Estimation of the seedbank of Striga spp. (Scrophulariaceae) in Malian fields and the implications for a model of biocontrol of Striga hermonthica

Striga seeds were extracted from soils collecled in Mali and the viahility of these seeds was estimated. Striga seeds were found in 45 of 46 samples taken from 23 fields. Siriga hermonthica (Del.) Benth. growmg on the host crop millet, was present at all 46 sites sampled. The a size uf the Striga seedbaiik measured lo a depth of 15 cm was estimated to be 38 800 m ^-2 of surface area (geometnc mean 11 500). The average number of S. hermonthica plants observed per millei hill was 8.83 (geometric mean 3.89) or 13.98 m^-2 (gcometrie mean 5.69 m^-2). Higher crop hill densities tended to result in higher densities of emerged S. hermonthica per hill. The relationship between seeds m^-2 and S. her-monhica plants m^-2 was fitied to a reetungular hyperbola and used to reassess an existing model of S. herniouhica control.     

Pathogenicity of fungi collected in northern Ghana to Striga hermonthica

Thirteen fungal species were isolated from diseased plants of Striga hermonthica (Del.) Benth in northern Ghana. The pathogenicity of 12 isolates of the fungal species including Curvularia fallax Boed, Fusarium equiseti (Corda) Sacc., Fusarium equiseti var. bullatum (Sherb.) Wollenw., F. oxysporum Schlecht, F. solani (Mart) Sacc., Macrophonuina phasealina (Tassi) Goidan, and Sclerotium rolfsii (Sacc.) were evaluated against S. hermonthica under controlled environmental conditions. All isolates were pathogenic to S. hermonthica when propagated on wheat (Triticum aestivum L.) grains and incorporated pre-planting into the soil. However, their virulence differed. Two isolates of F. oxysporum and one isolate of F. solani reduced the emergence of S. hermonthica by 88%, 98%, and 76%, respectively. Sorghum (Sorghum bicolor (L.) Moench) yield was increased by 26% when S. hermonthica was controlled. In contrast, in the control treatment with S. hermonthica no yield could be obtained. The F. oxysporum isolates were not pathogenic on sorghum. Germination tests indicated that the F. oxysporum isolates were highly pathogenic to S. hermonthica seeds. Although the F. solani isolate reduced the emergence of S. hermonthica in the pot experiments, it did not influence germination. This indicates that pathogens may attack different stages in the life cycle of S. hermonthica.     

Induction of Striga seed germination by thidiazuron

Thidiazuron at 0.1 to 10 mg 1^?1 induced concurrent germination and haustorium initiation in Striga asiatica (L.) Kuntze and S. hermonthica (Del.) Benth, but it had no effects on S. gesnerioides (Willd.) Vatke. Both millet and sorghum strains of S. hermonthica were equally responsive. The response of Striga seeds to thidiazuron increased with conditioning. Early applications of the compound induced some germination, but had adverse effects on the conditioning process. Induction de la germination des graines de Striga par le thidiazuron Le thidiazuron de 0,1 à 10 mg 1^?1 a induit de façon conjointe le germination et l'initiation d'haustoria chez Striga asiatica et S. hermonthica, mais n'a pas eu d'effet sur S. gesnerioïdes. Les souches de S. hermonthica liées au millet et au sorgho ont un comportement équivalent. La réponse des graines de Striga au thidiazuron a augmenté par conditionnement préalable. Des applications précoces de produit ont induit quelques germinations, mais ont eu des effets contraires sur Ie processus de conditionnement. Einleitung der Samenkeimung bei Striga-Arten durch Thidiazuron Mittels Thidiazuron-Lösungen von 0,1 bis 10 mg 1^?1 konnten eine gleichzeitige Keimung und Ausbildung der Haustorien bei Striga asiatica (L.) Kuntze und S. hermonthica (Del.) Benth. herbeigeführt werden, nicht jedoch bei S. gesnerioides (Willd.) Vatke. Der Rispenhirsen-und der Sorghum-Typ von S. hermonthica reagierten in gleichem Maße. Konditionierung der Striga-Samen förderte die Wirkung von Thidiazuron. Frühe Anwendung des Wirkstoffs führte zu einer geringen Keimung, beeinträchtigte jedoch die Konditionierung.     

