MODE OF ACTION Frequently Asked Questions

Transcription

1 MODE OF ACTION Frequently Asked Questions

2 MODE OF ACTION Q: How do entomopathogenic fungi kill pests? A: The spores of an entomopathogenic fungus must land on a target pest, which it recognizes as a host before it will even germinate. Once it has germinated, it will grow over the surface of the host in search of a soft area where it is easier to penetrate the insect pest. It then forms a special structure, called an appressorium, which helps it to drill down and enter the pest s body. Once inside the pest, it must be able to overcome the pest s own immune system in order to enter the insect s haemolymph. The insect s haemolymph is full of nutrients, which feed the EPF allowing it to grow. In the process of growing, the EPF gradually kills the pest and eventually the fungus grows and sporulates on the outside of the dead insect. Q: How long does it take for an EPF to kill the pest? A: The speed of death depends on the application rate of the biopesticide, the temperature and the susceptibility of the pest to that particular isolate of the biopesticide. It may take 3 to 10 days for the pest to die, but during this period it may feed less on the crop plant and lay fewer eggs. Q: When is it advantageous for an EPF to kill the host more slowly? A: If the EPF is being used in an auto-dissemination device and is relying on the host to re-distribute the EPF within the pest population the ADD is more efficient, the longer it takes for the contaminated pest to die. The infected pest will come into contact with more pests, the longer it lives and more pests will receive EPF spores. (see FAQ on ADD). Q: Why do EPFs not kill predators and parasitic wasps? A: The parasitic wasps have defense mechanisms, which protect them from infection by EPFs. The EPFs tend to be very specific in their target pest and parasitoids come from very different insect families to most pests. Parasitoids often have the ability to distinguish between hosts that are infected or not infected by EPFs and avoid coming into contact with the EPF. Q: How important is secondary pick up of EPF spores sprayed onto leaf surfaces? A: Whilst there is some secondary pick of EPF spores by host insects passing by spores previously deposited on the leaf by commercial spray applications - this is of minor significance compared to direct application of the spores onto the host insect s body. This requires optimum application coverage and application at a time of day when the pest is exposed to spray coverage (on top of the leaf). 1.

3 REAL FACT There is so much natural competition in the soil, that applying non-indigenous microbes to protect plants from pests and diseases - will not have a long-term effect on biodiversity. Bees can even be used to deliver biopesticides to flowers. They pass through a foot bath containing the biopesticides and collect the spores on their legs. The spores dislodge in the flower when they are pollination the flower. Pests such as thrips feed in the flowers and can pick up the Real Metahizium. Botrytis spores also land in the flowers and will have to compete with the Real Trichoderma. 2.

4 Metarhizium infection of stink bug (left) Beauveria infection over 5 days of pine weevil (below). Q: Why do fungi and bacteria colonise roots? A: Roots release nutrients made up of organic acids and inorganic hydrocarbons that microbes use as a food source. Soil microbes will colonise hot spots on the roots surface where they can feed on sloughed-off root cap and border cells, mucilage, and plant exudates. Q: What are plant growth-promoting rhizobacteria (PGPR)? A: Rhizobacteria are root-colonizing bacteria that form symbiotic relationships with many plants. The name comes from the Greek rhiza, meaning root. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties (mutualism). The relationship of PGPRs with host plants are either rhizospheric (limited to the outside of roots) or endophytic (within the host tissues). Biostimulants = PGPR = Biofertilisers. Q: What are the modes of action of PGPRs? A: PGPR can (a) increase nitrogen availability to the plant (b) precipitate insoluble compounds from the soil and sequester these in their own cell components thereby cleaning up heavy metal pollution in the soil (c) migrate form the rhizoplane to the rhizosphere where they can bind ions in biologically unavailable forms (d) assist in the formation of iron-chelating siderophores to improve the fitness of plants by increasing iron uptake. (e) reduce the intake of sodium into the plant and avoiding sodium toxicity in high saline soils (f) increase the availability of nutrients to the plant by production of organic acids that change the ph of the soil near the root. 3.

5 Q: How can PGPR affect the morphology of roots? A: Some PGPRs produce phytohormones (e.g. auxin), which promote the formation of lateral roots. Increased lateral root formation leads to an enhanced ability to take up nutrients for the growth of the plant. Q: Do microbes colonise all types of plant roots to the same extent? A: Plants are able to shape their rhizosphere microbiome by secreting different exudates attractive to different soil microbes. Different plant species host specific microbial communities when grown on the same soil. Rhizobium colonise legume roots but not other types of plants. Q: How does symbiotic relationship between legumes and Rhizobium work? A: A symbiotic relationship between the plant and the microbe requires both the plant and the microbe to benefit. This requires some compromises to take place. For example: For the plant to be able to benefit from the added available nutrients provided by the rhizobacteria, it needs to provide a place and the proper conditions for the rhizobacteria to live. Creating and maintaining root nodules for rhizobacteria can cost between 12 25% of the plants total photosynthetic output. 4.

