Late in the 19th century, a Polish scientist by the name of Franciszek Kamienski made a remarkable discovery. He found there were soil-borne fungi that formed a mutually beneficial (symbiotic) relationships with the root systems of plants. Today, those fungi carry the common name of mycorrhizae which, literally interpreted, means "fungus-roots". Nearly 150 years later, scientists continue to make novel discoveries about these unique micro-organisms, and the benefits they bring to modern agriculture.
A symbiotic relationship can be defined as two living organisms living in close physical association, most often to the benefit of both. It is estimated that nearly 80 percent of all plant species on earth form mycorrhizal associations of one type or another. Mycorrhizal classification is based on the relationship of the hyphae (branching filamentous structure that form the main body of the fungus) and the roots of plants.
Ectomycorrhizae, commonly found on the roots of woody plant species, produce hyphae primarily on the exterior of plant roots. The result is a hyphal sheath known as a mantel. In contrast, endomycorrhizae (a.k.a. arbuscular mycorrhizae) grow inside the roots both between and within root cells. The relationship between fungus and plant of endomycorrhizae is a more invasive then that of ectomycorrhizae. Endomycorrhizae colonize a wide array of plants species.
At one time skeptical about the importance of mycorrhizae, the scientific community now acknowledges their benefits as both numerous and important to plant growth. For example, because of an improved "connection" of a plant's root system and the soil that surrounds it, mycorrhizae allow for increased uptake of both water and essential mineral elements, especially phosphorus. These benefits lead to improved drought tolerance, a reduction in the amount of fertilizer need to be applied to soil and increased disease resistance.
The benefit of increased disease resistance imparted by mycorrhizae has been the result of much research. It has long been theorized that a healthy, vigorous plant is better able to withstand disease pressure when compared with a malnourished, stressed plant. Causing a plant's root system to be able to take in additional nutrients and water, undoubtedly makes for a healthier plant. However, there are additional reasons why mycorrhizae help plants to resist diseases.
Since some mycorrhizae form a mantel enveloping roots, their presence represents a physical shield against invasion by other soil-borne microbes. In short, they compete with microbial pathogens for both space and root exudates. Additionally, they cause cell walls to thicken, making pathogen invasion more difficult.
Additional to the above, it has been demonstrated that mycorrhizae excrete enzymes that are toxic to soil-borne pathogens such as nematodes. Disease suppressive effects against soil-borne fungi such as Fusarium, Verticillium and Phytothora also have been documented.
Of great curiosity is the defense response plants exhibit when mycorrhizal affiliations are present. In short, plants respond with countermeasures when under the attack of disease organisms. For example, certain chemical compounds with anti-microbial actions (e.g. alkaloids) are released by plants when disease organisms attack. Again, these responses appear to be stronger in plants having mycorrhizal associations compared with those that do not.
Although most mineral soils contain mycorrhizae, their numbers often are insufficient for adequate root colonization. Additionally, soilless media used in container production lack mycorrhizae unless blended into the mix as an additive.
In light of the many benefits of mycorrhizae, supplements of the latter are available to make certain sufficient populations are present in the root zone area. Brand names* include but are not limited to Asperello® (Trichoderma asperellum, strain T34), Obtego® (Trichoderma asperellum, strain ICC 012 + Trichoderma gamsii, strain ICC 080), PreStop® (Gliocladium catenulatum), RootShield® (Trichoderma harzianum), RootShield Plus® (Trichoderma harzianum + Trichoderma virens) and SoilGard® (Gliocladium virens). All are OMRI listed and labeled for use on both vegetable and ornamental crops. Although natural, the above products are considered (bio)pesticides and should be handled with care. Always read and follow label directions.
Additional biofungicides labeled for vegetable crops include Actinovate® (Streptomyces lydicus) and Cease® (Bacillus subtilis). However, the latter two contain beneficial bacteria rather than mycorrhizae.
In most cases, the above products are applied both before and after transplanting crops such as vegetables. Typically, the first application is made as a drench to transplants (e.g. tomato) growing in a greenhouse. Additional applications normally are made after setting plants in the field (or production greenhouse/high tunnel) via "chemigation", using drip irrigation equipment. Frequency of repeated field application depends both upon product and disease pressure.
*Mention of brand names does not imply endorsement by the author or University of Missouri Extension.
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REVISED: February 10, 2020