Biological control of Striga hermonthica (Del.) Benth. using formulated mycoherbicides under Sudan field conditions
Year: 2008
Type: dissertation
Abstract: Striga hermonthica is a parasitic flowering plant belonging to the family Orobanchaceae. It is a root parasite that attacks sorghum, maize, millet and several grass weeds in the semi-arid Tropics. In Sudan, Striga is widespread in irrigated and rainfed areas and considered the main biotic constraint in production of sorghum, the main staple food for the majority of Sudanese people. More than 500,000 hectares under rainfed cultivation are heavily infested with Striga, which commonly results in significant yield losses of 70 ? 100%. It has become obvious that there is no simple, fast and inexpensive solution to the Striga problem in Africa. Biological control is considered a potential cost-effective and environmentally safe means for reducing weed populations in crops, forests, or rangelands where low profit margins prevent large herbicide expenditure.\nBiological control using microorganisms (especially phytopathogenic fungi) showed a high efficacy in controlling S. hermonthica under controlled and field conditions. However, so far it did not come to practical field application. This could be attributed to environmental obstacles or due to the lack of appropriate delivery systems. The pathogenicity of two fungal isolates indigenous to Sudan (Fusarium nygamai [FN] and F. ?Abuharaz? [FA] isolate) against Striga has been studied using infected sorghum grains or a spore suspension as inoculum. These formulations were very effective in controlling Striga under controlled and natural conditions; however, a high level of fungal inoculum (approximately 800 kg ha-1 for the grain inoculum) would be required for effective control, which arises a lot of problems e.g. concerning sterilization and transportation. Such problems can be overcome by adopting an appropriate formulation technology. Granular formulations such as ?Pesta? and alginate pellets were found to be suitable delivery systems for controlling weeds. ?Pesta? granules are made by encapsulating bioagents in a gluten matrix. Alginate formulations are prepared by incorporating the biocontrol agent?s propagules in a sodium alginate solution, which is dripped to a calcium chloride or calcium gluconate solution. Alginate pellets are then formed by ionotrophic gelation. \nThe main objectives of this study were: (a) to study the efficacy of the two Fusarium species in controlling Striga under field conditions using ?Pesta? and alginate formulations, (b) evaluate the effect on sorghum yield, (c) determine the optimum dose of the formulated material, (d) investigate the persistence of the formulated fungal isolates in the soil, and (e) study the efficacy of seed treatments as an alternative delivery system. Furthermore, for environmental safety reasons the newly isolated F. ?Abuharaz? isolate was tested for its ability to produce some of the most important mycotoxins. Harvested sorghum seeds out of the fungus-treated plots were also investigated for their mycotoxins content.\nA prerequisite to be able to formulate biocontrol fungi is the development of an inexpensive method of inoculum production that yields sufficient biomass containing viable, highly virulent propagules. Chlamydospores are the soil-persisting propagules of many Fusarium species and considered as ideal propagules to be used in granular formulations. For this reason, finding a medium suitable for the production of chlamydospores by the two Fusarium isolates was one of the specific objectives of this study. Different media were tested among them Special Nutrient-poor Broth (SNB) + yeast gave the highest number of chlamydospores (105 ml-1) in both isolates throughout the incubation period. However, both isolates generally did not form sufficient chlamydospores to be used within a bioherbicide formulation. Richard?s solution gave the highest number of microconidia (108 ml-1) after five days of incubation and hence it was selected as growth medium for formulation purposes throughout this study.\nFN and FA were successfully formulated in ?Pesta? and alginate granules amended either with 10% wheat flour or 6% sorghum flour or yeast extract. Alginate granules generally gave higher numbers of colony forming units (cfu) per g of formulated material compared to ?Pesta?. Alginate preparations amended with 6% sorghum flour or yeast extract had significantly higher cfu compared to the alginate formulation using 10% wheat flour. Yeast extract amendment further increased the number of cfu by about 38 and 32% for FN and FA, respectively, compared to sorghum amendment.\nIn the first field experiment (2003/04), a screening for the suitable dose of ?Pesta? granules per planting hole to control Striga was conducted together with the investigation of a seed coating treatment as an alternative delivery system of the biocontrol agents. The ?Pesta? technology showed a potential to be used as a delivery system to control S. hermonthica under field conditions. Both ?Pesta?