Proper identification of fungi is challenging and the most important part in microscopic analysis. For spore count, only spore morphological characteristics are available. For microscopic analysis on other samples, additional fungal structures may be available. It is important for analysts to understand how to utilize available fungal structures for accurate identification and to recognize the significance of presence of certain fungi and fungal structures from perspectives of fungal biology and fungal ecology. The followings are common airborne, indoor fungi, and fungal structures for quick reference. Since colony characteristics are not available for direct microscopic analysis of most of the fungal samples, description of the colony will not be included in the following fungal descriptions. Readers should try to obtain and reference mycological books, journals, and monographs of important indoor fungi:
1. Acremonium and Gliomastix (Hyphomycetes). Conidia are usually single-celled, (they rarely have more than one cell) and may be colorless (Acremo-nium) or have dark pigmentation (Gliomastix). Their sizes and shapes vary from spherical, ovoidal, tear-shaped, subspherical, falciform, reniform, ellipsoidal, to cylindrical; their surfaces range from smooth to rough; they are usually thin-walled, developing mostly in a slimy mass, occasionally in dry conidial chains. Phialides range from being colorless to darkly pigmen-ted, are erect, and occur mostly singly from hyphae with inconspicuous col-lorettes.1,2 These two genera are morphologically very similar, and some mycologists, in fact, treat them under the genus Acremonium. The primary differences separating the two genera are pigmentation and roughness of tips of phialides. Acremonium is moniliaceous with smooth tips of phialides, while Gliomastix is dematiaceous with rough tips. Acremonium strictum (Fig. A.1) is a common indoor species, favoring high water activity, and
Sampling and Analysis of Indoor Microorganisms, Edited by Chin S. Yang and Patricia A. Heinsohn Copyright © 2007 John Wiley & Sons, Inc.
grows on drywall, cellulose-based ceiling tiles, wood materials, and other surfaces. Occasionally it may grow on building materials under high humidity without direct water damage due to repeated condensation. These genera can be identified with intact conidiophores and conidia, but it is very difficult to identify the conidia of this genus from air samples because of wide variations within the genera and similarity with other fungal spores. More often its airborne conidia were considered unidentified single-celled spores.
2. Alternaria (Hyphomycetes). Alternaria conidia are relatively characteristic because of their size, shape, and septation. Its conidia are large, light brown to brown, smooth or rough, multiple-celled with transverse and frequently longitudinal septation, (viz., brickwall pattern with flat bases and elongated tips), and range in size, up to 300 mm long. Many species develop conidia in chains. Conidiophores are light brown to brown, simple or branched, and have multiple cells, developing conidia in sympodial mode. Mycelia are colorless to brown. Common indoor species are A. alternata (Fig. A.2) and
A. tenuissima. This genus comprises a large number of species, most of which are saprobic or plant pathogenic. Several other fungal species and genera (e.g., Mystrosporiella, Phoma glomerata, P. pomorum, Ulocladium) develop spores with some similar morphological characteristics.