Genetic diversity of East and West African Striga hermonthica populations and virulence effects on a contrasting set of sorghum cultivars

The root hemiparasite Striga hermonthica causes very significant yield loss in its dryland staple cereal host, Sorghum bicolor. Striga‐resistant sorghum cultivars could be an important part of integrated S. hermonthica control. For effective resistance breeding, knowledge about the diversity of the parasite is essential. This study aimed (i) to determine the genetic diversity within and between seven S. hermonthica populations from East and West Africa using 15 microsatellite markers and (ii) to assess the virulence and host–parasite interactions of these Striga populations grown on 16 diverse sorghum genotypes in a glasshouse trial. Most of the genetic variance (91%) assessed with microsatellite markers occurred within S. hermonthica populations. Only a small portion (8%) occurred between regions of origin of the populations. A positive correlation (R² = 0.14) between pairwise geographic and genetic distances reflected the slightly increasing differentiation of S. hermonthica populations with increasing geographic distance. East African S. hermonthica populations, especially those from Sudan, had significantly greater average infestation success across all sorghum genotypes than West African populations. Some specific host–parasite interaction effects were observed. The high genetic variation among individuals of each S. hermonthica population underlines the high potential adaptability to different hosts and changing environments. This points to the need to manage sorghum resistance alleles in space and time and to employ resistant varieties as part of integrated S. hermonthica control, so as to hinder the parasite overcoming resistance.     

Genetic diversity of Striga hermonthica and Striga asiatica populations in Kenya

The parasitic angiosperms, Striga hermonthica and Striga asiatica, severely constrain cereal production in sub-Saharan Africa by causing huge losses in grain yield. Understanding the diversity of Striga populations is important because it allows identification of races or biotypes thus improving chances of breeding success. Amplified fragment length polymorphism (AFLP) analysis was used to study genetic diversity among 17 populations of S. asiatica and 24 populations of S. hermonthica from Kenya. A total of 349 DNA fragments ranging from 51 to 500 bp were obtained from four EcoRI and MseI primer combinations. Genetic distances for S. asiatica populations ranged from 0.009 to 0.116 with a mean of 0.032. S. hermonthica populations had a genetic distance that ranged from 0.007 to 0.025 with a mean of 0.015. Only two clusters were found in S. asiatica populations whereas no apparent structure was evident in S. hermonthica populations. There was no evidence of isolation by distance for the two species. Although the low genetic diversity suggests Striga is relatively uniform across the populations studied, it is possible that pathogenicity and virulence genes may be located in genomic regions that were not sampled. The data, however, does not provide evidence to support diversification of both Striga species in the region where the study was conducted.     

Genetic diversity and population structure of Striga hermonthica populations from Kenya and Nigeria

Striga hermonthica is a parasitic weed that poses a serious threat to the production of economically important cereals in sub‐Saharan Africa. The existence of genetic diversity within and between S. hermonthica populations presents a challenge to the successful development and deployment of effective control technologies against this parasitic weed. Understanding the extent of diversity between S. hermonthica populations will facilitate the design and deployment of effective control technologies against the parasite. In the present study, S. hermonthica plants collected from different locations and host crops in Kenya and Nigeria were genotyped using single nucleotide polymorphisms. Statistically significant genetic differentiation (F_ST = 0.15, P = 0.001) was uncovered between populations collected from the two countries. Also, the populations collected in Nigeria formed three distinct subgroups. Unique loci undergoing selection were observed between the Kenyan and Nigerian populations and among the three subgroups found in Nigeria. Striga hermonthica populations parasitising rice in Kenya appeared to be genetically distinct from those parasitising maize and sorghum. The presence of distinct populations in East and West Africa and in different regions in Nigeria highlights the importance of developing and testing Striga control technologies in multiple locations, including locations representing the geographic regions in Nigeria where genetically distinct subpopulations of the parasite were found. Efforts should also be made to develop relevant control technologies for areas infested with ‘rice‐specific’ Striga spp. populations in Kenya.     

