GENUS KEY

IDENTIFICATION TO GENUS

Four morphological character sets are complementary in identifying a fungus (or fungi) to genus: (1) mycorrhizal structures when roots are available for study and (2) mode of formation of spores extracted from a soil sample (culture or field), (3) properties of spore subcellular structures (spore wall layers, flexible inner walls), and (4) mode of spore germination. Each is considered separately below. All data sets are not needed, but the more matching (congruent) data sets, the stronger the determination. Underlined words or phrases are linked to pages with explanatory text and/or photos.

(1) MYCORRHIZAE (= MYCORRHIZAS)

All internal mycorrhizal structures stain darkly in trypan blue, chlorazol black E or acid fuschin.
Glomus, Gigaspora, Scutellospora

Arbuscules generally having cylindrical or slightly flared trunks with branches progressively tapering in width toward tips. Vesicles, if present (abundance and timing of appearance varies greatly with species and host-environment conditions), are generally thin-walled and ellipsoid. Intraradical hyphae rarely coiled except near entry points, usually spreading parallel to the root longitudinal axis, with cross-connecting branch hyphae at varying angles. Infection units often merging for form continuous colonization. Older colonization consisting mostly of hyphae (and abundant vesicles if present)
Glomus

Arbuscules generally having swollen trunks with branches tapering abruptly at tips; in pot cultures, still abundant long after plants (and roots) have ceased growth. No intraradical vesicles formed. Instead, transitory spiny auxiliary cells are formed abundantly on external hyphae around roots during early colonization and becoming less frequent as sporulation increases. Intraradical hyphae often coiled throughout, but most prominent at entry points; often knobby or with projections, varying greatly in width. Infection units merge to form fairly uniform colonization throughout the root cortex in broad regions.
Gigaspora

Arbuscules, entry points, intraradical hyphae and extraradical hyphae similar in morphology to that in Gigaspora. Auxiliary cells, however, do not have surface spines but form broad concavities to varying degrees, thus appearing almost smooth to having wide knobs.
Scutellospora

Mycorrhizae vary considerably in staining intensity, but over 50% of colonization stains weakly. Arbuscules generally similar to that of Glomus. Intraradical vesicles highly polymorphic, varying considerably in shape, often with knobs and concavities on vesicle surfaces. Intraradical hyphae both straight and coiled, with coiled hyphae most consistently formed at entry points. Infection units merge somewhat, but many remain isolated and give a mycorrhiza a very patchy distribution.
Acaulospora,
Entrophospora

Arbuscules and intraradical hyphae consistently stain lightly, with neither vesicles or auxiliary cells formed in any isolates of species examined to date in culture or in the field. Intraradical hyphae often coiled within and between cortical cells.
Archaeospora, Paraglomus

(2) MODE OF SPORE FORMATION

Spores borne terminally or intercalarily from a cylindrical to flared subtending hypha (occasionally with constrictions at the hyphal juncture with spore), with continuity between wall layers and all layers of the spore wall. Continuum of organizational complexity of subtending hyphae, from a single hypha to hyphae in loose aggregates, to hyphae bound together by an external hyphal peridium, or to a central plexus of interwoven hyphae in a sporocarp.
Glomus,
Paraglomus

Spores formed from the subtending hypha of an inflated sac called a "sporiferous saccule", sloughing the saccule after maturation after which the spores are "sessile" (lacking a subtending hypha).
Acaulospora,
Entrophospora

Spores borne laterally from the hypha of the sporiferous saccule
Acaulospora

Spores borne within the hypha of the sporiferous saccule
Entrophospora

Spores formed terminally on a bulbous "sporogenous cell" of the subtending hypha.
Gigaspora, Scutellospora

Spores borne terminally from a cylindrical to flared subtending hypha directly or as a branch from a structure identical in appearance to a sporiferous saccule. The only truly dimorphic species are found in this taxon.
Archaeospora

(3) SUBCELLULAR STRUCTURE OF SPORES

Spores having only a spore wall (all layers originating from the wall of the subtending hypha and breaking away only when thin and fragile), with a variable number of layers (1-4 so far). One or more outer layers usually slough, the outermost layer in many species mucilagenous and staining pinkish red in Melzer's reagent (over 70% of described species). No flexible inner walls are formed. The spore wall is not continuous, with a pore at the subtending hypha which may or may not be occluded.
Glomus, Paraglomus

Spores having an unbroken spore wall (no pore) with evidence of the originating subtending hypha usually being a ridge that forms one or two ovoid scars (cicatrix) on the surface. Spores also contain two colorless flexible inner walls. Each inner wall consists of two thin layers that often can be adherent and appear as a single structure. Unique to these genera is a beaded surface on the second (innermost) flexible wall.
Acaulospora,
Entrophospora

Spore wall consisting of three layers: the outer sloughing layer evident only in immature spores, a laminate layer (usually the thickest and often pigmented), and a layer of varying thickness depending on species which also separates to varying degrees. This third layer has been the most difficult to interpret because when thin and separated, it has the same properties as a flexible inner wall and it doesn't have the hyphal connection (e.g., in Glomus) to make its association with the spore wall obvious.
Acaulospora

