Fungi and Plants

Fungi are generally multi-cellular organisms with a nucleus and a cell wall made of chitin. They acquire nourishment by absorbing their food, principally carbon. As opposed to plants, which make their own food via chlorophyll, and animals, which eat food in the form of plants or other animals, fungi neither eat nor make food. Instead, they send strand-like parts of their body, called hyphae, directly into their food; they secrete chemicals to break the food down into simpler molecules; and then they absorb the food directly into their cells. The body of a fungus is built out of many threads of hyphae, collectively called the mycelium. The mycelium grows within the substance of its food—being undigested soil, plant root environs, deadwood, decaying animals—and, in the case of disease-causing fungi, living plant cells. When growing conditions are favorable, the fungus sends up fruiting bodies, which we recognize principally as mushrooms.

Different groups of fungi do different things for plants. Mycorrhizal fungi form a symbiotic relationship with feeder roots, extending the nutrient reach of the tree a hundred times over. These microscopic organisms cover the surface of colonized roots, forming a sheath that reaches between the cells of the root for nutrient exchange. Some mycorrhizal species actually penetrate the root cortex itself. Long-threaded hyphae extend outward from these fungal bodies to form a biological communications network throughout neighboring ground with other plants. In exchange for their receiving carbon-rich sugars—the product of plant photosynthesis taking place up above in the leaves—these hyphae supply to the tree nutrients like phosphorus, nitrogen, trace minerals, and even water. The surface area of mycorrhizae belowground far surpasses the surface area of the tree's leaves, making this a very fair trade indeed. The key to tree vitality is this nutrient exchange between the roots—via these fungal helpers—and the soil. Mycorrhizae also secrete growth factors that stimulate root growth and branching, as well as antibiotics that protect the roots from pathogenic organisms.

The decomposers work the top portion of ground. Here in this shallow zone, saprophytic fungi convert dead organic matter into fungal biomass and organic acid chains. Complex substrates like cellulose and lignin in wood are their principal food, which makes complete sense on the forest's edge, where fungal dominance paves the way for woodsy species. The so-called sugar fungi in this group like the same simple substrates, as do many bacteria. This breaking-down process ultimately allows nutrients to be retained in the soil in a form readily available for plant uptake. The importance of this humus bank cannot be overstressed: Acid-rich organic matter is resistant to degradation and will remain in the soil for centuries, fueling cycle after cycle of healthy plant growth.

Which brings us to a third group of fungi that has garnered far too much grower attention, relatively speaking: the problem fungi. These pathogenic organisms cause what we humans call disease. Fungal pathogens cause reduced production and/or death when they colonize roots, leaf and fruit surfaces, and other organisms. We could go on about Venturia (scab) and Phytophthora (root, crown, and collar rot), but there are two things I want you to know. Some opportunistic fungi actually help control worse problems; this is the case with Beauveria bassiana, which parasitize insect pupae in the soil. More important, a diverse and thus competitive state of affairs in the tree environs reverses problematic potential from the get-go. We have ways of dealing with intransigents in the holistic orchard.

Far less is known about arboreal fungi. Frankly, we humans do not come to holistic perception all that readily. The soil food web in a conceptual sense extends upward into the leaf canopy of trees and other fruiting plants. Bacteria and fungi alike coat and interact with every inhabitable surface on the planet. These are allies in the finest sense of the word. I like to use the term beneficial fungi here, much as we do when we refer to beneficial insects or beneficial weeds in discussing biodiversity considerations.