Host specificity of Fusarium oxysporum Schlect (isolate PSM 197), a potential mycoherbicide for controlling Striga spp. in West Africa

The ability of Fusarium oxysporum (PSM 197), a potential mycoherbicide for control of Striga hermonthica, to control different Striga species (S. hermonthica, S. asiatica and S. gesneroides) and another parasitic weed, Alectra vogelli, was investigated under glasshouse conditions. Significant reductions in the total number of emerged plants of S. asiatica (91.3%), S. gesneroides (81.8%) and S. hermonthica (94.3%) were achieved in the presence of F. oxysporum (PSM 197). The pathogen only caused a reduction of 8.5% in A. vogelli. This high susceptibility of the three Striga species provides a possible opportunity to control these parasites simultaneously with this mycoherbicide.     

Evaluation of integrated crop management strategies employed to cope with Striga infestation in permanent land use systems in southern Benin

Striga hermonthica and S. gesnerioides pose serious threats to cereal and cowpea production, endangering peoples' livelihoods on the Abomey plateau, Benin. A 2-year joint experiment was undertaken with farmers in two hamlets to investigate the potential of managing sowing dates of cowpea, sorghum transplanting, and trap cropping as ways of increasing agricultural production and reducing Striga damage. Early sowing of cowpea failed due to dry spells. Late sowing reduced cowpea yield due to water deficiency at the end of the growing season. Transplanting sorghum seedlings raised in fertilised or Striga-free nurseries doubled or tripled cereal yield and substantially reduced S. hermonthica infestation compared to direct early-sown sorghum. Transplanting sorghum from plant hills to fill gaps was unsuccessful. Trap crops such as cowpea and groundnut increased subsequent maize yield. Trap cropping had only a small effect on S. hermonthica infestation. The very poor soils in Somè central were a major constraint upon yield improvement to acceptable levels even after the introduction of the new crop (and Striga) management methods.     

The effect of nitrogen on the growth and development of giant witchweed,Striga hermonthica Benth.: effect on cultured germinated seedlings in host absence

For the first time in sterile nutrient media in the absence of the host plant, different forms and rates of nitrogen compounds were screened for their effect onS. hermonthica Benth. shoot development beyond seed germination. There was no shoot formation beyond the inoculation stages whenS. hermonthica germlings were grown in media devoid of nitrogen source. In culture media containing some nitrogen sources, healthy shoots were formed. Increasing concentrations of KNO₃, NaNO₃, Ca(NO₃)₂, Mg(NO₃)₂ and asparagine resulted in a significant increase inS. hermonthica shoot development. Inversely, increasing concentrations of (NH₄)₂SO₄, NH₄H₂PO₄, NH₄Cl and urea led to increasing significant reduction ofS. hermonthica shoot development. The amino acids, glycine and asparagine supported reduced shoot development ofS. hermonthica, indicating that organic nitrogen cannot replace inorganic nitrogen forStriga growth. The ammonium nitrogen compounds, (NH₄)₂SO₄ and NH₄H₂PO₄, suppressed further shoot elongation and total dry weight of 20 and 40 days oldStriga hermonthica plants, in sterile culture media. The organic compounds urea, allylthiourea and thiourea had an effect similar to ammonium compounds. Arginine and glycine on the other hand did not suppress the further development of the parasite. The suppressive effect of nitrogen however, was greater when the parasite was 20 days old than when it was 40 days old. This work provides data to show that some nitrogen compounds reduce the severity ofS. hermonthica attack by direct suppression ofStriga growth and development at the post-germination stage and after shoots have been formed.     

Host plant preference of Diuraphis noxia (Kurdj.) (Hom., Aphididae)

Host plant preference of the Russian wheat aphid Diuraphis noxia (Kurdj.) was studied on 11 cultivars of 9 plant species: winter barley, spring barley, winter wheat, spring wheat, rye, oat, Triticale, canary grass, red millet yellow millet and maize. Seeds of the host plants were sown in a circle near the edge of pots. The host plant choice was evaluated 24 hours after releasing 55 Diuraphis noxia female adults in the middle of each pot. The suitability of different hosts for aphid development was evaluated 2, 7 and 14 days after infestation based on the mean number of Russian wheat aphid individuals per plant. Red millet, yellow millet and maize were chosen by significantly fewer aphids than grain crops. Winter and spring barley were chosen as hosts most frequently, and the progeny production was also the highest on these plants. The growth rate of D. noxia was significantly affected by the host plants and the date of assessment and their interaction.     

