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► Foxy 2 destroys all tissue types (vessel, parenchyma) of Striga seedlings belowground. ► Hyphae follow Striga haustoria into sorghum central cylinder to ensure its destruction. ► In ...contrast to Striga seedlings, Striga shoots above ground react to Foxy 2 infection. ► Striga shoots produce an electron dense wall layer which prevents digestion of vessels. ► Clogging of vessels by Foxy 2 hyphae contributes to death of Striga shoots. ► Seed treatment is a suitable delivery method for Foxy 2.
The fungal strain, Fusarium oxysporum f. sp. strigae (Foxy 2) has proved to be specific and effective in controlling Striga hermonthica (Del) “Benth.” However, little is known on its mode of action. The objective of this study was to evaluate the time and extent of colonization of the parasite by the fungus which is of critical importance considering the mutual interaction between S. hermonthica and its hosts. Anatomical investigations using light and electron microscopy were performed to assess the pattern of colonization and control of S. hermonthica seedlings and Striga shoots by Foxy 2. The connection between the sorghum root and the S. hermonthica seedlings showed that Striga haustorial intrusive cells penetrated the central cylinder of sorghum forming a continuous xylem connection but further growth of the S. hermonthica plant was prevented by Foxy 2. Hyphae invaded and digested all Striga seedling tissues belowground including haustorial intrusive cells, hyaline tissue, vessels, central xylem elements and Striga cortical parenchyma within 26days after sowing Foxy 2 coated sorghum seeds. In S. hermonthica plants that managed to emerge, hyphae had penetrated and colonized vessels clogging them over long distances, and were identified even in the top of the shoots. In contrast to young seedlings, there was no digestion of vessels in the diseased shoots above-ground. Transmission electron microscopy revealed that some hyphae were embedded in an electron dense wall coating which was formed along the secondary vessel walls in diseased S. hermonthica plants. This electron dense wall layer was uneven in thickness and also occurred along xylem parenchyma pit membranes apparently preventing invasion of xylem parenchyma cells. Penetration of intertracheal pit membranes by hyphae was observed from vessel to vessel while attempted penetration of xylem parenchymal pit membranes was foiled by production of wall apposition beneath the pit. Thus, two mechanisms were identified by which Foxy 2 controls S. hermonthica. (i) Complete digestion of S. hermonthica seedlings inside the host and (ii) clogging of vessels of emerged S. hermonthica plants by hyphae contributing to wilting and subsequent death.
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The application of the potential
Striga-mycoherbicide
Fusarium oxysporum f. sp.
strigae (Foxy 2) by seed coating is an appropriate option for delivering and establishing the biocontrol agent in the ...rhizosphere, the infection zone of the root parasitic weed
Striga hermonthica. Cytological investigations using light and transmission electron microscopy were performed to assess the pattern and extent of colonization of sorghum roots and shoots by Foxy 2, applied as film-coat on seeds. Germination of Foxy 2-treated plants was similar to un-treated plants. During sorghum root development, mycelia of Foxy 2 started from seed coat colonizing root surfaces. The intensity of root colonization increased with time. However, hyphae were not found in the apical zone of the roots. Hyphae growing on the surface of the roots were observed penetrating rhizodermal cells including root hairs, and colonizing the intercellular space and the cells of the cortical parenchyma. Even after four weeks, hyphae were not penetrating the endodermal layer to invade the tissues of the central cylinder. In addition, hyphae of Foxy 2 were also completely absent in the xylem vessels in sorghum shoots even after 11
weeks of sowing, which further proves the non-pathogenicity of the fungus to sorghum. Foxy 2 showed high efficacy in controlling
Striga in combination with both tolerant and susceptible sorghum varieties in a root chamber experiment, as reflected by the high percentage of diseased
Striga seedlings (95% and 86%, respectively), recorded after 26
days of sowing. The intensity of fungal root colonization, which coincides with the peak of
Striga attachments to its host root, will ensure high fungal efficacy and will facilitate the practical use of Foxy 2 against
Striga using seed treatment delivery. Indeed, this proof of sorghum non-pathogenicity of the fungus will encourage the acceptance of Foxy 2 by authorities and farmers in Africa.
