Root media play a role in plant health

Fungal pathogens such as species of Pythium and Phytophthora can attack young seedlings and established plants. They can cause pre-emergence damping of seedlings, post-emergence damping off of young plugs and can attack the root systems of established plants. Significant plant losses result from these organisms, and growers often spend considerable time and effort in an attempt to minimize crop losses.

Growers often rely on fungicides to control these pathogens. However, over the past couple of decades a significant amount of research has been conducted on alternative methods for controlling soil-borne fungal pathogens. One of these areas has been in the development of disease-suppressive root media.

Plant pathologists often refer to the “disease triangle “ in which three components must be in place for disease to develop: a susceptible plant host, a virulent pathogen or disease-causing organism, and a conducive environment for the pathogen. When developing disease-suppressive root media, we’re attacking the third component of the disease triangle. Because disease suppressiveness is, in fact, a function of what the root medium is being compared against, and because many root media have some degree of disease suppressiveness depending upon the control against which they are compared, it is best to say that we are trying to design root media so as to increase their level of disease suppressiveness.

Physical properties of the root medium

During irrigation (if top watering), all of the pores of a substrate in a container are filled with water (saturated condition). Immediately after irrigation, the larger pores are unable to hold water against gravity and they drain and become air-filled. These air-filled pores allow for gas exchange between the roots and the outside atmosphere. Without this gas exchange, the roots would be deprived of oxygen required for respiration. If air-filled pore space is too low, the roots can suffer stress that makes the plants more susceptible to fungal pathogens.

If the air-filled pore space is too low, most pores will be filled with water and the root media may require longer to dry between irrigations (“the root medium stays too wet”). This provides a very conducive environment for fungal pathogens. Adequate air-filled pore space should be designed into the root medium so there is proper balance between air-space and water-holding capacity.

Although there are no absolute recommendations regarding air-filled pore space, many researchers recommend 10-20 percent (by volume) air-filled pore space for most containerized crops. If a substrate has too low an air-filled pore space and is holding too much water, increasing the amount of perlite, 3/8 screened bark, PBH, calcined clay, pumice or other large particles will help.

Selection of root media components

Because disease suppressiveness is a function of what the root medium is being compared against, and because many root media components have some degree of disease suppressiveness if compared against a specific control, it is best to say that we are trying to design root media so as to increase the level of disease suppressiveness. Several common root media components have been demonstrated to have some level of disease suppressiveness.

Sphagnum peat. In most cases, research conducted on disease-suppressive root media components uses sphagnum peat-based root media as a control. The assumption has often been that sphagnum peat was not disease suppressive. However, compared to sand or field soils, sphagnum peat has been shown to be disease suppressive. The origin and level of decomposition of the peat can impact its degree of disease suppressiveness. Typically younger or “blonde” peats have been shown to be the most disease suppressive while more decomposed darker peats are significantly less disease suppressive.

The primary mechanism for the disease suppressiveness of sphagnum peat comes from the populations of beneficial microorganisms that it contains (i.e., Trichoderma). These beneficial microorganisms may parasitize or outcompete the fungal pathogens for resources. Because the mechanism of disease suppression in peat is biological, pasteurization of the peat or peat-containing root medium can greatly reduce or eliminate its disease suppressiveness.

Composted barks. For bark to be disease suppressive, it must go through a proper thermophilic composting process because its primary mechanism for disease suppression is biological. It’s best if the composting is conducted near trees, fields or other natural vegetation. Composted barks often contain beneficial microorganisms just like many peats. Composting bark near natural vegetation allows for these organisms to colonize the bark once the composting process is completed.

When bark is added to the root medium, the beneficial microorganisms are also added. Just as with peat, pasteurization of the bark or bark-containing root medium will reduce or eliminate the disease suppressiveness of the bark since the beneficial microorganism population may be reduced or killed. Composted barks are not always disease suppressive. It was reported that even when composted properly, 20 percent of bark sources did not exhibit significant disease suppressiveness in root media. Under a high level of disease pressure, the disease suppressiveness of composted bark-based root media may not provide adequate control.

Coconut coir. Coconut coir has been shown to be highly disease suppressive, although the mechanism is chemical rather than biological. Pasteurization of the coir doesn’t reduce or eliminate its disease suppressiveness. From research, it appears that small amounts of coconut coir in a root medium do not increase its disease suppressiveness. In most cases, a root medium needs to be at least 60 percent coconut coir to show significant increases of disease suppressiveness. A root medium containing 20 percent perlite and 80 percent coconut coir was the most disease suppressive. Not all coconut coir sources are disease suppressive. Younger, fresher and “red” coconut coir is much more disease suppressive than older and darker coconut coir sources.

Root media amendments

Wetting agents. Certain wetting agents inhibit disease development caused by Pythium and Phytophthora in hydroponic systems, but most research on wetting agents added to peat or bark-based root media has not shown them to be effective controls for soil-borne fungal pathogens.