Competition between Solanum nigrum and pepper in the presence of Meloidogyne incognita

The influence of the root-knot nematode Meloidogyne incognita (Kofoid & White) Chitwood on the competitive relationships between pepper (Capsicum annuum L.) and Solanum nigrum L. (black nightshade) was investigated in a glasshouse experiment. Seven competition treatments were set up: two intraspecific for the crop and the weed and five interspecific treatments in which the emergence of S. nigrum plants was progressively delayed with respect to that of the pepper. Nematodes reproduced in every inoculated plant and their multiplication rates were high in both pepper and S. nigrum. The parasite reduced all growth and yield parameters of the crop, but did less harm to the weed. The negative effect of S. nigrum on pepper peaked in the treatment in which the weed and pepper plants emerged together. S. nigrum was a stronger competitor than pepper under both nematode-infested and nematode-free conditions. The effect of nematodes on pepper yield was less than that of competition, but both appeared to be additive.     

Studies on competition between Setaria anceps and Eleusine indica

Experiments were conducted at Kairi. Queensland. Australia on the competitive effects or Eleusine indica (L.) Gaertn. on Setaria anceps Stapf. ev. Nandi. In one experiment S, anceps top dry weights and seed yields were reduced to a similar extent by 20, 40. 80 or 164 (unthinned) E. indica plants/m². Similarly, the dry matter yields from populations of 20–164 plants/m² of E. indica did not differ significantly. Varying the density of S. anceps from 6 to 9.3 plants/m² had little effect on the yield of either species. The mean production of E. indica seed was 1968 kg/ha or approximately 5.3 10⁹ seeds/ha. In two experiments where equal populations of both species were planted at varying times in relation to each other the above ground dry matter yield of S. anceps was not significantly affected when E. indica emerged 2 weeks or later than S. anceps. This was true whether the S. anceps was planted in rows or uniformly throughout the plot. Ina third, similar experiment S. anceps top dry matter was significantly reduced by 21% when the E. indica emerged 2 weeks after the crop. In a time of weeding experiment, removal of E. indica for the first 13 days after planting was sufficient to prevent a significant reduction in S. anceps top dry matter and seed yields. In this experiment the two species were shown to be mutually exclusive.     

Discovery of an isolate of Fusarium oxysporum with potential to control Striga hermonthica in Africa

Native fungi of West Africa were evaluated as a means to control Striga hermonthica (Del.) Benth., a troublesome parasitic weed of several gramineous crops. An isolate of Fusarium oxysporum, grown on sorghum straw and incorporated into pots, successfully prevented all emergence of S. hermonthica, and resulted in a 400% increase of sorghum dry weight. The fungus inhibited germination and attachment of S. hermonthica to sorghum roots in Plexiglas root chambers. Several crop species, including sorghum, inoculated with the fungus showed neither any disease symptoms nor any reduction in biomass.     

Fusarium oxysporum Foxy 2 shows potential to control both Striga hermonthica and S. asiatica

Under glasshouse conditions, the ability of Fusarium oxysporum isolate Foxy 2, a fungal pathogen isolated from Striga hermonthica, to control S. asiatica and S. gesnerioides, was investigated on potted plants. In the experiment, the target weed of the fungus, S. hermonthica, was included as a standard. In the present study, S. asiatica was the only species that, besides S. hermonthica showed high susceptibility to Foxy 2. This susceptibility was demonstrated by almost complete prevention of emergence of the parasite. In contrast, S. gesnerioides did not show any susceptibility at all. The susceptibility of two Striga species to the fungus provides an opportunity for simultaneous control of both parasites in those regions where they are co‐existing (e.g. Tanzania and Kenya).     

