Foliar Sprays/Foliar Feeding
Foliar feeding refers to the application of fertilizers to a plant’s leaves.
In hydroponic systems nutrients are highly bioavailable and plants receive the large majority of nutrients very effectively through the roots.
This differs somewhat from soils where minerals are less bioavailable and can become locked due to the Cation Exchange Capacity (CEC) and Total Exchange Capacity (TEC) properties of various soil types. For instance, soil colloids tend to be negatively charged and like a magnet they attract positively charged elements (cations such as calcium etc) reducing their mobility in soil.
Other than this, clay and humus have electrostatic surface charges that attract the solution ions, and hold them. This holding capacity varies for the different clay types and clay-blends present in soil, and is very dependent of the proportion of clay+humus that is present in a particular soil. A way to increase CEC is to favor the formation of humus. In general, the higher the CEC, the higher the soil fertility.
Okay, I’ve possibly oversimplified things here – however, the long and short of it is that nutrients and far more bioavailable in hydroponic systems than they are in soils. This is just one of the reasons that hydroponic crops tend to produce larger yields. I.e. Hydroponics allows the plant to expend less energy in the pursuit of nutrients and to divert more energy towards plant growth. The result in increased plant size, better health and biomass (eg. plants grown in hydroponics tend to produce more nodes because of the optimum growing conditions involved etc).
So are there benefits to foliar feeding in hydroponics? Well yes! Certainly!
Ultimately, it depends upon your goals, your plants, and the condition of your plants. Here are several of the most common reasons why growers foliar feed.
Root damage can cause serious uptake problems in soils and soilless systems. If the roots are damaged this quickly results in nutrient deficiencies, as the roots are unable to uptake key critical nutrition. Foliar feeding can help correct these deficiencies.
Overcoming Deficiencies Caused by Inadequate Nutrition
The balance of factors in a hydroponic garden is very delicate, so it can be fairly easy for your plants to suffer a nutrient deficiency. Perhaps the nutrient balance isn’t optimized, or the pH level is either too high or too low. In some cases too much of an element may be introduced via the overuse of an additive which results in one element locking out other key elements (eg. Too much phosphorous will lock out iron, zinc, and calcium etc).
To Give Your Plants a Boost
Something doesn’t necessarily have to be wrong with your plants for you to choose to feed them with foliar fertilizer. Many choose foliar feeding for their plants simply to give them the extra nutrients that they need to grow to their maximum potential.
Some Elements are Better Delivered via Foliar Feeding (eg.Triacontanol, amino acids, complex carbohydrates etc)
In very simple terms, the Casparian strip is located inside each and every root and acts as a barrier that blocks or reduces the uptake of many elements or compounds except simple sugars (eg. glucose, sucrose, fructose1), simple amino acids that the plant recognizes and nitrogen (e.g. glycine) or sulphur, and regular plant nutrients like nitrogen, calcium and potassium. Studies have indicated that plants cannot efficiently uptake some amino acids and complex carbohydrates through their roots. Root-based supplements or growth and flowering stimulants containing these elements are, for this reason, best applied via foliar application.
Additionally, it has been well documented in studies that some elements are more effectively delivered to the plant via foliar feeding. This applies to both plants grown in soils or soilless systems. For instance, it has been well documented in research that Triacontanol is best applied via foliar applications.
Foliar Feeding Is an Effective, Safe Way of Delivering Organics To Plants
As previously discussed, where hydroponics is concerned, particularly water based systems (e.g. NFT, deep tank, and aeroponics) it’s important not to overdo it with organic matter or additives. In adding too much organics into the hydro system the proliferation of unwanted microbial life may potentially rob oxygen from the root zone creating a situation where roots are suffocated and pathogenic microbe numbers explode under oxygen starved conditions.
For this reason, foliar feeding organic additives becomes a viable method to use organics in hydroponic settings without risk of overloading the solution with organic components. For instance, kelp is often used in foliar feeding formulations as it contains growth hormones and amino acids as well as macro‐ and micronutrients. Many of these hormones and amino acids are more readily absorbed through the foliage than the roots.
1) Saglio, P.H. and Xia, Jian-Hua (1988) Characterization of the hexose transport system in maize root tips.
BENEFICIAL BACTERIA AND FUNGI AND STERILIZING AGENTS
Beneficial microbe science is an extremely complex subject that is, too often, oversimplified by those with interests in selling beneficial bacteria and fungi products through the hydroponics industry. Just one of the practices that is common is manufacturers/suppliers present positive findings from soil-based research and apply this to an entirely different growing methodology –hydroponics – where, among other things, microflora, pH, EC, media matrix, microbial food levels, nutrient levels, and the bioavailability of nutrients are extremely different from soils. It is important to note that even in soil based research the benefits that a bacteria or fungi species may demonstrate in one soil type may not be replicated in another soil type. Additionally, many biocontrol agents perform well in the laboratory and green house conditions but fail to do so in the field.1 Similar outcomes are also demonstrated in soiless growing where inconsistencies arise between different systems .2
What is clear is that beneficial microbes offer hydroponic growers benefits beyond other methods of pathogen control/prevention. I.e. the use of beneficial microbes is not only demonstrated to control/eradicate pathogens but also to enhance yields through hormone stimulation, enzyme production and other mechanisms. The same cannot be said for sterilisation methods such as UV, ozone, monochloramine, chlorine and hydrogen peroxide.
