By Dr. Florence Sessoms
From the April 2023 Issue
Mycorrhizae are fungi found in soils that form symbiotic relationships with plants. This symbiosis is very old, and there is even fossil evidence of mycorrhizal association with the very first terrestrial plants. It’s thought that arbuscular mycorrhizal fungi (AMF) helped plants to colonize land when their root systems were primitive. Because this symbiosis was present prior to the diversification of plants, it may explain why 80% of terrestrial plants can have interactions with mycorrhiza and why this association is found all over the world.
Give & (Up)Take
There is a lot of exchange and communication that happens between a plant and the mycorrhizal fungus. In particular, minerals such as phosphate and nitrogen are delivered from the fungus to the plant. Plants clearly cannot move and their nutrition depends on the availability of the mineral resources around their root system. The mycorrhizal fungi form a large mycelium network, called extraradical hyphae, around the plant’s roots which then forage for nutrients far beyond the plant’s reach. As a result, the plant receives more nutrients, resulting in healthier plants.
More specifically, the process works like this: mycorrhizae spores present in soil germinate, infect (or colonize) the plant root system, and form arbuscule structures inside the cells (Figure 1). These arbuscules are the site of nutrient exchanges between the plant and the fungi. In exchange for minerals, the plant provides sugars and lipids to the fungus. It’s important to note that mycorrhizae are obligate biotrophs; in other words, they are dependent on their host plant for nutrients and cannot survive without the plant.
Other Benefits
Besides helping with plant nutrient uptake, other benefits of mycorrhizal interactions can include:
• Improved plant drought tolerance and resistance. The extraradical hyphae of AMF are very thin and can access water in soil pores that fine plant root hairs cannot. As a result, AMF-colonized plants often demonstrate greater water uptake. In addition, many research studies on AMF- colonized plants during drought stress have revealed the plants are better able to regulate carbon and water (somatal regulation) and have improved water transport from the roots (hydraulic properties).
• Better plant tolerance to certain diseases. Previous studies on AMF have shown reduced incidence of diseases such as fusarium, rhizoctonia, pathogenic bacteria, phytophthora, pythium, and parasitic nematodes. The reason for this has several possible explanations: AMF can result in what’s called mycorrhizal-induced resistance (MIR); alteration to the soil microbes could block development of certain soil pathogens; or the extensive root colonization could prevent the infection and establishment of other root pathogens.
(The effect of AMF colonization on shoot pathogens is less understood. In some cases, AMF seems to lead to greater disease severity. AMF’s effect on foliar pathogens may depend on the pathogens’ seasonality.)
• Positive impacts on soil biology. Mycorhizal fungi can produce and secrete a substance called glomalin that can protect soil from desiccation during drought.
• Improved decomposition of organic matter in soil. Mycorrhizal fungi can partner with other soil microorganisms, such as bacteria, that can improve decomposition, releasing mineral nutrients that can later be delivered to the plant.
Watch The Phosphate
It’s important to realize that nutrient availability in the soil can have a tremendous impact on the symbiosis of plants and mycorrhizae. Soil available phosphate is a very important regulator of mycorrhizal establishment. Numerous research studies have shown that increased phosphate applications lead to the reduction and disappearance of mycorrhizal fungal establishment inside the plant root system. High to moderate available phosphate level in the soil (>10 PPM) will result in reduced fungal colonization. A high soil- available phosphate level will not actually kill the fungus, but under such conditions, plants will not allow fungal colonization inside their root systems.
Therefore, if you are planning to use mycorrhizal inoculants in your fertility program, soil testing should be performed to assess available phosphate and care should be taken to reduce phosphate inputs during the growing seasons.
AMF In Turfgrass
While research has been done studying mycorrhizal fungi in grasslands and its ecological benefits for plant diversity and soil, fewer studies have been done with turfgrass species. However, as members of the grass family, cool- and warm-season turfgrass species can be colonized with mycorrhizal fungi. For example, perennial ryegrass, tall fescue, Kentucky bluegrass, creeping bentgrass, fine fescue, Bermudagrass, and zoysiagrass have been shown to form mycorrhizal associations.
Research studies have demonstrated that mycorrhizal colonization in grasses could lead to increased seedling establishment, greater biomass production, and better resistance to stresses such as drought or disease. However, all these benefits are not straightforward and can vary in function of the turfgrass and fungal species used in the bio-inoculant, the soil properties, the land use (golf course versus home-lawns), the environmental conditions, and the time of year when the application is made.
Moreover, it’s important to ensure the mycorrhizal inoculant reaches the location of the active root system for successful establishment. Application at the time of seeding can be more successful because the inoculant can be mixed with the loose soil. In mature or established lawns, mycorrhizal inoculants should be added when the lawn is actively growing and when the soil is aerated. Additional fertility applications and/or methods of soil incorporation should follow the packaging directions.
Fertilizer Label Literacy
Types Of Inoculant
It’s better to select a mycorrhizal inoculant containing various species of mycorrhizal fungi since different species could have different impacts with various turfgrass species, soil types, and management styles/practices. This type of mycorrhizal inoculant can be found on mycorrhizae.com. Or Sustane has an inoculant, called Bolster MycoBio®, which contains a mix of mycorrhizal fungi and soil beneficial bacteria (Bacillus sp.); these bacteria are known for their participation in nutrient cycling and improve plant tolerance against diseases or abiotic stresses.
However, another company, Harrell’s, has an interesting type of mycorrhizal inoculant, called Bio-MAX Myocrrhizae Pro. Though it contains just a single species of mycorrhizal fungus (Rhizophagus irregularis), it also includes the liquified agar on which the fungus was grown. This means the inoculant should contain not just the mycorrhizal fungus, but all initial chemical signalization between the fungus and the roots. This results in faster establishment of the introduced fungus and the likely activation of soil-born mycorrhizal fungi.
The carrier of the mycorrhizal inoculant is important to consider, as it will define the method of incorporation in the turfgrass system. Never forget that the mycorrhizal inoculant needs to reach an active root system to establish properly.
Sessoms, PhD, is a researcher in Turfgrass Science in the Department of Horticultural Science at the University of Minnesota. During her Ph.D. in Switzerland and postdoctoral position at the Boyce Thompson Institute (Cornell, USA), she worked on understanding the nutritional regulation allowing mycorrhizal establishment inside roots. She has developed various projects on heat stress performance of several turfgrass cultivars, on consumer-available seed mixtures performance under acute drought, and weed suppression ability of certain types of fine fescue. She has also started projects including: a screen for better turfgrass establishment under foliar shade, testing the potential biological nitrification inhibition in turfgrass, and assessing several mycorrhizal-containing bioinoculants on bentgrass greens. For more detailed information, visit turf.umn.edu/news/arbuscular-mycorrhizal-fungi-tiny-friends-big-impact and turf.umn.edu/news/arbuscular-mycorrhizal-fungi-amf-and-their-interactions-turfgrass-species.
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