The role of infection time in the differential response of sorghum cultivars to Striga hermonthica infection

Striga hermonthica is an important parasitic weed that severely reduces yields of sorghum in sub‐Saharan Africa. Pot experiments with the sensitive sorghum cultivar CK60‐B and the tolerant Tiémarifing were conducted in 1999 and 2000 to investigate the role of infection time on the interaction between sorghum and Striga hermonthica. Timing of Striga inoculation was used to establish delays of one and two weeks in first attachment of the parasite. In 1999, early Striga inoculation resulted in a relatively early first Striga attachment on CK60‐B. Although first infection of Tiémarifing occurred one week later, an identical final number of emerged Striga plants was observed. Plants of CK60‐B were more severely affected and supported a higher total Striga biomass. Only with this cultivar the interaction between host and parasite was significantly affected by delayed infection. Parasite biomass was most sensitive and already significantly reduced following a 1‐week delay in infection time. With a further 1‐week delay, an additional reduction in parasite biomass was accompanied by a strong and significant increase in total and panicle dry weight of the host plant. In 2000, first infection of CK60‐B was relatively late and occurred simultaneously with first infection of Tiémarifing and no significant effect of delayed infection on Striga biomass or host‐plant performance was observed. The results indicate that the influence of delayed infection strongly depends on actual infection time and confirm that earlier observed differences in time of first infection between the two cultivars do contribute to the more tolerant response of Tiémarifing to Striga infection.     

Prediction of Heteranthera reniformis competition with flooded rice using day–degrees

In field studies carried out in 1992 and 1993, several durations of growth of Heteranthera reniformis Ruitz et Pavon were established in flooded rice (Oryza sativa L.) to evaluate weed influence on crop yield using day-degree predictive models for weed growth. H. reniformis was allowed to emerge at 7-day intervals from rice emergence until 49 days later. Weeds that emerged with the rice accumulated 403 day-degrees during the first 49 days. Weeds allowed to utilize 308 day-degrees had a dry weight reduction of 20% compared with plants that emerged with the rice. With the loss of all 403 day-degrees dry weight was reduced by about 95%. Maximum leaf area index (LAI) was 2.8, reached at 308 day-degrees. Weed density ranged from 48 to 5 plants m^-2 when all 403 day-degrees were accumulated or lost by the plant respectively. When the weed lost only 95 day-degrees out of the possible 403 (1 week's delay in emergence) rice yield was 34% and 39% lower in 1992 and 1993 respectively. When the weed was allowed to accumulate ail 403 day-degrees yield reduction reached 62% in 1992 and 68% in 1993, very similar to the control plot, which were maintained weedy throughout (64% and 70.5%). After an accumulation of 403 day-degrees the weed reduced the number of rice panicles by 45% in 1992 and 38% in 1993.     

Spore cell wall components ofAspergillus niger elicit downy mildew disease resistance in pearl millet

Elicitors derived from the cell wall of fungi are shown to be active in eliciting resistance in plants against a wide range of pathogens. In the present study carbohydrate components from the autoclaved spore cell wall ofAspergillus niger were prepared as aqueous suspensions and tested for defense response in pearl millet (Pennisetum glaucum (L.) R.Br.) against the oomycetous downy mildew pathogenSclerospora graminicola (Sacc.) Schroet. The aqueous suspension derived from the spore cell wall ofA. niger was used as a seed soak treatment at concentrations of 0.25, 0.5, 1.0, 1.5 and 2.0 mg ml⁻¹ for time intervals of 3, 6, 9 and 12 h. The concentration of 0.5 mg ml⁻¹ for a 6 h soaking period offered 94% seed germination and seedling vigor index increased to 1526. The seed germination and the seedling vigor were significantly higher than the untreated check. Spore cell wall suspension as seed treatment at a concentration of 0.5 mg ml⁻¹ required a 3-day time interval to provide 67% protection against downy mildew. Histological and biochemical studies were conducted to elucidate the mechanism of defense response in treated seedlings uponS. graminicola infection. Resistance host response was detected in the form of lignin and callose deposition in the epidermal cell wall of pearl millet seedlings, which is the site ofS. graminicola infection. A time course study showed rapid and localized deposition of lignin and callose in epidermal cell wall of carbohydrate components-treated pearl millet seedling coleoptiles. Increased levels of the defense-related enzyme peroxidase were detected in the treated seedlings. Peroxidase activity in elicitor-treated samples reached a peak at 8 h post-infection, which was 45% more than in their respective uninoculated control. Characterization of peroxidase isoforms by isoelectric focusing revealed 16 different isoforms, of which pI 6.8, 7.2 and 8.7 increased in elicitor-treated samples uponS. graminicola infection. http://www.phytoparasitica.org posting Nov. 14, 2005.     