Additionally, It is generally agreed that bio inoculants control diseases more stably under the better controllable conditions than in the open field. Thus, hydroponic systems offer a unique environment for control of pathogens since various parameters can be managed to favour friendly (beneficial) microorganisms over pathogenic bacteria and fungi.3
In the following material I have endeavoured to focus as much as possible on hydroponic specific content. The material covers just some of the ‘beneficials’ that have been shown to colonize efficiently in hydroponic growing environments and are proven to reduce plant disease and/or provide other benefits.
Root disease, root disease prevention, root disease cure, and sterilisation methods are among other subjects covered.
Pythium is a specific type of organism but the term Pythium has become the generic name for describing a large number of water moulds or damping off fungi. For the purposes of this paper I will refer to many rhizosphere pathogens as Pythium. In other cases where a specific pathologen has been demonstrated to be eradicated or controlled by a given beneficial bacteria or fungi species I will refer to the pathogen using its scientific name. Other names, besides Pythium, that you will find are Fusarium oxysporum, Fusarium spp., Phytophthora spp etc.
spp. refers to species (plural). For instance, if you see Trichoderma spp. this refers to Trichoderma species
Host refers to the plant. I.e. The plant is a host for the beneficial bacteria or fungi
Bio inoculant – A formulation containing one or more beneficial bacterial or fungi strains
When referring to beneficial microbes or beneficial microbe products (bio inoculants) various terminology may be used. E.g. bio inoculant, beneficials, beneficial microbe products, friendly bacteria, beneficial bacteria and beneficial fungi. While the terminology isn’t scientifically correct this terminology is used because it is commonly used throughout the hydroponics retail sector (I.e.it is culturally appropriate to the readership).
Root Disease in Hydroponics (In Brief)
When science first conceived of hydroponics it was believed that the new artificial growing method would exclude soil borne pathogens. This was quickly disproven and it was soon discovered that a microflora, similar to that found in soils, rapidly established itself in hydroponic systems. Among the microflora were the plant pathogens Pythium, Phytophera and Fusarium.
Phytophthora (pronounced Fy-tof-thora – meaning plant destroyer) is a water mould, also known as an oomycete.
Phytophthora is an aggressive plant pathogen. When a plant is infected, it is unable to absorb nutrients.
Fusarium oxysporum is a common soil fungus, and can become a pathogen causing a wide variety of wilt diseases in plants (usually called Fusarium wilts). Fusarium wilt can be identified with symptoms such as wilting, chlorosis, necrosis, premature leaf drop, browning of the vascular system, stunting, and damping-off.
The most common root disease found in hydroponics is caused by Pythium. Pythium attacks the root system and severely limits the plant’s capacity to uptake food. What this ultimately means is an unhealthy crop and a low yield. In severe cases it can lead to crop death.
Pythium disease can be recognized by a brown root system that breaks away when pulled. This may also be accompanied by a musty smell as the root system decays.
Pythium can take hold of a weak, stressed crop far more easily than it can a healthy crop. Making sure that your plants remain healthy through the correct nutrition (particularly during heavy fruiting) and optimum conditions (air temp, water/nutrient temp, RH etc) will give your plants increased resistance against Pythium. I.e. plants grown in optimal conditions (i.e. optimal air temperature, optimal water/nutrient/media temperature, optimal nutrition, optimal RH) will be more resistant to root disease than plants that are subjected to stress as a result of less than optimal growing conditions.
Pythium spores are soil inhabitants. This is why hydroponics and soil don’t mix. Avoid introducing soil into your hydroponics environment! This means taking precautions such as not dragging soil from outdoors into your (indoor) growing environment on your shoes, clothes or hands.
Pythium are water moulds. Because of this, untreated water such as stream, dam, and shallow bore water are high-risk products. If you are going to use stream, dam or bore water in your system you will need to sterilise it prior to use. Rainwater should also be treated because of the likelihood of it collecting wind blown soil.
Managing disease suppression in hydroponics represents the best way of controlling Pythium. Three main strategies can be used: (1) increasing the level of suppressiveness by the addition of antagonistic microorganisms; (2) using a mix of microorganisms with complementary ecological traits and antagonistic abilities, combined with disinfection techniques; and (3) amending substrates and nutrient to favour the development of a beneficial microflora. 1
Friendly Bacteria and Fungi in Hydroponic Settings
Hydroponic systems offer a unique environment for control of pathogens since various parameters can be managed to favour friendly (beneficial) microorganisms over pathogenic bacteria and fungi. Given this, the addition of beneficial bacteria and fungi in hydro systems, when handled correctly, promotes a dynamic microculture that prevents harmful organisms damaging the crop.