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•Efficacy of 2 aflatoxin biocontrol products reported across crops, years and zones.•Aflatoxin reduced by 99% in 800 maize and groundnut farmers’ fields in Ghana.•Results are the most consistent and ...highest aflatoxin reductions reported to date.•Displacement of toxigenic fungi by the active ingredients caused the reductions.•Having both products offers greater versatility to modulate Aspergillus communities.
Biological control is one of the recommended methods for aflatoxin mitigation. Biocontrol products must be developed, and their efficacy demonstrated before widespread use. Efficacy of two aflatoxin biocontrol products, Aflasafe GH01 and Aflasafe GH02, were evaluated in 800 maize and groundnut farmers’ fields during 2015 and 2016 in the Ashanti, Brong Ahafo, Northern, Upper East, and Upper West regions of Ghana. Both products were developed after an extensive examination of fungi associated with maize and groundnut in Ghana. Each product contains as active ingredient fungi four Aspergillus flavus isolates belonging to atoxigenic African Aspergillus Vegetative Compatibility Groups (AAVs) widely distributed across Ghana. An untreated field was maintained for each treated field to determine product efficacy. Proportions of atoxigenic AAVs composing each product were assessed in soils before product application, and soils and grains at harvest. Significant (P < 0.05) displacement of toxigenic fungi occurred in both crops during both years, in all five regions. Biocontrol-treated crops consistently had significantly (P < 0.05) less aflatoxins (range = 76% to 100% less; average = 99% less) than untreated crops. Results indicate that both biocontrol products are highly efficient, cost-effective, environmentally safe tools for aflatoxin mitigation. Most crops from treated fields could have been sold in both local and international food and feed premium markets. Adoption and use of biocontrol products have the potential to improve the health of Ghanaians, and both income and trade opportunities of farmers, aggregators, distributors, and traders.
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•Striga hermonthica is a serious parasitic weed of maize in Western Kenya.•Fusarium oxysporum (Foxy 2) is imported into Kenya for biological control.•Efficacy of Foxy 2 is tested ...under post-entry quarantine (PEQ) conditions in Kenya.•Results show that Fusarium oxysporum (Foxy 2) is ineffective in Kenya.
The production of maize, a major staple food crop in sub-Saharan Africa is being constrained by the parasitic weed Striga hermonthica. The fungus Fusarium oxysporum f. sp. strigae (Foxy 2) that causes fusarium wilt of Striga in Ghana, West Africa, is being considered for biological control of the weed in Western Kenya. The present study investigated the efficacy of F. oxysporum f. sp. strigae (Foxy 2) for S. hermonthica management in Western Kenya. Research was conducted in post-entry quarantine (PEQ) facilities at Alupe, Busia, Homabay, Kibos and Siaya field stations for two seasons. Each PEQ was a split-plot, with 4main blocks each having 6 treatment subplots. The treatments included seeds of two S. hermonthica-susceptible maize varieties, either coated with Foxy 2 using gum Arabic, gum Arabic alone, or left untreated. Data was collected over seven sampling periods on S. hermonthica population per plant, percentage of those that were wilting, and the severity of wilting. Maize plant growth parameters assessed included duration to 50% anthesis and 50% silking, plant height, number of leaves, stover and cob weights, and maize yield per hectare. Statistical analysis was done using SAS 9.1 software. Data on S. hermonthica population were analyzed by χ2-test using Proc Genmod (Poisson); while the other parameters were analyzed by Proc Mixed using study location, season and blocks as random effects, and the sampling periods as repeated effects. All the assessed parameters were similar between plants grown from seeds inoculated with F. oxysporum f. sp. strigae (Foxy 2), those coated with gum Arabic, and the ones without any coating. These parameters were also not different between the maize varieties. There are varying reasons for the disparities between results on F. oxysporum f. sp. strigae (Foxy 2) obtained in this Kenyan study, and those from researches outside this country. In conclusion, F. oxysporum f. sp. strigae strain Foxy 2 is predominantly safe on maize growth, but its efficacy in controlling S. hermonthica was not evident on the tested Kenyan soils.
