on Mycorrhizae in Greenhouse Plants
By Brian Sparks, Editor, Greenhouse Grower
“Myco – rhiza” literally means “fungus – root” and describes the mutually beneficial relationship between the plant and root fungus. These specialized fungi colonize plant roots and extend far into the soil. Mycorrhizal fungal filaments in the soil are extensions of root systems and are more effective in nutrient and water absorption than the roots themselves. Approximately 95% of plant species on the planet form a symbiotic relationship with the beneficial mycorrhizal fungi.
Mycorrhizal fungi increase the surface-absorbing area of roots, thereby greatly improving the ability of the plant to access soil resources. According to Blair Busenbark, Sales and Marketing Manager at Mycorrhizal Applications, mycorrhizae are an essential part of a healthy soil microbiome, and several miles of fungal filaments can be present in less than a thimbleful of soil.
Mycorrhizal fungi form an intricate web that captures and assimilates nutrients, conserving the nutrient capital in soils.
Mycorrhizal fungi increase nutrient uptake not only by increasing the surface-absorbing area of the roots, but also by releasing powerful organic compounds into the soil that help to solubilize hard-to-capture nutrients, such as organic nitrogen, phosphorus, iron, and other tightly bound soil nutrients. This extraction process is particularly important in plant nutrition and explains why non-mycorrhizal plants require high levels of fertilization to maintain their health. Mycorrhizal fungi form an intricate web that captures and assimilates nutrients, conserving the nutrient capital in soils.
Plants propagated and raised in most commercial greenhouse environments receive intensive care and feeding. Since mycorrhizae increases a plant’s nutrient and water uptake, many growers can significantly cut back on these inputs and reduce the ever-rising costs of water and fertilizers. When you consider all the benefits vs. the low cost and ease of application, mycorrhizal inoculation clearly can be a cost-effective option for greenhouse and nursery operations.
Two Types of Mycorrhizae
There are two major types of mycorrhizal fungi. Endomycorrhizal fungi form symbiotic relationships with about 85% of plant families and pair with most commercially produced plants, including green, leafy, and fruiting or flowering plants. They penetrate the root cortex and form nutrient exchange structures within the root cells.
Ectomycorrhizal fungi form relationships with about 10% of plant families, mostly conifers and American hardwoods. They do not penetrate the root cell walls, but form a sheath around the root and nutrient exchange structures known as a Hartig net.
High nitrogen levels increase vegetative growth at the expense of root growth, so reducing nitrogen levels in your fertility program benefits both the mycorrhizae and the plant.
Influence on Phosphorus Uptake
One of mycorrhizae’s benefits is its ability to provide significant amounts of nitrogen and phosphorus to the plant in exchange for carbohydrates. If the plant receives extensive amounts of fertilizer, it will not signal the mycorrhizae to germinate and form the symbiotic colonization of the root system. High nitrogen levels increase vegetative growth at the expense of root growth, so reducing nitrogen levels in your fertility program benefits both the mycorrhizae and the plant.
High levels of soluble phosphorus can inhibit colonization and mycelial growth, so it is recommended to use fertilizers with a low phosphorus concentration. For example, a fertilization program of 200 ppm nitrogen (N) using a 20-20-20 fertilizer could be reduced to 100 ppm N using a 20-2-20 fertilizer, which would reduce nitrogen by 50% and phosphorus by nearly 95%. Growers should consider these potential savings and plant enhancements based on their growing conditions. The use of controlled-release fertilizers is also a consideration for a lower fertility regime.
Considerations for Choosing the Right Mycorrhizae Product
There are several mycorrhizae products on the market for professional growers. When evaluating the available options, here are some important points to consider from Jozsef Racsko, Senior Product Development Manager at Valent U.S.A., and Hugh Poole, an international agricultural consultant based in Anderson, SC:
Know your need. Mycorrhizal fungi can be used to increase water and nutrient uptake by enhancing root growth, maintaining optimal plant growth under stressful conditions, increasing general plant health, and reducing transplant shock.
Established product performance. Make sure the product has been on the market for some time and its performance has been published in technical magazines/journals (i.e., tested by growers and independent researchers).
Product quality. The product must contain viable and infective mycorrhizal inoculum at a rate that is listed on the label.
Producer credibility. The producers/manufacturers have delivered what they promise on their label.
Technical service. The technical staff of the producer/manufacturer can be reached for various technical questions and information regarding product use. Larger, more reputable companies offer services such as testing the mycorrhizal colonization of roots after application to check product efficacy.
Diversity of mycorrhizal fungal species. This is critical to achieve maximum efficacy. There are several species of mycorrhizal fungi in the soil. The mycorrhizal fungal community changes over time with plant phenology, season, microclimate, and soil conditions. Furthermore, different species are responsible for different functional benefits to the plant. Therefore, products containing multiple mycorrhizal species appear to provide more consistent benefits to the plant and ultimately to the grower.
Optimizing the Greenhouse Environment
Mycorrhizae are designed to enhance or benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, crop quality, and yield in plants. So how can growers make sure their greenhouse is optimized to make the best use of these materials?
Dr. Michelle Jones, The Ohio State University DC Kiplinger Floriculture Chair, and a panel of industry experts addressed this and other questions during a discussion at Cultivate’21. The panel included Matthew Krause (Lallemand Plant Care), Troy Buechel (Premier Tech), and Blair Busenbark (Mycorrhizal Applications), among others. One of the topics was the beneficial plant responses to colonization with mycorrhizae and how long it takes to observe these responses.
Krause: Depending on the plant type, time in production, stage of growth, and growing conditions as well as type of mycorrhizal active ingredient(s) and nutrient status of the growing medium or soil, positive responses range from improved plant vigor to increased rooting density to greater resistance to drought stress to increased phosphorus or micronutrient content in the foliage. How long it takes to observe such responses depends on the plant type, variety, and growth stage, and the mycorrhizal product’s active ingredients, formulation, and application timing.
Busenbark: Mycorrhizal fungi have the ability to survive in soils that would be otherwise toxic to plants. This includes heavy metal and sodium-containing soils that are at levels that the plant cannot survive and/or be productive in by itself. Mycorrhizal fungi have the ability to acquire nutrients and water in these conditions and then transport nutrients and water to the plant through the hyphal network safely. The hyphal network provides this filtration and transportation which allows the plant to be productive under conditions that would otherwise be non-habitable. The general time frame on association formation, functionality, and visual benefit measurements varies depending on the plant’s needs and the growing environment. The association, once developed, can start to provide benefits to the plant within 10 to 28 days. The visual benefit can develop sooner than 30 days; however, in most controlled environment growing systems, it is more often in the 45- to 60-day range.
Beuchel: Plants benefit from endomycorrhizal fungi through increased nutrient uptake, improved plant growth, increased flowering, drought tolerance, and mitigation of excess uptake of heavy metals from contaminated soils. These benefits are more pronounced where these stresses are more significant. Typically, it takes from two to four weeks for endomycorrhizae to completely colonize plant roots and for benefits to occur, depending on plant species. However, if the growing medium temperature is below 50 to 55°F or endomycorrhizal fungi inoculum levels are low in the growing medium, then colonization will take longer.
Krause: This largely depends on the plant’s type and variety, growth stage, and time in production. Generally, plants in production less than three to four weeks that have been treated with mycorrhizae at seeding or transplant will not exhibit the possible benefits during this period, but the effects can be observed in the next level of production or by the end consumer. For woody crops, results can typically be observed after the first or second season of production in the field, depending on the product, the plant species and variety, and soil and environmental conditions.
