Tree Disease: Mycorrhizae

MYCORRHIZAE

Mycorrhizae are mutually beneficial associations between certain highly specialized fungi and the roots of plants (and trees). In these associations the fungi and the host plant roots grow together in such an intimate relationship as to literally form "fungus-roots" (i.e., mycorrhizae), entities distinctly separate from either the fungi or the roots alone. The roots are roots, the fungi are fungi, and together they from mycorrhizae. The fungi benefit by way of receiving carbohydrates and certain other necessities (for example, vitamins) from their hosts. In turn, the fungi enhance the host roots' ability to absorb nutrients (esp. phosphorus) from the soil. Indeed, in certain cases the fungi literally feed nutrients from the soil to their host root systems. Sometimes the presence of mycorrhizal fungi imparts a degree of protection or resistance against certain root disease microorganisms (e.g., nematodes, fungi). In general, plants and trees with mycorrhizae are healthier than those without.

Mycorrhizae are the rule, not the exception in nature. Only a few plant species and fewer, if any, trees are known to be nonmycorrhizal. In Florida, it would be safe to say that one would be hard pressed to find a nonmycorrhizal tree. Only in special situations such as disturbed or chemically treated (e.g., fumigated) soils are mycorrhizal deficiencies likely to occur. Such deficiencies are often accompanied by stunting and/or poor growth of the host.

Mycorrhizae are generally divided into two large groups. ECTOMYCORRHIZAE occur on pines and other conifers as well as oaks, eucalyptus, beech, and birches. Ectomycorrhizae are characterized by the presence of an external fungal sheath or "mantle" on the surface of infected feeder roots. Mantles can vary in color from white to black, often a function of the particular fungus involved. Ectomycorrhizal fungi also grow between the cortex cells of their hoot roots, forming what is referred to as a "hartig net" (visible only with a microscope).

Ectomycorrhizae on pines are often forked or branched (bifurcate), but may occur unbranched (monopodial) or repeatedly branched (coralloid) as well. At certain times of the year many ectomycorrhizal fungi produce distinctive sporophores (mushrooms, toadstools, puffballs, etc.) on the ground, always in association with the roots of their host.

ENDOMYCORRHIZAE, as a rule, cannot be detected with the naked eye. Endomycorrhizal fungi do not form a mantle on the surface of the infected roots. Growth of these fungi is primarily within (in contrast to ectomycorrhizae) the cortex cells of their host roots. Distinctive changes in root morphology (branching, etc.) do not occur with the formation of endomycorrhizae. Microscopic observation is required to observe endomycorrhizal infections.

Endomycorrhizal tree species include maples, oaks, sycamore, dogwoods, yellow-poplar and sweetgum

Mycorrhizal fungi reproduce by means of various kinds of spores. Those of ectomycorrhizal fungi are most frequently produced in aboveground sporophores (mushrooms, toadstools, puffballs, etc.) and are disseminated by the wind. Accordingly, it is usually not long before ectomycorrhizae "reappear" in soil disturbed by practices such as methyl bromide fumigation in forest tree nurseries. Nearby forest and shade trees usually support sufficient ectomycorrhizae (and sporophores of the ectomycorrhizal fungi) to insure recolonization of disturbed soils in a matter of months.

In contrast, the spores of most endomycorrhizal fungi are produced underground in association with their host roots. These spores are not aerially disseminated and, as a result, the build up of endomycorrhizae in disturbed soils is not uncommonly a slow process. Endomycorrhizal deficiencies are occasionally a problem in nurseries where soil is regularly sterilized or fumigated as cultural practice.

Both ectomycorrhizal and endomycorrhizal fungi are moved in infested soils and on (in) the roots of mycorrhizal plants. Both types of mycorrhizal fungi are also capable of vegetative growth from root to root within and between host plants or trees. In trees, mycorrhizal infections are limited to fine, feeder roots less than 1-2 mm in diameter.