While the mechanisms that beneficial microbes use against pathogens are complex these mechanisms can be defined as:
Microbial antagonism results from direct interactions between two microorganisms sharing the same ecological niche. Three main types of direct interaction may be characterized: parasitism, competition for nutrients or plant tissues, and antibiosis.
Parasitism of a plant pathogen by other microorganisms is a widely distributed phenomenon. It involves specific recognition between the antagonist and its target pathogen and several types of cell wall-degrading enzymes (CWDEs) that enable the parasite to penetrate the cell wall (hyphae) of the pathogen.
Competition for nutrients
Competition for nutrients is a general phenomenon regulating the dynamics of microorganisms sharing the same ecological niche and having the same physiological requirements when resources are limited. Competition for nutrients, especially for carbon, is common in as soils and other media, and is considered to be responsible for the phenomenon of fungistasis which is the inhibition of fungal spore germination. Competition for nutrients is one of the modes of action of many beneficial micros.
Antibiosis is the antagonism resulting from the production by one microorganism of secondary metabolites toxic for other microorganisms. Antibiosis is a very common phenomenon responsible for the biocontrol activity of many beneficial microorganisms such as fluorescent Pseudomonas spp., Bacillus spp., Streptomyces spp. and Trichoderma spp. A given strain of beneficial microbe may produce several types of secondary metabolite, having different functions and effective against different species of fungal pathogens.
Induced resistance of the plant
Plants react to physical stresses such as heat, frost, drought, salt, and inoculation with pathogenic or nonpathogenic microorganisms by expressing defence reactions. These defence reactions are SAR (systemic acquired resistance) and ISR (induced systemic resistance). We’ll talk more about this in a moment.
Overview of Microbial Inoculants
Microbial inoculants are used in agriculture as soil amendments that use beneficial bacteria and fungi to promote plant health and nutrition. Various microbe species can be used as biological control agents and may provide effective activity against various pathogenic microorganisms. Just some examples:
Trichoderma harzianum has biocontrol potential against Botrytis cineria, Fusarium, Pythium and Rhizoctonia; Ampelomyces quisqualis, – a hyperparasite of powdery mildew. Bacillussubtillis has antifungal potential against Phytophthora parasitica Dast, Alternaria solani, Pythium aphanidermatum, C. gloeosporioides, Verticillium dahliae Klebahn, Fusariumoxysporum f.sp melongenae, Botrytis cinerea Pers, Fusarium oxysporum, and Lycopersici.1 Fluorescent pseudomonads produce “highly potent” broad spectrum antifungal molecules against various phytopathogens. 2 Application of Trichoderma viride, Pseudomonas and Bacillus spp. have been found to substantially control seedling, root and stalk rots of maize caused by Fusarium graminearum. Pseudomonas cepacea has been found to inhibit a range of soil borne fungal pathogens including Fusarium graminearum, Fusarium moniliforme and M. phaseolina.3Pseudomonas putida and Trichoderma atroviride have been found to promote the reproductive growth of tomato plants under typical hydroponic growing conditions,4 while numerous studies have demonstrated that rhizosphere bacteria can stimulate plant growth in both soils and hydroponic settings.
Foliar sprays can be used for leaf coverage and they are applied through irrigation to inoculate the soil. While they are applied to improve plant nutrition and health their exudates can also promote hormone production in plants, therefore promoting plant growth. Many of the beneficial bacteria and fungi form symbiotic relationships within the plant that are mutualistic. Roots themselves release exudates into the soil that are beneficial to the microorganisms which suggests a degree of co-evolution between microorganisms and plants that form the ecosystem of the rhizosphere.
The use of inoculants in agriculture has been shown to extend beyond their benefits as biological fertilizers. Research into the disease resistance of microbioinnoculants in crop species shows they can initiate systemic acquired resistance (SAR) to several common crop diseases.
In plants SAR is a resistance response that occurs following a previous localized exposure to a plant pathogen. Once stimulated SAR can provide resistance for several days to a wide variety of pathogens. When a plant recognizes a pathogen it induces a rapid defence response called the hypersensitive response (HR). HR results in localized cell or tissue death at the site of infection, which limits further spread of infection.
This localized response provides non specific resistance throughout the plant; a phenomenon known as systemic acquired resistance – SAR (Ryals et al 1996).
Plants produce salicylic acid as a result of the HR and this increase in concentration of salicylic acid is an activator of SAR. Research has shown that aspirin (acetyl salicylic acid) can work as a trigger for SAR.
There is also induced systemic resistance (ISR). ISR corresponds to the resistance induced by plant growth-promoting rhizobacteria involving the jasmonic acid (JA) and ethylene (ET) pathways. The two pathways are not independent and there are some commonalities between SAR and ISR. For example, both SAR and ISR are controlled by the same regulatory protein non-expressor in the plant. Cross-communication between defense pathways enables the plant to fine-tune its defense response. Based on recent research, the phenomenon of ‘priming’ defense appears to be a common feature of the plant’s immune system that offers protection against disease. When ISR is induced the plant shows a faster or greater activation of defense responses after infection.5
Beneficial microbes such as plant growth promoting bacteria (PGPB) and fungi can improve plant resistance to pathogens and even some insects by inducing
systemic defence responses. Beneficial bacteria and fungi exudates are recognized by the plant, which results in a mild activation of plant immune responses.
