress better than the strain BR 2001 and the uninoculated control. The leghaemoglobin concentration was not inhibited in the drought-stressed plants at ψ _m -70 kPa when inoculated with the strain EI 6, which confered a differential degree of drought resistance in plants. The ψ _w declined in the stressed plants reaching values of -1.0 MPa which was sufficient to cause disturbance in nodulation and biomass production. This revised version was published online in June 2006 with corrections to the Cover Date.     

Diversity of arbuscular mycorrhizal fungi in Brazil's Caatinga and experimental agroecosystems

Cowpea (Vigna unguiculata) is a nutritious legume crop for both its grain and leaves and comprises an important component in both human and animal nutrition. In Brazil, the use of mulch, such as coconut fiber, and organic fertilizers to maximize cowpea production offers an alternative to conventional mineral fertilizer strategies. Farming practices affect the diversity and activity of soil microorganisms, including arbuscular mycorrhizal fungi (AMF), important plant growth promoters for legumes. Our objective was to determine the effect of mulching with coconut fiber and manure on AMF diversity in cowpea. Soil samples were collected from an Experimental Station in Petrolina, NE Brazil: one Caatinga (natural dry‐forest vegetation), one fallow, and one experimental site established in the fallow area and cultivated with cowpea receiving cattle manure and four doses (0, 12, 24, 48 t/ha) of coconut fiber. AMF species richness, abundance, and diversity were evaluated. Sixty‐four AMF species were recorded, with predominance of Glomeraceae and Acaulosporaceae. Highest species richness (47) was recovered from the Caatinga but AMF diversity was also high in the cultivated sites, demonstrating the importance of mycotrophic plants, such as cowpea, in crop production systems for the maintenance of AMF species richness. Although several species, such as Claroideoglomus etunicatum, Acaulospora scrobiculata, Glomus trufemii, and Paraglomus pernambucanum, revealed pronounced sporulation patterns, even high doses of coconut fiber did not affect AMF richness and diversity, compared to fallow. Consequently, cultivation of mycotrophic plants and use of organic manures are able to maintain high AMF species richness in tropical agroecosystems.