Satisfactory bioherbicides products should generally have a long shelf life, ease of use, low cost, and high efficiency. Bioherbicides products contain living microorganisms and are more demanding on the dosage form.
Many fungal biological herbicides can be stored in the form of dry spore powder at room temperature for 1 to 2 years before use. Spores are fermented on a liquid or solid substrate. After mechanically harvesting spores and drying, they can be processed into spore powder preparations. Formulations have become a primary issue in recent studies, and starting with dosage forms can reduce the dependence of biological herbicides on dew.
Solid dosage form
Many kinds of cereals are used as materials for the production of fungal growth carriers and biological herbicides, including rice, barley, millet, wheat, and the like. After the fungus has been cultured for a period of time, the culture material is dried, ground and refined for use.
Walker and Connick invented a new method of loading a bioherbicidal fungus into a calcium alginate capsule, the fungal propagule dissolved in a sodium alginate solution, and then the mixture was added dropwise to the calcium chloride solution. A fungal propagule is included in each droplet of calcium alginate. The dried granules can be used directly as a preparation or as a carrier for fungal spore preparations for other dosage forms. This technique has been widely used and has been further improved in trials.
Connick et al. developed another novel batter embedding method called "Pesta". The fungal suspension is mixed with wheat flour and kaolin to form a dough, which is processed into a thin layer and then made into granules. This technical improvement includes the use of different flours, the addition of rapeseed oil, starch, pyrophyllite, vermiculite, or the use of different pressing, fluidized bed drying procedures.
Quimby et al. reported another solid dosage form, using materials such as absorbent starch, sucrose powder, and hydrous silica, called the “Stabileze Processâ€. The composite packaging method described by Baker et al. uses ion-free polymer particles, immersing the spore suspension in 6 mol/L sorbitol, mixing with 5% ethylene polymer in ethanol, and spraying into n-hexane at -30 ° C to form granules. Recently, Chittick et al. reported that when Phomopsis sp. was used to control saffron, the hyphae fragment was made into a dry powder for water spray.
The main disadvantage of solid form bioherbicides is that suitable humidity conditions must be encountered after application to allow weed pathogens in the bioherbicides to grow and infect plants. During this time weed pathogens must be able to survive in the field and prevent ants from eating.
It is generally appropriate to apply a bioherbicidal solid dosage form when it is not necessary to urgently control weeds such as grasslands, pastures, and natural ecosystems. However, for crops, timely control of weeds minimizes disasters, and the focus of bioherbicidal formulation research should be placed on herbicide formulations similar to conventional spray applications.
2. Liquid dosage form
CollegoR is a model for the development of many potential bioherbicides. Both DeVineR and CollegoR herbicides are fermented in bioreactors (fermenters). DeVineR products are liquid and have a short shelf life. CollegoR is made by collecting spore dry powder and needs to be rehydrated before spraying.
The simplest liquid dosage form is an aqueous spore suspension, usually with 0.1% Tween 20 as a wetting agent. This preparation is usually compared as a standard control and a new complex dosage form. This simple aqueous suspension also achieves good results under ideal conditions for pathogenic fungi to infect weeds. Wetting agents such as Tween can help spread the bacterial suspension on the surface of the leaves. Silicone-polyester copolymers such as Silwet L-77 can help small spores penetrate into the pores and lenticels of plants. However, the wetting agent reduces the surface tension of the bacterial suspension on the one hand, and increases the evaporation rate of water on the other hand.
A number of substances have been tested which are capable of reducing the water loss rate of liquid dosage forms, including simple humectants such as glycerol and sorbitol, however, the results demonstrate that the use of only humectants has not achieved significant results. The complex polysaccharide has been tried as a binder, but it has not been widely used in practice, and it has been used as a moisturizing material as a polymer.
(1) Polymer
Shabana et al. evaluated eight hydrophilic polymers, including alginate, resin, polyacrylamide, cellulose, and the like. Experiments in a controlled environment have shown that the addition of Gellan resin and polyacrylamide to the mycelial inoculum of Alternaria eichhormeae enhances the pathogenicity of the suspension.