Plant Growth Promoting bacteria (PGPB) are considered to promote plant growth directly or indirectly. PGPB can exhibit a variety of characteristics responsible for influencing plant growth. The common traits include production of plant growth regulators (e.g. auxins), siderophores (iron chelating compounds), HCN (amino acid precursor) and antibiotics. Indole acetic acid (IAA) is one of the most physiologically active auxins. IAA is a common product of L-tryptophan metabolism by several microorganisms including PGPB. Microorganisms inhabiting rhizospheres of various plants are likely to synthesize and release auxin as secondary metabolites because of the rich supplies of substrates exuded from the roots compared with non-microbe inhabited soils.
There is evidence that the growth hormones produced by microbes can in some instances increase growth rates and improve yields of the host plants. It is also possible that microbes capable of phosphate solubilization may improve plant productivity both by hormonal stimulation and by supplying phosphate. However, because of the capacity of beneficial microbes to confer plant beneficial effects, efficient colonization of the plant environment is of utmost importance. This is often a fact that is greatly oversimplified by those with interests in selling beneficial microbe products to the agricultural and/or hydroponic sectors. One must consider that many microbes require optimal conditions in which to sufficiently produce benefits and in many instances soil-based research is used to substantiate the merits of bacteria and/or fungi benefits in hydroponics.
Take for example, Arbuscular Mycorrhizal Fungi (AMF) …
About Arbuscular Mycorrhizal Fungi (AMF)
The term “mycorrhiza” literally means fungus-root. It is estimated that 80 to 90 percent of all plant species form mycorrhiza. The relationship between plant and micorrhizae is a symbiosis, the main function of which, while complex, is the transfer of carbon produced by plants to fungi (sugars created in leaves of the plant move downward and into the fungal hyphae via the roots) and the transfer of nutrients acquired by fungi to plants (the plant receives phosphorus, nitrogen, potassium, and micronutrients such as copper, sulfur and zinc).
Elements that are critical in the plant/mycorrhizae symbiosis are CO2 concentration, nitrogen levels, phosphorous levels, soil matrix, pH and carbon.
Phosphorous, Nitrogen and AMF
One of the key functions of AM fungi is they increase the uptake of poorly soluble P sources, such as iron and aluminium phosphate and rock phosphates by converting non bioavailable phosphates in their organic form to inorganic, bioavailable H2PO4– (Pi) and HPO42- phosphorous.
AM fungi colonize the root cortex of the host plant in which the fungi are able to acquire organic carbon as food to build ‘the infrastructure’ for P uptake and transport. The mycorrhizal system is able to take up P more efficiently and transport P over longer distances than the plant root system, overcoming P depletion in soils.1
AM fungi also acquire substantial quantities of N from organic sources and play an important role in the nitrogen cycle, intercepting inorganic N released from decomposing organic matter before roots can acquire it and passing some of this on to plants as arginine (CH2CH2CH2NH-C(NH)NH2). Additionally, a plant ammonium (NH4 N) transporter that is mycorrhiza-specific and preferentially activated in arbusculated cells has recently been discovered, suggesting that N transfer to the plant may operate in a similar manner to P transfer. 2
Pitched this way AM fungi sound impressive.
The benefits of AM fungi are greatest in systems where inputs of phosphorous are low. Heavy usage of phosphorus fertilizer can inhibit mycorrhizal colonization and growth. As a soil’s phosphorus levels available to a plant increases, the amount of phosphorus also increases in the plant’s tissues, and carbon drain on the plant by the AM fungi symbiosis become non-beneficial to the plant. 3
A comprehensive literature review conducted byKathleen K. Treseder (2004) concludes mycorrhizal abundance declines in response to adequate N (-15%) and P (-32%) fertilization by average across numerous studies.4
Under even moderate P levels that prevail in the majority of field crop systems, early season colonisation by AMF may often be parasitic, creating a carbon drain on crops and reducing yields.5
In research with AMF (Glomus intraradice), Schenck et al (1993) show citrus grown in adequate P environments had lower relative growth rates than non-mycorrhizal plants of equivalent P status.6 Similar findings have been established in other plant species.7
Author’s note: Carbon drain occurs when there is adequate available phosphorous, however, AMF continue to metabolise plant produced carbon thus placing unnecessary energy drain/burden on the host plants which are receiving low benefits via the mycorrhizae/plant symbiosis.