Spore wall consisting of two layers, appearing the lack the third inner layer found in Acaulospora. Otherwise, subcellular structure is identical to that of Acaulospora.
Entrophospora

Spore wall consisting of 3-4 layers, with at least two unbroken (no pore). The outer most layer tends to slough, varying considerably in thickness; may or may not stain in Melzer's reagent. No true bi-layered flexible inner walls formed, but the innermost layer of the spore wall is semi-flexible; no reaction in Melzer's reagent.
Archaeospora

Spores have a spore wall consisting of at least two permanent layers: an outer layer varying in thickness and color and usually adherent to an inner layer which is laminate, thicker, and often pigmented.
Gigaspora,
Scutellospora

Outer and laminate spore wall layers have smooth surfaces; prior to germination a third thin layer will be seen which thickens in areas to form warts on the inner surface that vary considerably in width and depth. Warts can be intersperse over the entire inner surface of the spore, but are concentrated near areas where the germ tube forms. Healthy spores are never darker than yellowish green (but they all can turn brown to black when old or parasitized). No flexible inner walls formed.
Gigaspora

Spore wall consisting of only the outer and laminate layers, with the laminate layer being smooth to covered with surface ornamentations. Spores contain 1-3 colorless flexible inner walls that except with rare exception consist of two layers that may be adherent and thus difficult to separate. Surfaces of all flexible inner walls are smooth.
Scutellospora

(4) GERMINATION

Spores usually germination through the lument of the subtending hypha. Germ tube originates from the occlusion structure or from the innermost layer of the spore/hyphal wall (at their juncture). Germ tubes can emerge from any external hyphae connected to the spore (or separated from the spore, for that matter) and are synthesized from regrowth of the innermost layer of the hypha wall. Germ tubes may develop from the spore wall in some species, but they also germinate through the subtending hypha.
Glomus,
Paraglomus, Archaeospora?

Spores germinate from a flexible, colorless, plate-like "germination orb" that always forms on the surface of the innermost flexible inner wall. Multiple germ tubes may form and penetrate through the spore wall. Hard to detect except on older spores that have undergone plasmolysis of spore contents.
Acaulospora,
Entrophospora

Spores germinate only by direct penetration of a germ tube through the spore wall, originating from the innermost warty layer of the spore wall. Multiple germ tubes can form in the same area, usually near the sporogenous cell at the base of the spore.
Gigaspora

Spores germinated from a flexible, colorless, plate-like "germination shield" that always forms on the surface of the innermost flexible inner wall. Multiple germ tubes may form and penetrate through the spore wall. Shields often have thickened edges and lobes which make them easily detected in intact spores.
Scutellospora

All internal mycorrhizal structures stain darkly in trypan blue, chlorazol black E or acid fuschin.		*Glomus, Gigaspora, Scutellospora*
	Arbuscules generally having cylindrical or slightly flared trunks with branches progressively tapering in width toward tips. Vesicles, if present (abundance and timing of appearance varies greatly with species and host-environment conditions), are generally thin-walled and ellipsoid. Intraradical hyphae rarely coiled except near entry points, usually spreading parallel to the root longitudinal axis, with cross-connecting branch hyphae at varying angles. Infection units often merging for form continuous colonization. Older colonization consisting mostly of hyphae (and abundant vesicles if present)	*Glomus*
	Arbuscules generally having swollen trunks with branches tapering abruptly at tips; in pot cultures, still abundant long after plants (and roots) have ceased growth. No intraradical vesicles formed. Instead, transitory spiny auxiliary cells are formed abundantly on external hyphae around roots during early colonization and becoming less frequent as sporulation increases. Intraradical hyphae often coiled throughout, but most prominent at entry points; often knobby or with projections, varying greatly in width. Infection units merge to form fairly uniform colonization throughout the root cortex in broad regions.	*Gigaspora*
	Arbuscules, entry points, intraradical hyphae and extraradical hyphae similar in morphology to that in Gigaspora. Auxiliary cells, however, do not have surface spines but form broad concavities to varying degrees, thus appearing almost smooth to having wide knobs.	*Scutellospora*
Mycorrhizae vary considerably in staining intensity, but over 50% of colonization stains weakly. Arbuscules generally similar to that of Glomus. Intraradical vesicles highly polymorphic, varying considerably in shape, often with knobs and concavities on vesicle surfaces. Intraradical hyphae both straight and coiled, with coiled hyphae most consistently formed at entry points. Infection units merge somewhat, but many remain isolated and give a mycorrhiza a very patchy distribution.		Acaulospora, Entrophospora
Arbuscules and intraradical hyphae consistently stain lightly, with neither vesicles or auxiliary cells formed in any isolates of species examined to date in culture or in the field. Intraradical hyphae often coiled within and between cortical cells.		**Archaeospora, Paraglomus**