Chittick and Auld added three natural resins and four synthetic hydrophilic polymers as auxiliaries to the herbicides, and evaluated their effects on herbicidal effects. The test results show that the dosage form with the hydrophilic polymer has only a little synergistic effect compared to the single aqueous suspension lacking the wetting agent. The dosage form to which the hydrophilic polymer was added still had a small amount of water remaining after 8 hours under low humidity (RH 33%), and the activity of the residual water was high, indicating that the hydrophilic polymer had a certain moisturizing effect. However, although some polymers are used to maintain a certain amount of water after 6-8 hours, the herbicidal effect of the fungus Colletotrichum orbiculare has not been improved because the polymer itself The germination of fungal spores has a hindrance effect.
Experimental results by Shabana et al. demonstrated that the polymer-containing solution was able to hold water for several days under RH (55 ± 5)%. However, the use of a large amount of polymer solution (100 g) is close to the maximum amount of water on the surface of the leaves compared to the amount of 0.5 g in the Chittick and Auld experiments.
(2) Simple emulsion
The control effect of C. orbiculare on Xanthium spinosum L. under controlled conditions in indoor conditions showed that a simple vegetable oil emulsion (containing 10% oil and 1% emulsifier) ​​can reduce the circle. The dependence of the thorn spore on dew, but the opposite result in the field trial. Zhang and Watson used an improved method to add 1% (w/v) glucose and 0.2% (w/v) carboxymethylcellulose to the oil phase for spraying the Exserohilum monoceras test. The result of 90% control of alfalfa was obtained. Other improvements on the basis of simple emulsions have also achieved significant synergies. Boyette uses a blend of unrefined grain oil and water 1:1 (V/V) to make a simple emulsion, which can be used in the control of the genus Sesbania exaltata (Raf.) Cory in C. truncatum. Reduce dependence on dew. Why the simple emulsion can reduce the dependence on water, the mechanism is still unclear. Greaves et al. believe that this is due to the formation of a conversion emulsion on the surface of the leaves, while the oil phase penetrates into the mesophyll tissue and binds to the water component of the tissue.
(3) Modified emulsion
The modified emulsion consists of a continuous oil phase containing small droplets. Quimby et al. first recognized that the production potential of this dosage form is capable of reducing the evaporation of biological herbicides. This dosage form overcomes the dependence on dew while reducing the concentration of spores of the desired pathogen, but it still has drawbacks. Due to its high oil content (>30%), the cost is increased and the viscosity of the formulation is increased, requiring special spraying devices such as air-assisted spray heads. In addition, high doses of oil may be toxic to plants and affect the growth of non-target plants.
(4) Water-in-water emulsion
Auld invented a new type of compound emulsion herbicide formulation and applied for a temporary patent called Water Water Ingredients (WOW). It is made by dispersing fine water droplets of water in a continuous aqueous phase. This emulsion contains at least one lipophilic surfactant and a hydrophilic surfactant. The pathogen spores may be present in the innermost aqueous phase or the outermost aqueous phase, or in both aqueous phases. The oil content of this emulsion can be adjusted between 1% and 5% as needed. The composite emulsion can be sprayed using a conventional spray device. WOW emulsions have been known for a while, and although they have been used in the pharmaceutical, cosmetic, and food industries, they have not been used in agriculture and horticulture. Repeated tests were conducted under controlled conditions, and C. orbiculare was applied to control weeds in the genus, indicating that the WOW emulsion reduced the dependence on dew. Compared with the 13 h treatment of the aqueous suspension and dew, the WOW in the field lacking dew There was no significant difference in the effect of emulsion control.
(5) Other liquid dosage forms
Attempts have been made to use long-chain fatty alcohols as outer wrappers to reduce water evaporation, but the problem is that the outer wrap is sensitive to contaminants and easily decomposes. Pathogen spore germination stimuli such as iron chelators and nutrients can shorten the sensitive period of pathogenic bacteria to low humidity. Phytotoxins and a range of enzymes and enzyme inhibitors can be used as auxiliaries, which are usually specific to any pathogen and herbicidal compound.
3. Special dosage form
Special dosage forms are required under some special environmental conditions. For example, a dispersible dosage form is suitable for aquatic weeds and rice field weeds. Gohbara and Tsukamoto introduced a new type of dispersible formulation in which the herbicidal microbes are encapsulated in an emulsion and the outer layer is coated with a powder of exceptional gravity. The dispersed dosage form consists of a 2:1 (V/V) mixture of NaHCO3 and citric acid, a concentrated spore suspension, and a vegetable oil containing a lipophilic surfactant.
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