3. Aspergillus (Hyphomycetes). Conidia colorless, green, bluish green to brown or black, spherical, subspherical to cylindrical, 2.5-7.5 mm, smooth to rough, in dry chains, forming columns or diverging. Some species develop very thick-walled cells (Hülle cells) or sclerotia. Phialides flask-shaped, without collorettes borne directly on vesicle (uniseriate) or on metulae (biseriate). Conidiophores erect, unbranched, with enlarged apical vesicles. Some species develop teleomorphs (sexual state) also. This is a very large genus and some species are very common contaminants and important indoor fungi. Some species are clinically significant to human health because of their pathogenic, mycotoxigenic, and allergenic nature. Species of this genus are mostly xerophilic to mesophilic, but rarely hydro-philic. This genus is easy to identify, due to the presence of vesicle and arrangement of phialides borne on it, but very difficult to identify to species. Several species may implicate human mycoses.4 For instance, A. flavus and A. fumigatus (Fig. A.3) may cause pulmonary aspergillosis.2 For direct microscopic analysis, it is relatively easy to identify the members of Aspergillus to genus. However, it is not practical to identify them to species without culturing them on required growth media and conditions. To identify common members of this genus to species, please refer to Identification of Common Aspergillus Species by Klich.5
4. Aureobasidium (Hyphomycetes). Aureobasidium pullulans is a very common black yeastlike fungus indoors and is often found in bathrooms, in kitchens, or on exterior building walls under shade. Conidia colorless, ellipsoidal, varying in size (mostly 9.0-11.0 x 4.0-5.5 mm) and shape, one-celled, in a slimy mass (Fig. A.4), and not very characteristic. Hyphae
colorless, 3-12 mm, locally developing into dark brown, thick-walled chlamydospores. Budding is often present and endoconidia are present in intercalary cells. Undifferentiated conidiogenous cells are in colorless hyphae intercalarily. It is not practical to identify spores of this fungus on spore trap samples without culture characteristics. For microscopic analysis on other samples, with more fungal structures, this fungus may be identified.
5. Bipolaris (Hyphomycetes). Conidia brown, mostly smooth, ellipsoidal, straight or slightly curved, multicellular, with a flat dark scar at base (Fig. A.5). Conidiophores brown, simple, erect, multicelled, developing conidia in sympodial mode. Germination is bipolar. Nearly all species are pathogens of grasses. Some species are saprobes. It is very difficult to differentiate the conidia of this genus from those of Drechslera, based solely on the morphology of conidia. Often the spores of these two genera are lumped together for spore count.
Fig. A.6. Botrytis cinerea: (a) conidiophore; (b) conidia.
Fig. A.6. Botrytis cinerea: (a) conidiophore; (b) conidia.
6. Botrytis (Hyphomycetes). Conidia colorless, gray to pale brown, smooth, ellipsoidal, obovoid, or subspherical, mostly nonseptate. Conidiophores gray to brown, straight or flexuous, smooth, branched, often dichotomously or trichotomously, with branch ends often enlarged. A conidiophore forms a long stipe and a dense head. Botrytis spp. are common fungi outdoors and not common indoors. The most common species is B. cinerea (Fig. A.6), which is a plant pathogen causing gray mold on a very broad range of hosts including some common ornamental plants, such as geranium, begonia, rose, lily, dogwood, rhododendron, dahlia, Magnolia, and camellia6,7 and fruits and produce. This fungus is mainly of outdoor origin, although it may be from growth on fruits or flowers brought in from outdoors. Some houseplants can be infected by this fungus, such as cyclamen, poinsettia, chrysanthemum, and gerbera.7 Other species of Botrytis may be present, such as B. peoniae on peonies, B. squamosa on onion, and B. tulipae on tulips. These species of Botrytis share some common characteristics in pathology and ecology. With the presence of a conidiophore, it is not difficult to identify this genus. However, it is a quite challenging to identify its conidia in the samples of spore count. Conidia of this genus are often described as unidentified fungal spores.
7. Cercospora (Hyphomycetes). Conidia colorless, filiform (scolecospores), multiple-celled, with a conspicuous thick dark scar at base (Fig. A.7). Con-idiophores colorless or dark, simple, in clusters and bursting out of leaf tissue. Conidia develop successively on the new growing tips of conidiophores. Species of Cercospora are plant pathogens on a wide variety of higher plants, commonly causing leaf spots. Conidia of Cercospora are common in outdoor air during warm seasons. These spores do not have a significant effect on indoor air quality. The presence of these spores in indoor air are due to infiltration from outdoor sources. Pseudocercospora, and ascospores of Balansia, Cochliobolus, and Gaeumannomyces, and
other genera develop spores sharing some similarities in morphology. Understanding the development of conidia and ascospores will be useful in differentiating them.