Spores borne terminally or intercalarily from a cylindrical to flared subtending hypha (occasionally with constrictions at the hyphal juncture with spore), with continuity between wall layers and all layers of the spore wall. Continuum of organizational complexity of subtending hyphae, from a single hypha to hyphae in loose aggregates, to hyphae bound together by an external hyphal peridium, or to a central plexus of interwoven hyphae in a sporocarp.		Glomus, Paraglomus
Spores formed from the subtending hypha of an inflated sac called a "sporiferous saccule", sloughing the saccule after maturation after which the spores are "sessile" (lacking a subtending hypha).		Acaulospora, Entrophospora
	Spores borne laterally from the hypha of the sporiferous saccule	*Acaulospora*
	Spores borne within the hypha of the sporiferous saccule	*Entrophospora*
Spores formed terminally on a bulbous "sporogenous cell" of the subtending hypha.		**Gigaspora, Scutellospora**
Spores borne terminally from a cylindrical to flared subtending hypha directly or as a branch from a structure identical in appearance to a sporiferous saccule. The only truly dimorphic species are found in this taxon.		*Archaeospora*

Spores having only a spore wall (all layers originating from the wall of the subtending hypha and breaking away only when thin and fragile), with a variable number of layers (1-4 so far). One or more outer layers usually slough, the outermost layer in many species mucilagenous and staining pinkish red in Melzer's reagent (over 70% of described species). No flexible inner walls are formed. The spore wall is not continuous, with a pore at the subtending hypha which may or may not be occluded.		*Glomus, Paraglomus*
Spores having an unbroken spore wall (no pore) with evidence of the originating subtending hypha usually being a ridge that forms one or two ovoid scars (cicatrix) on the surface. Spores also contain two colorless flexible inner walls. Each inner wall consists of two thin layers that often can be adherent and appear as a single structure. Unique to these genera is a beaded surface on the second (innermost) flexible wall.		Acaulospora, Entrophospora
	Spore wall consisting of three layers: the outer sloughing layer evident only in immature spores, a laminate layer (usually the thickest and often pigmented), and a layer of varying thickness depending on species which also separates to varying degrees. This third layer has been the most difficult to interpret because when thin and separated, it has the same properties as a flexible inner wall and it doesn't have the hyphal connection (e.g., in Glomus) to make its association with the spore wall obvious.	*Acaulospora*
	Spore wall consisting of two layers, appearing the lack the third inner layer found in Acaulospora. Otherwise, subcellular structure is identical to that of Acaulospora.	*Entrophospora*
Spore wall consisting of 3-4 layers, with at least two unbroken (no pore). The outer most layer tends to slough, varying considerably in thickness; may or may not stain in Melzer's reagent. No true bi-layered flexible inner walls formed, but the innermost layer of the spore wall is semi-flexible; no reaction in Melzer's reagent.		*Archaeospora*
Spores have a spore wall consisting of at least two permanent layers: an outer layer varying in thickness and color and usually adherent to an inner layer which is laminate, thicker, and often pigmented.		Gigaspora, Scutellospora
	Outer and laminate spore wall layers have smooth surfaces; prior to germination a third thin layer will be seen which thickens in areas to form warts on the inner surface that vary considerably in width and depth. Warts can be intersperse over the entire inner surface of the spore, but are concentrated near areas where the germ tube forms. Healthy spores are never darker than yellowish green (but they all can turn brown to black when old or parasitized). No flexible inner walls formed.	*Gigaspora*
	Spore wall consisting of only the outer and laminate layers, with the laminate layer being smooth to covered with surface ornamentations. Spores contain 1-3 colorless flexible inner walls that except with rare exception consist of two layers that may be adherent and thus difficult to separate. Surfaces of all flexible inner walls are smooth.	*Scutellospora*

Spores usually germination through the lument of the subtending hypha. Germ tube originates from the occlusion structure or from the innermost layer of the spore/hyphal wall (at their juncture). Germ tubes can emerge from any external hyphae connected to the spore (or separated from the spore, for that matter) and are synthesized from regrowth of the innermost layer of the hypha wall. Germ tubes may develop from the spore wall in some species, but they also germinate through the subtending hypha.	Glomus, Paraglomus, Archaeospora?
Spores germinate from a flexible, colorless, plate-like "germination orb" that always forms on the surface of the innermost flexible inner wall. Multiple germ tubes may form and penetrate through the spore wall. Hard to detect except on older spores that have undergone plasmolysis of spore contents.	Acaulospora, Entrophospora
Spores germinate only by direct penetration of a germ tube through the spore wall, originating from the innermost warty layer of the spore wall. Multiple germ tubes can form in the same area, usually near the sporogenous cell at the base of the spore.	*Gigaspora*
Spores germinated from a flexible, colorless, plate-like "germination shield" that always forms on the surface of the innermost flexible inner wall. Multiple germ tubes may form and penetrate through the spore wall. Shields often have thickened edges and lobes which make them easily detected in intact spores.	*Scutellospora*