Biological agents. Many biological products have been designed to be added to root media to suppress soil-born fungal pathogens. These include such products as Rootshield, SoilGuard, Mycostop, Actino-Iron and Actinovate. The majority of these are incorporated into the root medium. Some biological agents are strains of specific species of Trichoderma, Bacillus or Streptomyces. These beneficial organisms may cause induced plant resistance, directly parasitize the disease-causing organism, compete for space or food sources or produce antibiotics that inhibit development or kill the disease-causing organism.

Significant evidence has been generated by private companies as well as by universities that support the efficacy of such biological amendments, but recent research published has raised doubts regarding the efficacy of such biological amendments. Many factors affect the efficacy of biological components added to substrates. These include the nature of the substrate, the environmental conditions and the crop. The best course for growers interested in such amendments would be for them to experiment on a small scale to determine their efficacy on the crops being grown and under the cultural conditions being used. When conducting trials, include control groups so that the specific effect of the biological agent may be determined.

Silicates. There has been a great deal of excitement in recent years regarding the potential of silicates for disease suppression. Silicates have been demonstrated to control root rot-causing organisms such as Pythium in hydroponics production. Research is underway to evaluate silicates sprays for the control of soil-borne fungal diseases in greenhouse root media.

Silicates appear to inhibit disease development in at least two ways. Silicates may cause induced resistance in plants to make plants more able to resist disease-causing organisms. The second possible mechanism is that silicates absorbed and stored in the cell walls of the plant makes them stronger and more difficult for fungal pathogens to invade. Regardless of the mechanism, silicates have been show to suppress pre- and post-emergence damping-off of young plants caused by several fungal diseases.

A primary limitation to the use of silicates in root media is a limited understanding how silicates affect root media pH. Silicates act as bases in root media and increase pH. Commercial silicate products on the market include AgSil and Sil-Matrix. They have recommendations for use as foliar sprays, in hydroponics or in nutrient solutions. Recommendations for use in root media have not been developed at this time.

Root media require proper handling

Selecting the appropriate components, the proper ratio of the components and the proper amendments are important in designing a root medium with maximum disease suppressiveness. These decisions are only part of the process. Proper mixing and handling of the root medium can have a significant impact on the  medium’s physical and chemical properties and its ultimate performance. Even a well-designed root medium may perform poorly if mixed and handled improperly.

Mix root media components thoroughly. Containers filled with a poorly mixed root medium may have different combinations of components and amendments, so their physical and chemical properties may vary. Air-filled pore space, drainage and water-holding capacities can vary significantly if the root medium is not uniform. Variations can result in some areas of the crop drying out more rapidly while other areas may remain wet for too long. This uneven drying makes water management difficult.

Do not overmix. As the root medium is turned or tumbled, particles may be broken down. This creates smaller particle sizes that result in a reduced air-filled pore space and drainage and a higher water-holding capacity. Although high-quality components were selected and mixed at the appropriate ratios, the excessive mixing changed the components’ physical properties and the resulting root medium. There’s no absolute rule regarding the amount of time required for proper mixing.

Add water during mixing. Water allows the root media components to expand, maximizing the volume of the root media and its total pore space (including air-filled pores). If the root medium is placed into containers dry and is then watered, it will not expand to its fullest potential. When water is added to a dry root media in a container, the surface may appear wet, but the water will often channel through the root medium and out of the container. It may make take many waterings to wet up the entire root medium. It is commonly recommended that a root medium be at 50% (v/v) moisture level when it is placed into the container.

Store unused root medium moist in a clean location. Place the root medium in sealed bags or totes or place it in bins or other clean storage containers. Don’t place it on bare ground or unclean surfaces to avoid contamination. Cover the medium to prevent contamination and to minimize moisture loss.

Don’t stack containers on top of one another. The weight of the containers compresses the root media. In a stack, not all of the containers are compressed equally. If containers or flats are stacked, stagger them or use boards to separate layers so that the weight is supported.

Pasteurize carefully, only when needed. Root media containing organic materials such as sphagnum peat and composted bark generally should not be pasteurized. Pasteurization eliminates or reduces beneficial microorganisms. Where sand or field soils are added to a substrate, and there is concern that disease organisms might be introduced, pasteurize the mineral components and then add them to the organic components. Components such as perlite, vermiculite and PBH don’t require pasteurization because they are sterilized during production. When pasteurization is required, the temperature should be increased to 160°F throughout the volume of material for 30 minutes and then allowed to completely cool before use.

- Michael R. Evans

Michael R. Evans is associate professor at the Department of Horticulture, University of Arkansas, Fayetteville; (479) 575-3179; mrevans@uark.edu.

This article was presented at the Society of American Florists’ 24th Annual Pest Management Conference in Atlanta.

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June 2008