Selection of sorghum (Sorghum bicolor (L.) Moench) varieties resistant to the parasitic weed Striga hermonthica (Del.) Benth

Pot and field experiments were performed in Burkina Faso in 1987 and 1988 to evaluate the resistance of selected ‘low-stimulant’ sorghum (Sorghum bicolor (L.) Moench) varieties to the parasitic weed (Striga hermonthica (Del.) Benth. In a pot experiment, the variety IS-7777 supported the lowest number and had the latest emergence of Striga, compared with the other varieties tested. The varieties IS-14825, IS-6961, IS-7739, IS-14928 and IS-14975 also had signifi cantly lower numbers of emerged Striga per pot than the resistant control Framida. The resist ance of IS-7777 was confirmed in field experi ments, as was that of IS-7739, IS-6961 and IS-14928. However, the yield potential of these poorly adapted varieties was low in Striga-infested fields. The varieties IS-14975, IS–14825 and Seguetana Niarabougou exhibited a low susceptibility associated with a grain yield equivalent to that of the other varieties in farm fields infested by Striga. As Seguetana is already grown by Sahelian farmers, its use could be recommended in the absence of resistant varieties adapted to Sahelian agroclimatic conditions. The exceptionally high level of restance exhibited by IS-7777 could be exploited in studies on the genetics and mechanisms of resistance of the host plant to the parasite, as well as in sorghum improvement programmes.     

Effect of phosphate‐based seed priming on strigolactone production and Striga hermonthica infection in cereals

Strigolactones, plant‐secreted underground signalling molecules, play an important role in agricultural ecosystems, because they mediate the interaction of crops with symbiotic AM fungi and parasitic weeds like Striga hermonthica. Cereal host plants secret these signalling molecules particularly under nutrient‐deficient conditions and especially when phosphate (P) is limiting. The objective of the present study was to see the potential of P seed priming for S. hermonthica management in cereals in relation to strigolactone production. It has been demonstrated that P fertiliser application down‐regulates the production of these signalling molecules in the rhizosphere, which results in lower S. hermonthica infection of cereals. The laboratory study showed maximum production of strigolactones from dry and water‐soaked seeds, while seed soaking in P solution reduced their production. Similarly, maximum S. hermonthica infection was observed under control treatments with dry sowing or water soaking, while P seed soaking decreased S. hermonthica germination, emergence and dry biomass in all cereal crops. Our study shows that P seed priming resulted in lower exudation of strigolactones, which induced less S. hermonthica seeds germination and hence may lead to lower S. hermonthica infection. P‐based seed priming could prove to be an effective and affordable strategy to reduce S. hermonthica infection in cereals. Further research for practical field application is needed.     

Life-table analyses for the tomato leaf miner,Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae): effects of plant genotype

BACKGROUND

Host plant resistance plays an important role in integrated pest management programs. Crop resistance assessments commonly focus on only a single dependent variable, such as larval survival/plant damage, which limits the ability to appreciate the impact of host plants on pest populations in the full sense. Therefore, we performed life-table analyses for tomato leaf miner Tuta absoluta, on 19 Solanum lycopersicum genotypes and a wild Solanum habrochaites accession. These analyses assess the ability of the pest to attain a high population density on different tomato genotypes. Based on the resulting ranking of tomato resistance at the vegetative stage (45-day-old plants), we tested the resistance of six selected genotypes at the reproductive stage (4-month-old plants).

RESULTS

T. absoluta performance was significantly inferior on vegetative-stage S. habrochaites plants (LA 1777); time taken for the first instars to mine the leaves (5 ± 0.14 days), development time of early- and late-stage larvae (8.8 ± 0.2 and 4.2 ± 0.2 days, respectively), pupal period (11.2 ± 0.58 days), and total developmental time (29.4 ± 0.83 days) were significantly longer, fecundity was significantly lower (18.66 ± 7.24 days), and the highest total mortality (63.33%) also recorded compared with other genotypes, resulting in the lowest net reproductive rate (R₀) (11.20 ± 2.51). For the six selected genotypes, the ranking of plant resistance did not change between plants at the vegetative and reproductive stages.

CONCLUSION

This study suggested that of 20 screened tomato genotypes, LA 1777 and EC-620343 are the least suitable hosts for T. absoluta to establish fast-growing populations, and thus can be employed in integrated T. absoluta management. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.