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Fusarium oxysporum f. sp. strigae (Foxy 2) is a mycoherbicide against Striga hermonthica. To ensure the safe use of this biocontrol agent, and as part of the risk assessment, this study was aimed at ...providing cytological evidence that Foxy 2 does not possess pathogenic behavior towards the non-target host sorghum. Therefore, we compared the infection processes and sorghum root tissue reactions towards the pathogenic F. proliferatum to that of Foxy 2 using light- and transmission electron microscopy. Given that during the growth process, hyphae could get into the central cylinder, tissue specific reactions of sorghum to Foxy 2 were also investigated by wounding the roots (exposing the vascular system), and testing for proliferation of hyphae within the vessels. Results showed that 2 weeks after sowing, F. proliferatum had invaded and destroyed all cell types including the central cylinder while Foxy 2 hyphae were located around the outer endodermal layer and were not able to penetrate the latter. There was an increase in blue autoflourescence in the central cylinder and especially the endodermis, probably due to increased phenolics in Foxy 2 infected roots which was not the case for F. proliferatum. This might contribute to the inability of Foxy 2 to penetrate the endodermis. Transmission electron microscopy showed extensive degradation of endodermis and vessel walls into thin translucent layers by F. proliferatum but not by Foxy 2. In the mechanically wounded and infected roots, Foxy 2 could invade the central cylinder close to the wound but was not identified a few millimeters away from the wound. This implies that it was not able to grow within or destroy the central cylinder even when already present within it; probably due to the observed increased phenolics. Thus, exposure of the vascular system did not serve as a route for the invasion of Foxy 2 which therefore could not further cause tracheomycosis. Therefore, Foxy 2 could be seen as a fungus well suited for biocontrol.
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Fusarium nygamai has been isolated and identified from diseased Striga hermonthica plants collected from sorghum fields in the Sudan. Fumonisin B
1
, produced by this fungus, was isolated and ...purified. In in vitro experiments, the bioherbicidal potential of this mycotoxin was evaluated against S. hermonthica and S. asiatica. Different concentrations (250, 500 and 1000 μg L
−1
of water) of fumonisin B
1
, applied during the conditioning phase of the Striga seeds significantly reduced the germination of both species in a range of 19.8-32.2 and 34.5-47.6%, respectively. In addition, the length of the germ tubes was reduced and the germ tubes became brownish. Fumonisin B
1
slightly delayed the emergence of Striga when 250 mg fumonisin B
1
dissolved in 1 L of water were applied into the soil per pot before sowing sorghum. By the way of contrast, fumonisin B
1
was very phytotoxic when Striga seedlings (leaves and stems) of different heights and ages were sprayed with a concentration series of 250, 500 and 1000 μg fumonisin B
1
mL
−1
until run-off, or when plants were injured with a scalpel before fumonisin B
1
was applied into the injury at concentrations of 250 and 500 μg mL
−1
. One day after application, the plants showed symptoms of wilting, first at the top and then moving downwards. Leaves turned black and stems desiccated at the point of injuries. Within 4 days after the application, 55% of the plants tested were completely destroyed. In total, at the end of the experiment, 40 days after the treatment, 85% of the treated plants were killed by fumonisin B
1,
regardless of the concentration applied and the application technique used.