Hydroponics and AM Fungi
1) The benefits of AM fungi are greatest in P deficient environments
2) Where adequate P is present AM fungi colonization is reduced (average 32%)
3) Bioavailable N plays a pivotal role in AM fungi colonization
4) Where high bioavailable N is present, AM fungi colonization is reduced (average 15%)
5) Yields may be detrimentally affected where adequate P exists (due to carbon drain)
H.J. Hawkins et al (2004) note that a nutrient medium containing a P concentration of 0.9 mM (27.876384ppm P) failed to produce viable mycorrhizal colonisation.8 Similar findings by G.Nagahashi (1996) demonstrates that mycorrhizae grown in the presence of P at 1.0mM (30.973ppm) showed significantly less hypal branching than in lower P environments.9
While the symbiosis between plants and AM fungi is complex and while more hydroponic specific research is needed, based on current knowledge it seems probable that any potential benefits of AM fungi in hydroponics is negated by the presence of high bioavailable P in hydroponic solutions. Additionally, high bioavailable N in hydroponic solutions likely reduces the efficiency of AM fungi further. It is also possible the presence of AM fungi in hydroponic settings may be detrimental to growth rates and yields as a result of carbon drain.
Back to the story…
The majority of soil living beneficial bacteria require oxygen for cellular respiration (also termed “oxidative metabolism”). Bacteria that require oxygen are classed as aerobes. Aerobes also require organic material or molecules (such as glucose) to produce energy. For this reason this class of bacteria are also called aerobic heterotrophs (i.e. aerobic heterotrophs are organisms that cannot live without free oxygen and do not produce their own food).
The main elements required for beneficial bacterial nutrition are C, H, O, N, S, P, K, Mg, Fe, Ca, Mn and traces of Zn, Cu and Mo.
‘Aerobic heterotrophs’ require a source of organic carbon, gaseous oxygen (air) and water along with the aforementioned mineral elements. Their source of energy is produced by the aerobic oxidation of organic material by metabolism to water and carbon dioxide. The energy released is stored in the phosphoanhydride bonds of ATP. When the energy is required it is released from ATP by hydrolysis. Certain environmental conditions are also required for the growth and division of bacteria like O2 concentration, pH and temperature.
ATP stands for Adenosine Tri-Phosphate. ATP consists of an adenosine molecule and three inorganic phosphates. ATP is the most important energy-transfer molecule in all living cells. ATP transports chemical energy within cells for metabolism. ATP is produced during photosynthesis and cellular respiration and used by enzymes and structural proteins in cellular processes, including biosynthetic reactions and cell division.
Phosphorous/phosphate plays a vital role in the ATP chain. Inorganic phosphorus in the form of the phosphate PO43- plays a major role in biological molecules DNA and RNA where it forms part of the molecular structure. Living cells use phosphate to transport cellular energy in the form of ATP. Nearly every cellular process that uses energy obtains it in the form of ATP. ATP ——-> ADP (Adenosine Diphosphate) + Pi (orthophosphate) + energy.
For beneficial bacteria to survive in a hydroponic environment they will need ideal environmental conditions. Most hydroponic nutrients lack organic carbon sources for beneficial bacteria to survive. They can metabolise humic and fulvic extracts but one of the best sources of food for beneficial bacteria is molasses. Molasses typically contains ‘Total Digestable Nutrients’ (TDN) in excess of 60%, as well as containing a number of the major elements and trace elements required by bacteria, molasses is very high (50%+) in sugars. The sugars contained in molasses are an ideal source of carbon for heterotrophs. Cobalt and molybdenum, which are not usually listed in the typical analysis of molasses, will still be found in small traces. Another property of molasses, due to the high percentage of sugars, is its’ sticking ability when used in foliar sprays. Molasses, along with a wetting agent, increases the coverage and surface holding, optimising foliar nutrition. While discussing foliar sprays and biological inputs, saponins can be used as an organic wetting agent that not only reduces the surface tension of water (i.e. surfactant – surface active agent) it also has bio stimulating properties. Saponins are chemical compounds (phytochemicals) found in abundance in various plant species. To be specific they are amphipathic glycosides. The foaming ability of saponins is because of their surfactant like structure with hydrophillic (water soluble) and hydrophobic (fat soluble) chains. Their name is derived from the plant soapwort (genus Saponaria). Most commercial saponins are extracted from Yucca schidigera (Spanish Dagger) and Quillaja saponaria (the soap bark tree).
Two other prominent organic additives that act as microbial nutrients/stimulators and plant fertilisers are kelp and fish products.
It is important to note that where hydroponics is concerned, particularly water based systems (e.g. NFT and aeroponics) it’s important not to overdo it with organic matter or additives. In adding too much organics into the hydro system the proliferation of unwanted microbial life may potentially rob oxygen from the root zone creating a situation where roots are suffocated and pathogenic microbe numbers explode under oxygen starved conditions.
Beneficial Bacteria and Fungi Species for Hydroponics
Trichoderma spp. including T. harzianum, T. viride, T. koningii, T. hamatum and other spp.
Trichoderma spp. are free-living fungi that are very common in soil and root ecosystems. Recent discoveries demonstrate that they are opportunistic plant symbionts as well as parasites of other fungi. 1
For many years, the ability of these fungi to increase the rate of plant growth and development, including, especially, their ability to cause the production of more robust roots has been known. The mechanisms for these abilities are only just now becoming understood.
