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The Root of Tree Health: Beneficial Fungi (F1782)

This is a 2 page informational publication on beneficial fungi, as they pertain to tree health and management.

Derek Lowstuter, Forest Restoration Specialist, North Dakota Forest Service

Aaron Bergdahl, Forest Health Manager, North Dakota Forest Service


The word Mycorrhizae comes from the Greek words for fungus and root, meaning “fungus roots.” These fungus roots develop as a mutually beneficial relationship between plant roots and fungi that live in healthy soils. This relationship is found on about 90 percent of all land plants around the world, including most conservation and landscape trees in North Dakota. These fungi that attach themselves to plant roots and act as extensions of plant root systems are called Mycorrhizal fungi.

A single square inch of soil can have miles of these fungus root extensions, called mycelium. This dense web of threads can improve plant health in different ways. In exchange for what the fungi offer, plants feed them with sugars produced through photosynthesis.

Tree Roots

Benefits of Mycorrhizal Fungi

Water and Nutrient Uptake

Roots with mycorrhizal fungi are able to use water and nutrients that “naked” roots cannot reach by themselves. The extremely small fungal threads are able to reach water trapped in small soil pores. This not only improves plant growth, but it also can increase the chance of survival during drought.

Likewise, the threads can access nutrients in mineral deposits, soil aggregates and organic matter with pores too small for roots to grow into. Many forms of mycorrhizal fungi also are able to produce chemicals that break down soil compounds into forms that are easier for trees to absorb and use.

Pest and Disease Resistance

By improving water and nutrient uptake, especially phosphorus, fungi can improve tree health and increase trees’ abilities to fight off pests and disease. Some fungi also protect trees from soil-borne disease and pests by producing antibiotic compounds, stimulating other beneficial microorganisms in the root zone, activating plant defense genes, out-competing harmful fungi and acting as armor around fragile roots.

Survival in Harsh Soils and Overall Soil Health

Some mycorrhizal fungi can help trees tolerate soils with high or low pH, high salt content, low fertility or heavy metals. The way these fungi help trees survive in such difficult sites is not completely understood. However, much of the benefit comes from trees’ increased ability to take up extra moisture and nutrients, and by locking away potentially harmful chemicals in plant or fungus tissue.

Mycorrhizal fungi not only can increase tree survival in difficult soils, they can improve soil through time. Fungal roots help chemically and physically break up compacted soils. These processes also can help improve soils structure by promoting soil aggregates. The improved soil structure can reduce erosion, increase water infiltration and promote the growth of other beneficial soil-dwelling life forms.

Food for Wildlife

Some forest fungi rely on animals to spread their spores. These fungi produce fruiting bodies, commonly called mushrooms or sporocarps, directly above or below the soil surface. These can be excellent sources of protein, carbohydrates, minerals and water for many kinds of wildlife.

The spores are spread as animals dig for the fruiting bodies or as they carry the spores on their bodies or in their stomachs. Some of the better known sporocarps, such as morels, can be eaten by humans and are very valuable.

Photo courtesy of ND Forest Service

Some well-known mycorrhizal fungi, such as these morels, are important for wildlife and can be collected to be eaten or sold. Never eat or handle a sporocarp (mushroom) that a knowledgeable person has not positively identified as nonpoisonous. (Photo courtesy of North Dakota Forest Service)

Encourage Beneficial Fungi

Maintain Native Fungus Populations

Beneficial fungi occur naturally in North Dakota. These fungi are adapted to the local soil, climate and plant species. However, activities such as tilling, leaving cultivated land fallow a long time, high fertilizer or pesticide applications, and the removal of plant litter or woody debris all can reduce fungal growth. If tree planting site preparation includes tilling or herbicide application, much of the beneficial native fungi likely have been destroyed, and adding a commercially available blend of beneficial fungi could improve overall tree health and survival rates.

Buy Inoculated Seedlings or Inoculate in the Field

Trees can be inoculated (infected) with beneficial fungi in the nursery, while being out-planted with granular or liquid products, or after planting using a root drench. Buying trees already colonized with fungi or applying fungi on seedling roots using an inoculant at planting time are the most effective ways to colonize tree roots with good fungi.

Limit Fertilizer and Fungicide Use

High soil fertility actually reduces the development of many mycorrhizal fungi. Large applications of fertilizer, especially ones high in soluble phosphorus and nitrogen, should be avoided. Most foliar fungicide sprays will not affect mycorrhizal growth. However, root drenches of systemic fungicide can kill good fungi along with harmful fungi. Carefully read product labels and follow label directions on any chemical being applied around trees and shrubs to reduce the chance of harming helpful fungi.

Photo courtesy of Mycorrihizal Applications, Inc

High-quality nursery stock often already is inoculated with a mix of beneficial fungi. These fungi may appear as a white fuzz on or around roots. This fuzz is not alarming as long as seedlings appear healthy. (Photo courtesy of Mycorrhizal Applications, Inc.)

January 2016

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