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ROOT DISEASE PREVENTION IN HYDROPONICS
As a warning, this article is long (over 10,000 words). However, root disease, root disease prevention and microbe science are subjects worth coming to terms with.
References for this article can be foundhere
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
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.
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 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:
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.
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
Evaluation of P in Hydroponic Working Solutions
We evaluated several off the shelf hydroponic nutrients to establish how many ppm of P (phosphorous) would be in working solution by average. It is important to note that the values were established using random mL/L dilution rates and do not reflect values at comparative ECs (although EC should be between approx 1.8 and 2.4). The aim of the analysis was to establish roughly what ppm of P would be in working solution across a broad range of ECs. In all cases ppm of P exceeded 62ppm, which is double the ppm that available research has shown AMF efficiency is reduced. The ppm data was calculated from lab analysis of concentrate formulas once diluted.
Samples (elemental P and not P as P2O5)
AN Sensi Bloom 4ml/L = 81ppm P
AN Connoisseur Bloom 4ml/L = 90ppm P
H and G Coco 5ml/L = 75ppm P
Canna Aqua Flores 5ml/L = 76ppm P
GH 3 Part = (full strength bloom as per manufacturer recommendations) 330ppm P
Average = 130.4ppm
Author’s note: When considering that many hydroponic growers use further P through the use of P and K additives during flowerset this too needs to be factored into the P equation. For instance, with a product that contained PK 13- 14 %w/w listed as P2O5 and K2O with a specific gravity of 1.25, used at 1.5mL/L this would equate to an additional 104.8ppm of P in working solution. More simply, additive + nutrient equals over 5 times the P that has been found to be detrimental to AM fungi colonization.
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.