Trichoderma spp. show a high level of genetic diversity, and can be used to produce a wide range of products of commercial and ecological interest. They are prolific producers of extracellular proteins, and are best known for their ability to produce enzymes that degrade cellulose and chitin — although they also produce other useful enzymes.2 In addition, different strains produce more than 100 different metabolites that have known antibiotic activities.3
Trichoderma spp. have been used as biological control agents against a wide range of pathogenic fungi e.g. Rhizoctonia spp., Pythium spp., Botrytis cinerea, and Fusarium spp. Phytophthora palmivora, P. parasitica and different species can be used (e.g. T. harzianum, T. viride, T. virens) to control the various pathogens. Among them, Trichoderma harzianum is reported to be most widely used as an effective bio inoculant.
Some strains of Trichoderma are highly rhizosphere competent (able to colonize and grow on roots as they develop). The most efficient rhizosphere competent
strains can be added to soil or seeds by any method. Once they come into contact with the rhizosphere, they colonize the roots. If added as a seed treatment, the best strains will colonize root surfaces even when roots are deep below the soil surface. Trichoderma can colonize for long periods of time in the right environments, so colonization can occur throughout the duration of a crops life cycle. However, in less conducive environments Trichoderma colonization will prove less efficient and reapplication of the fungi is necessary.
To the authors knowledge various strains of Trichoderma control every pathogenic fungus for which control has been sought. However, in contrast to other fungi, Trichoderma spp. have been reported to have limited applications in biocontrol of pathogenic bacteria. An immediate explanation would be that bacteria generally have a faster growth rate (i.e. they multiply faster) than fungi.4
This information becomes important when understanding that a broad-spectrum approach to preventing plant pathogens should be incorporated and species of both beneficial bacteria and fungi are likely the ideal. For instance, a good microbrial product should contain species of Bacillus spp. (bacteria) and Trichoderma spp (fungi). However, it isn’t a simple case of incorporating multiple strains of known to be beneficial bacteria and/or fungi as some species may outcompete others and the combinations may reduce overall efficiency. For instance, Pseudomonas fluorescens strain CHA0 which has demonstrated biofungicide qualities against a range of pathogens releases a compound (Phl) that has antibiotic activity against other beneficial microbes.5 Thus, Incompatibility of co-inoculants can arise because biocontrol agents may also inhibit the growth of each other as well as the target pathogen or pathogens.6
Trichoderma spp. and Plant Immune Response
Localized and systemic induced resistance occurs in all or most plants due to among other things, response to attack by pathogenic microorganisms, physical damage due to insects and other factors, and the presence of non-pathogenic rhizobacteria.
Trichoderma penetrate the cells of the root system – this triggers a response in the plant that effectively `walls off’ the Trichoderma and prevents it getting any further into the living root tissue. In triggering this response, the plants natural defence mechanism is activated and a systemic resistance is induced. The relationship between Trichoderma and plant roots is an `opportunistic avirulent symbiotic relationship’ meaning even though the Trichoderma has gained entry to the plant tissue, it does not cause any disease or damage. Both plant and Trichoderma benefit from the symbiosis.
The plant gets protection, while the Trichoderma receives an ecological niche and food from the plant.
The Pathogens Pathogen
In addition to colonizing roots for food, Trichoderma spp. attack, parasitize and gain nutrition from other fungi. Since Trichoderma spp. grow and proliferate best when there are abundant healthy roots, they have evolved numerous mechanisms for both attack of other fungi and for enhancing plant and root growth.
One of the most effective methods of pathogenic fungi control exhibited by Trichoderma is `mycoparasitism’. In this process the Trichoderma detect other fungi, grow towards them, and attach and coil around the fungus, then produce enzymes that destroy the cell walls of the target fungus.
Trichoderma release two types of enzymes in their quest for sustenance – these are cellulase and chitinase. Cellulase enzymes break down cellulose which is a component of plant cells and organic matter. Chitinase breaks down chitin which is a structural component of fungal cell walls.
The production of chitinases has been implicated as a major cause of Trichodermas biocontrol activity against pathogenic fungi.7
Viability and Benefits of Trichoderma Harzianum in Hydroponic Settings
T. harzianum are amongst the most effective of the beneficial microbes in hydroponic settings. Research demonstrates that where T. harzianum has been trialled in hydroponics their presence has controlled or eliminated all manner of pathogens in both inorganic and organic medias. This makes T.harzianum an obvious choice for hydroponic growers. Plant growth promoting benefits are also exhibited by some species of Trichoderma spp.
In research conducted in a controlled hydroponics system, Chet et al (2006) note an increase, at protein level, in the activity of chitinases, b-1,3-glucanases, cellulases and peroxidases in cucumber roots previously inoculated with T. harzianum strain T-203. The capability of T. harzianum to promote increased growth response was verified in the hydroponic system. A 30% increase in seedling emergence was observed and these plants exhibited a 95% increase in root area. Similarly an increase in P and Fe concentration was observed.8
Similarly, research with T.harzianum strain T-203 conducted with cucumbers grown in an axenic (free from other microorganisms) hydroponic system demonstrated increased growth response as early as 5 days post-inoculation resulting in an increase of 25 and 40% in the dry weight of roots and shoots. Similarly, a “significant” increase in the concentration of copper, phosphorous, iron, zinc, manganese and sodium was observed in inoculated roots. In the shoots of these plants, the concentration of zinc, phosphorous and manganese increased by 25, 30 and 70%, respectively.9
Ozbay et al note, T. harzianum strains T95 and T22 increased yield in the presence of measurable disease. Reduction of disease by the use of T. harzianum strains improved tomato yields between 6% and 37% in coir and between 2% and 25% in rockwool. However, Ozbay et al also note, T. harzianum had no effect on yield in the absence of the disease compared with an untreated and uninoculated control. Theses findings suggests that T. harzianum strains used in this experiment act only as biocontrol agents and, beyond this, offer no benefit to yields where disease is not present.10
Trichoderma harzianum are shown across a range of studies to be efficient biocontrol agents.