6 Q: How much nitrogen can Rhizobium fix in a legume crop? A: Legumes can fix more than 250 kg N per hectare. However, the amounts of N 2 fixed can vary considerably depending on pesticide applications to the soil, presence and effectiveness of rhizobia, pest damage, plant genotype and age, plant and rhizobia interactions and changes in soil physiochemical conditions. Nitrogen is an important building block of proteins and the seeds of legumes (beans and peas) are valued for their high protein contents. Q: How does having a legume in the rotation help other crops in the rotation? A: The fixed N2 is released when the plants die, making it available to other plants and this helps in fertilizing the soil. If the legume crop debris is removed from the field there is less benefit because most of the nitrogen is in the leafy and fruiting parts of the legume plant. Q: Are all Rhizobium the same? A: Even among rhizobia that can nodulate the same plant, there are many different, genetically distinct, strains. Some fix nitrogen better more efficiently than others, resulting in superior plant growth. Some also compete better with rhizobia that are already in the soil. This means that they can enter the plant's root hairs more efficiently, resulting in faster nodule formation. Commercial Rhizobia strains have been developed for specific legumes (peanuts, cowpeas, clover etc.) Q: How can root knot nematode galls and Rhizobium nodules be distinguished on the roots of legumes? A: The galls of rkn are swellings within the root, which cannot be rubbed off by hand (below right) and Rhizobium nodules are external balls which can be rubbed off by hand (below left). If the Rhizobium nodules are actively fixing nitrogen they will be pink in colour when dissected. 5.

7 Q: How do mycorrhizae work? A: Mycorrhizal fungi attach to the surface of the root and penetrate in or around the inside of the root cells. Filaments (called mycelium) extend into the surrounding soil, effectively extending the plant's roots and root absorbing capacity for nutrients and water, from ten to several thousand times. In return, the mycorrhizae receive from the plant roots sugars and other compounds. The mycorrhizal mycelium release enzymes that dissolve tightly bound minerals like phosphorus, sulphur, iron and all the major and minor nutrients used by plants. The nutrients are organically assimilated by the mycorrhizae and become readily available for use by the plants. The fungal filaments also bind soil particles into larger aggregates with organic glues such as humic compounds; the resulting soil structure allows air and water movement into the soil, encouraging root growth and distribution. Q: What s the difference between ecto-mycorrhizae (VAM) and endo-mycorrhizae? A: Ecto-mycorrhizae form a sheath around the plant roots and mainly colonise conifers and oaks. Endo -mycorrhizae (vesicular arbuscular mycorrhizae - VAM) will penetrate the root with a structure that enable exchanges to take place between the mycorrhizal network and the plant. Most vegetables, grasses, flowers, shrubs, fruit trees and ornamentals associate with VAM. Q: Are phosphorous levels too high in hydroponics for mycorrhizae to invade plant roots? A: Mycorrhizae spores remain dormant if phosphorous levels are above 70 ppm, however if you establish your mycorrhizae colony early on when phosphorous levels in the nutrient solution are lower, they may continue to grow later in the cycle when P levels are higher. Q: What are the different modes of action of mycorrhizae and Trichoderma and Bacillus subtilis? A: Mycorrhizae are an extension of the plant s root system and the Trichoderma and Bacillus subtilis solubilise phosphate, create a larger physical plant root with more branches and are antagonistic against plant pathogens and pests. Q: Are mycorrhizae compatible with Trichoderma and Bacillus subtilis? A: The mycorrhizae network in the soil may provide a physical structure for the Trichoderma and Bacillus to grown on. Mycorrhizae are usually only applied once at plant establishment and both Trichoderma and Bacillus are applied regularly as crop protection agents and bio-fertilisers. 6.