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Coating sorghum seeds with Fusarium oxysporum (Foxy 2) for control of the root parasitic weed Striga, appears to be an attractive option for minimizing the inoculum amount, establishing the ...biocontrol agent in the potential infection zone of the host plants, and offering a simple, easy and economical delivery system. Our investigations resulted in the selection of appropriate seed coating materials and a suitable type and form of fungal inoculum. The coating materials tested were arabic gum (AG10%, 20%, 40%), carboxymethylcellulose (CMC1%, 2%) and pectin (LS 440, LM-5 CS) 1%, while the fungal inoculum included microconidia and fresh and dried chlamydospores produced using different substrates. Foxy 2 survived the seed treatment processing and showed excellent viability on seeds for at least 8 months of storage after coating. In general, the performance of 40% arabic gum in combination with dried chlamydospores was the best among the other types of inoculum and coating material tested. Regardless of the type and form of inoculum and coating materials tested, Foxy 2 was able to colonize all roots, even root tips and hairs of the host (sorghum), thereby meeting important criteria of a promising candidate for controlling Striga when applied as a seed treatment. The efficacy of treated sorghum seed with Foxy 2 using different coating materials in reducing S. hermonthica infestation was evaluated in pot and root chamber trials. Foxy 2 markedly reduced Striga emergence and dry weight and increased the percentage of the diseased emerged Striga shoots. However, the efficacy of seed coating varied according to the type and form of fungal inoculum as well as coating material. Coating sorghum seed with dried chlamydospore inoculum homogenized into 20% arabic gum (as adhesive) showed the highest efficacy of 81 and 77% (i.e., percent reduction in healthy emerged Striga shoots compared to the control treatment) against Striga using either sterilized or non-sterilized soil, respectively. In root chamber bioassays, the application of Foxy 2 in combination with AG40% significantly caused disease in 77% of the germinated Striga seeds and in all tubercles after 25 days of sowing. These findings provide an optimized coating protocol as an attractive delivery system for bioherbicides for root parasitic weeds.
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The lack of an adequate shelf-life in mycoherbicidal products has been an obstacle to their commercialization. Therefore, experiments were conducted to study the effect of inoculum type and ...concentration on the viability of the encapsulated propagules of
Fusarium oxysporum, abbreviated as Foxy 2, in Pesta granules during storage. Pesta granules were made with different inocula of Foxy 2, including: microconidia, a mixture of mycelia and microconidia, and fresh as well as dried chlamydospore-rich biomass, each with three different inoculum concentrations. All granular preparations (0.5–2
mm, particle size) were stored at 4
°C or at room temperature (21
±
3
°C) for 1year. All Pesta granules containing chlamydospore-inoculum retained higher viability (up to 100%) than those with mycelial and/or microconidial inocula, irrespective of inoculum concentration and storage temperature. Microconidial and mycelial preparations were not viable after 1 year at room temperature. Throughout the year, the viability of fresh and dried chlamydospore-rich biomass was not significantly affected by the concentration of chlamydospores in the formulation at 4
°C or room temperature. Thus, 85–100% viability of Foxy 2 propagules can be achieved in Pesta granules (0.5–2
mm) for at least 1 year by formulating chlamydospore-rich biomass and storing at a temperature of 4
°C. This information has significant implications in enhancing shelf-life of Foxy 2 products thereby helping us to overcome this obstacle to commercialization.
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The effect of vacuum packaging on the shelf-life and handling of Pesta granules and seed treatment made with chlamydospores of Fusarium oxysporum strains Foxy2, PSM197 or their mixture was studied at ...4°C and 22±3°C over 1 year. In addition, the effects of co-incorporated amendments urea in Pesta or co-delivered fungicides (Ridomil Gold®, Apron XL®) on coated sorghum seeds, and coating material (Arabic gum 'AG', SUET Binder 'SB') on the viability of Striga-mycoherbicides were evaluated. Storage under vacuum packaging did not enhance shelf-life of the formulated Striga-mycoherbicidal products after 12 months of storage regardless of the treatment used. The co-incorporated urea into Pesta granules significantly reduced the viability of mycoherbicides, but less so at 4°C (58% strain-stability after 12 months). No significant differences between the coating materials in maintaining the viability of mycoherbicides were observed. The shelf-life of isolates on coated seeds significantly decreased when adding Ridomil Gold®. However, at 4°C, the fungicide Apron XL® allowed better survival of Foxy2 and PSM197 by maintaining their averaged half-lives (t
0.5
) by an additional 6 months compared to Ridomil Gold®. In general, Striga-mycoherbicidal product combinations exhibited a significantly higher shelf-life when stored at 4°C than at 22±3°C. The absence of a positive effect of vacuum packaging on shelf-life of Striga-mycoherbicidal products reflects the tolerance of the formulated fungal propagules (chlamydospores) to withstand an oxygen enriched environment and allows their handling and distribution through ordinary packaging systems in Africa. The high compatibility between Striga-mycoherbicides and the co-delivered fungicide Apron XL®, and the fungal storage stability allows simultaneous control of Striga and fungal cereal diseases within an integrated pest management (IPM).
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