Additionally, some strains of Trichoderma harzianum are demonstrated to increase the uptake and concentration of a variety of nutrients (copper, phosphorus, iron, manganese and sodium) in hydroponic culture, even under axenic conditions. This increased uptake indicates an improvement in plant active-uptake mechanisms.
However, what is also demonstrated is species, among other factors, will determine whether benefits beyond efficient root disease prevention will be exhibited.
Other Info – Trichoderma spp and Enzymes
Cellulases (enzymes) produced by Trichoderma spp. are the most efficient enzyme system for the complete hydrolysis of cellulosic matter (e.g. decaying root matter) into glucose.11
In research with Trichoderma asperellum, Brotman et al (2008) note the majority of proteins released by T. asperellum could be classified as plant cell wall-degrading enzymes: cellulases (cellobiohydrolase, endoglucanase), hemicellulases (glucan 1,3-β-glucosidase and arabinofuranosidases), and an aspartyl protease (an enzyme that breaks down proteins). glucoamylase, a starch-degrading enzyme, and swollenin, a protein first isolated from T. reesei were also detected.12
Trichoderma viride, T. reesei T. harzianum13 and T. asperellum14 have been demonstrated to produce high levels cellulase enzymes.
Trichoderma in Coco Substrate
Inorganic substrates are more effectively colonized by bacteria, while organic substrates are more effectively colonized by fungi. While Trichoderma spp. have been shown to establish and proliferate in a range of mediums, colonization may be greater in organic mediums such as coconut coir. When coconut coir and rockwool were compared after inoculation with T. harzianum it was found that colonization was greater in the coco fibre, while the rockwool system contained the highest amount of fluorescent pseudomonads bacteria.15 When T.harzianum strains were applied at transplanting to the mediums coir and rockwool, Fusarium crown and root rot incidence of greenhouse-grown tomatoes was reduced up to 79% in coir slabs and up to 73% in rockwool slabs with yield increases of 6% and 37% in coir and between 2% and 25% in rockwool.16
Materials that are high in lignocellulose are the organic medias, straw, wood bark, and coconut fibre. This makes coco fibre an ideal environment for Trichoderma spp. colonization.
Trichoderma spp. possess an innate resistance to most agricultural chemicals, including fungicides, although strains differ in their resistance. Most manufacturers with registered Trichoderma products have extensive lists of susceptibilities or resistance to a range of pesticides.
Food for Fungi
Potatoe starch in particular makes a good food for fungi. When they breed fungi in the lab they use potatoe starch to stimulate Trichoderma colonization.
Fulvic/humic acid in solution has been demonstrated in numerous studies to aid micro colonisation in hydroponic settings.
Milk sugar (soluble dissaccharide lactose) has been demonstrated to benefit enzyme production by Trichoderma fungi.
Optimum Nutrient and Media Temperature for Trichoderma
Like many microbial species Trichoderma spp. has temperature optimums for rapid colonization and bioactivity. For most of the commonly applied species this is 25-30o C (77-86 oF) (8) with 28o C (82.4 oF) being the ideal. 18 If conditions are too cold, the colonization of Trichoderma will slow and even cease; if too warm, then die back may occur and the Trichoderma may become out competed, leaving the door open for other forms of microbial species to take hold.
Optimum Water/Media Temp in Hydroponics vs. Optimum Water/Media Temp for Trichoderma
Often hydroponic growers attribute root browning/root disease to water borne pathogens (Pythium) when in fact one of the major causes of root browning is root zone oxygen starvation typically caused through overly warm nutrient or waterlogged media.
Nutrient salts don’t leak into the roots of the plant. Nutrient uptake is an active process which relies on several factors, one of which is that satisfactory levels of oxygen are available to the roots of the plant.
Roots “pump” nutrients from the outside of the root to the inside where they are transported to the leaves. This pumping process requires energy. The roots get their energy from respiration. In turn, respiration requires energy, which is achieved by burning sugar. Part of the sugar made in leaves by photosynthesis is transported to the roots to power the nutrient pumps.
Photosynthesis converts sugar and oxygen from carbon dioxide, nutrition and water using the energy from light.
Respiration is the opposite. Respiration makes energy by burning sugar (supplied by the leaves of the plant) and oxygen to make carbon dioxide. It is this energy that powers (among other things) the root nutrient pumps. In turn these pumps deliver the nutrition that is critical to sugar production within the plant.