-formulated Fusarium isolates were able to delay Striga emergence, reduce the total number of Striga shoots and induce disease symptoms on all growth stages of Striga plants, irrespective to the dose and method of application used. The highest control efficacy was achieved by applying FA at 1.5g, which reduced the total number of parasite shoots by 82 % and the number of healthy Striga shoots by 88% compared to the untreated control. As a consequence, sorghum biomass and sorghum 100-seed weight were increased by 86 and 110%, respectively. FN and the combination of the fungal isolates were slightly less efficient in controlling the parasites. 1.5 g ?Pesta? granules per planting hole was found to be the optimum dosage for Striga management since increasing the dosage did not result in a significant improvement of control.\nIn the second season (2004/05), the efficacy of alginate formulations amended with 10% wheat flour applied at 1.5g/planting hole was evaluated in addition to the ?Pesta? formulation for controlling Striga under field conditions. Alginate granules were able to delay Striga incidence significantly and reduce the total number of Striga shoots by 64 ? 78 % compared to the control early in the season. In contrast to the first season, fungal isolates formulated in ?Pesta? granules had no pronounced effect on delaying Striga emergence, however, ?Pesta?-granulated Fusarium species were able to reduce the total number of Striga shoots by 42 ? 55 % compared to the control early in the season. By the end of the season, the effect of both formulations on the total number of Striga shoots became negligible, but they significantly increased disease incidence on Striga shoots compared to the untreated control. FA formulated in ?Pesta? or alginate pellets was especially effective in this regard, causing disease in 74 and 80% of the Striga plants and reducing the total number of healthy Striga shoots by 55 and 60% compared to the control, respectively. FA applied as ?Pesta? granules was the most effective treatment in reducing Striga biomass by 58 % compared to the control which was positively reflected in an increased sorghum grain yield (63%) and sorghum straw yield (73%) compared to the control.\nThe reduction of the efficacy of the ?Pesta? formulation in controlling Striga in the second season compared to the first season can probably be attributed to three reasons. These include a) climatic conditions, which differed from the first season in higher temperatures coupled with lower rainfall and low relative humidity, b) sodicity problems in the fields which might have affected the proliferation of the fungi in the soil and c) an inhibitory effect of the metabolites of the applied insecticide Sevin (active ingredient Carbaryl (1-naphthyl N-methylcarbamate)) on the virulence of soil fungi.\n\nFurthermore, an outdoor pot experiment was conducted to study the efficacy of alginate formulations with different amendments (wheat flour, sorghum flour and yeast extract) in comparison to the ?Pesta? formulation and seed treatments on controlling Striga. In this experiment, FA formulated as ?Pesta? granules was the most effective treatment and successfully inhibited Striga emergence until the end of the season. This was reflected in a significantly increased sorghum plant height (by 80%) and sorghum shoot dry weight (400%) compared to the negative control. Fusarium species in alginate granules also delayed Striga emergence and reduced the total number of Striga throughout the growing season. The best efficacy was obtained by FA, which reduced the total number of Striga shoots by 71% (using 10% wheat flour) and 84% (6% sorghum flour or yeast extract) compared to the control. Likewise, it reduced the proportion of healthy Striga shoots by 71%, 88% and 84%, respectively, and Striga biomass by 50%, 81%, and 89%, respectively. Alginate formulations generally also significantly increased sorghum plant height by up to 80% and sorghum shoot dry weight by 200 to 400% compared to the control.\nIt can therefore be summarized that of the investigated fungal isolates and granular formulations FA formulated in ?Pesta? granules showed the best efficacy in controlling Striga under field and controlled conditions.\nThe ability of FA to produce trichothecene mycotoxins that could be a hazard to humans or animals was assessed from samples of the fungus growing on autoclaved wheat grains. Additionally, samples of harvested sorghum seeds from various plots inoculated with the biocontrol agents were investigated for the content of trichothecene mycotoxins. None of the following toxins were either produced by FA or translocated to harvested sorghum seeds under field conditions: nivalenol, fusarenon X, deoxynivalenol, 15-actetyldeoxynivalenol, scirpentriol, monoacetoxyscirpenol, diacetoxyscirpenol, T-2 triol, HT-2 toxin, T-2 toxin and neosolaniol. \nA simple seed coating treatment using fine ?Pesta? granules and gum Arabic as adhesive material also showed a potential to control Striga in the f
Author: Eldur B. Zahran
Language: English
Subfield: Plant Science
Domain: Life Sciences
SDGs: Zero hunger, Life on land
Open Access status: green
OpenAlex ID: W1920098911