8 Q: What is a green manure? A: Green manure crops uprooted or mown crop parts, which are left to wither on a field so that they serve as a mulch and soil amendment. Plants used for green manure are often cover crops grown which are ploughed under and incorporated into the soil while green or shortly after flowering. Green manure is commonly associated with organic farming and can play an important role in sustainable annual cropping systems Q: How do green manures work? A: Green manure crops act mainly as soil acidifying matter to decrease the alkalinity ph of alkali soils by generating humic and acetic acids. Incorporation of cover crops into the soil allows the nutrients held within the green manure to be released and made available to the succeeding crops. This results immediately from an increase in abundance of soil microorganisms from the degradation of plant material that aid in the decomposition of this fresh material. Microbial activity from incorporation of cover crops into the soil leads to the formation of fungal mycelium and viscous materials, which benefit the health of the soil by increasing its soil structure by aggregation. The increased percentage of organic matter improves water infiltration and retention, aeration, and other soil characteristics. Q: What is a biofilm? A: Rhizobacteria can form biofilms on root surfaces; a continuous colony of bacteria bound together by polysaccharides. This helps the bacteria bind to the root surface and makes it more difficult for other rhizobacteria to establish on the root. Biofilm formation is enhanced by the presence of malic acid, which is excreted by plant roots. Q: What is an endophyte? A: An endophyte is a bacterium or fungi able colonize the internal tissue of a plant without causing a disease or any harm to the plant. Of the nearly 300,000 plant species that exist on the earth, each individual plant is host to one or more endophytes. The photo shows endophytic bacteria colonizing Mays. Transverse section of a maize stem (red) colonized by K. pneumoniae (yellow-green). (b to d) Longitudinal section of a maize root (dark green) colonized by GFP-tagged cells (bright green) (Chelius & Triplett, 2000)[ Q: What is the difference between and phytopathogen and an endophyte? A: Phytopathogens are bacteria or fungi, often with part of their life cycle in the soil, which cause harm to the plant. Diseases are caused by phytopathogens. Endophytes colonize an ecological niche similar to that of phytopathogens, which makes them suitable as bio-control agents 7.

9 Q: Are any of Real IPM s biopesticides also endophytes? A: Real Trichoderma and Real Bacillus subtilis are endophytes. Real IPM is currently investigating the endophytic attributes of its EPFs (Metarhizium and Beauveria). Q: How does Bacillus subtilis control powdery mildew if sprayed in the canopy? A: Bacillus subtilis is known to produce lipopeptides, which destroy the cell membrane of powdery mildew spores on the leaf surface. (ref AgraQuest). These substances are produced by Bacillus during the manufacturing process and are present in the total fermented product. The live bacterium itself is less important than the substances in the liquid culture. Applications should not be made to run-off so that the natural active ingredients remain in contact with the leaf surface and prevent powdery mildew spores form germinating. Q: How do plants use the Bacillus subtilis applied to the soil to help protect the plant canopy from attack by diseases? A: Scientists have measured a peculiar interaction between a plant under attack from a disease in the canopy. Root colonization by Bacillus subtilis increases (below right). The increased level of Bacillus subtilis on the root system sends signals to the plants own chemical pathways that produce abscisic acid and salicylic acid which are involved in controlling the closure of stomata. When stomata close they reduce the entry points for the disease into the plant. 8.

10 REAL FACT Many diseases are able to enter the plant through wounds or open stomata. Therefore colonisation of stomata with beneficial microbes such as Bacillus subtilis may reduce the ability of pathogens to invade the plant. 9.

11 Q: How does Trichoderma control downy mildews? A: Trichoderma is an endophyte and needs to be applied to re-growth of rose plants (red leaves) after flushing or pruning. Downy mildews are also systemic which makes them difficult to control because they are not exposed to control agents. Systemic fungicides are more prone to resistance occurring and Trichoderma should be tank mixed when systemic fungicides are applied as part of a resistance management programme. Weekly foliar applications of Trichoderma have provided good control of downy mildews in roses and onions. Do not spray to run-off. Q: How does Trichoderma control root knot nematodes? A: Real IPM Trichoderma is Registered in Kenya and Ethiopia for the control of root knot nematode. Its probable mode of action is the excretion of a chitinase enzyme, which destroys the egg masses of nematodes on the outside of the plant roots. A root gall contains one large single female nematode. She extrudes eggs masses on the outside of the root and then dies. The gall will not disappear and remains as historical evidence of rkn presence. If Trichoderma is applied regularly with the irrigation water, the number of new galls should decrease. Q: Are biopesticides protective or curative? A: Biopesticides are more effective when used in a prophylactic preventative programme, not dependent on thresholds or scouting. The colonization of the rhizosphere by beneficial microbes in advance of the attack by pathogens and pests will reduce the living space on the roots where the pathogens might attempt to colonise. Beneficial microbes will secrete substances, which can slow down the growth of the competing pathogenic microbes or kill them. This will reduce the level of disease or pest attack on the plant. Q: What is the difference between Systemic Acquired Resistance (SAR) and Induced Systemic Resistance (ISR)? A: ISR is usually initiated as a result of beneficial microbes applied to the roots of the plant and increases the level of jasmonic acid in the plant tissues. Beneficial microbes that are applied to the canopy usually induce SAR and it tends to increase the level of salicylic acid in leaf tissue. Have you got some more questions? You can join us on our WhatsApp and facebook page, Realipm Visit us on

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