Unlike sugar, oxygen is not transported from the leaves to the roots. This means that the roots must get their own oxygen.
If the roots can’t get sufficient amounts of oxygen (because of excessively warm water/nutrient or because there isn’t enough air space in the growing medium) their pumping capacity is significantly reduced. The result of this is that the plant becomes starved of critical nutrition.
While there are various factors that determine dissolved oxygen levels in water, it can be simply stated as fresh (non saline) water can hold 8.26 parts per million of oxygen at 25OC (77 OF), while at 20O C (68 OF), water can hold as much as 9.09 parts per million of oxygen. The colder water gets the more oxygen it can retain. The warmer water gets the less oxygen it can retain. However, if water is too cold nutrient uptake (hence growth rates) will be reduced.
Oxygen content and water temperature are inextricably linked. As water warms up it loses its capacity to hold oxygen. To avoid root rot as a result of oxygen starvation you will need to keep the nutrient temperature below 25 degrees C (recommended 20 –22°C = 68 – 71.6 °F). In addition to this, aeration of the nutrient is advised.
Given this information, it is best to maintain optimum oxygen temperatures in solution and media and compromise somewhat on optimum temperature for Trichoderma colonization.
Optimum pH for Trichoderma spp.
Optimum pH for Trichoderma fungi may vary between species, however fungi thrive in semi acidic conditions. Optimum cellulase production by Trichoderma harzianum is demonstrated at pH 5.0 – 6.0 with 5.5 being the ideal. Above pH 6.0 reduced cellulase production occurs and, therefore, it is advisable that optimum pH for Trichoderma in hydro is 5.5 – 5.8.19
Bacteria in Hydroponics – Bacillus and Pseudomonas spp
Beneficial bacteria, like beneficial fungi, form a symbiotic relationship with the plant (host). The bacteria benefit from the ecological niche provided by the plant, while the plant receives protection from the beneficial bacteria.
Plant growth–promoting rhizobacteria, most of which are Pseudomonas and Bacillus species, are applied to a wide range of agricultural crops to enhance growth and act as disease control.1
Beneficial bacteria suppress pathogens by, among other things, producing hydrolytic enzymes and antibiotics.
Antibiotics act as micro toxins that can, at low concentrations, poison or kill other microorganisms. It is shown that some antibiotics produced by bacteria are particularly effective against plant pathogens and the diseases they cause.2 It is this antibiotic production that plays a central role in disease control.3
Additionally, these antibiotics are known to induce defence mechanisms in the host plant.4
Bacillus subtilis is able to produce more than two dozen antibiotics with an amazing variety of structures.5
Biocontrol activity of Bacillus strains against multiple plant pathogens have been widely reported and well documented.6 Their success as a biocontrol agent is associated with the prominent property of producing lipopeptide antibiotics which exhibit wide spectrum antifungal activity.7
Strains of Pseudomonas fluorescence have known biological control activity against certain soil-borne phytopathogenic fungi and are known to produce the antibiotic 2, 4-diacetylphloroglucinol (DAPG) which induces defence mechanisms in the host plant.8
There is a synergism between the micro toxins (antibiotics) and hydrolytic enzymes produced by bacteria. Firstly, the enzymes degrade the cell wall of the pathogen, and secondly, this enables the toxin to act more efficiently against the pathogen by gaining access at an intracellular level. I.e. bacteria are more able to effectively poison pathogens via the use of cell wall degrading enzymes.
Viability in Hydroponics
Pseudomonas putida strain PCL1760 has been demonstrated to have significant biological control over Fusarium oxysporum in eight independent laboratory experiments conducted in rockwool substrate.9
Similarly Pseudomonas spp. and Bacillus spp. have been demonstrated to have control over Fusarium oxysporum in hydroponic settings.10
In research with lettuce grown in recirculating gravel bed hydroponic systems Bacillis spp. were shown to control Pythium with Bacillis subtillis demonstrating the highest rate of control. Additionally, B. subtillis consistently enhanced the fresh leaf and root weight by 29.2 and 24.3% compared to the untreated control.11
Research conducted in inorganic and organic hydroponic medias showed the stimulating effect of Pseudomonas putida and T. atroviride (Trichoderma atroviride) on the reproductive growth of tomato plants in both growing medias. The plant growth stimulation was most likely the result of numerous modes of action exhibited by each microorganism tested. This study concluded that Pseudomonas putida and T. atroviride could be used as plant growth-promoting microorganisms to improve the productivity of greenhouse tomato crops under hydroponic conditions in inorganic or organic media.12
Pseudomonas putida strain PCL1760 has been shown to exercise significant biological control of tomato foot and root rot (TFRR), a disease caused by Fusarium oxysporum f. sp. radicis-lycopersici (Forl), in eight independent laboratory experiments in hydroponics (stonewool/rockwool substrate). Furthermore, its activity in stonewool was also tested in an industrial certified greenhouse with similar results. The research concluded that Pseudomonas putida strain PCL1760 acted as a bioinoculant via ‘‘competition for nutrients and niches” (CNN).13
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