8. Chaetomium (Ascomycota). Chaetomium species produce ascospores, which are very characteristic and relatively easy to identify. Ascomata spherical to pyriform, covered with characteristic hairs, which are branched or unbranched and undulate or spiral coiled. Asci typically produce eight ascospores, cylindrical to clavate, slenderly stalked, and with deliquescing walls. Ascospores aseptate, smooth, dark, varying in size and shape, but limoniform in most indoor species, release in a dark mass. The most common species indoors is C. globosum, a hydrophilic species, often growing with in long-term water damage environments (Fig. A.8). A similar genus, Chaetomidium, produces similar spores. Chaetomium species grow on wood and paper products, and are hydrophilic (moisture-loving or have high water activity). They are common on water-damaged drywall, wood, or materials with significant cellulose content. Spores of Chaetomium detected indoors are excellent indicators of water damage.
9. Cladosporium (Hyphomycetes). Conidia dark, mostly one- or two-celled, varying in size and shape, smooth to mostly rough, ovoid to cylindrical, some subglobal, some limoniform with conspicuous scars at both ends, frequently in simple or branched acropetal chains. Conidiophores tall, dark, erect, unbranched or branched near the top, single or in clusters. Cladosporium spp. are weak phytopathogens or saprophytic. C. cladosporioides and C. sphaerospermum (Fig. A.9) are the common species indoors. Spores
Fig. A.8. Chaetomium globosum (a) and its ascospores (b).
Fig. A.8. Chaetomium globosum (a) and its ascospores (b).
of Cladosporium are the most common airborne spores on the earth. Clados-porium is the dominant airborne fungus outdoors in many areas around the world.8 The spores of Cladosporium have well-defined seasonality with a peak in growing seasons. Their diurnal pattern has a midday peak.9 However, species of Cladosporium can grow indoors, such as on fibrous glass
insulation materials in the HVAC and on cold, condensing surfaces. Many other fungal spores, such as spores of Cladophialophora, Fulvia, Hormoconis (Amorphotheca), and Hyalodendron, are very similar to Clados-porium spores.
10. Curvularia (Hyphomycetes). Conidia dark, three- to five-celled, fusiform, curved, with an enlarged central cell and paler cells at both ends; conidiophores brown, simple, developing conidia on new sympodial growing points or api-cally (Fig. A.10). Curvularia, Drechslera, and Bipolaris are closely related. Spores of Curvularia are very characteristic, as its name implies, and are easy to identify. Species of Curvularia are pathogens on a wide variety of plants and saprophytic. Their spores can be found outdoors and indoors.
11. Drechslera (Hyphomycetes). Conidia, light brown to midbrown, smooth to finely roughed, straight, broadly rounded at both ends, multicelled, with dis-toseptate (Fig. A.11). Conidiophores dark brown, simple, erect, thick, and smooth, often developed from dark brown stromata. This genus occurs mainly outdoors as plant pathogens. The airborne spore of this genus indoors originate outdoors. Its conidia geminate from any cells, while the conidia of Bipolaris are from polar cells.
12. Epicoccum nigrum (Hyphomycetes). Synonym is E. purpurascens. Conidia developed singly, golden brown to dark brown, rough, spherical to pyriform, 15-25 mm, with funnel-shaped base and broad attachment scar; conidial septation is obscuring in various directions and dividing conidia into a maximum of 15 cells (Fig. A.12). Conidiophores straight or somewhat flex-uous, short, in clusters, colorless to pale brown. This is a common indoor fungus, often growing on water-damaged dry walls and paper products. Conidia of this species are very common in outdoor air, very characteristic, and easy to identify.
13. Fusarium. Fusarium may produce three types of spores: (a) macroconidia, fusiform, curved, multicelled, colorless, with a foot cell at the base end in most species (Fig. A.13); (b) microconidia much smaller, simpler, colorless, one- or two-celled, smooth, variable in size and shape, developed in chains or slimy heads in most species; (c) chlamydospores may develop or absent within hyphae intercalarily or terminally. Some species consistently
produce both macroconidia and microconidia. The macroconidia are easily identified, but the microconidia are difficult to identify. Microconidia may be similar to and confused with spores of some Acremonium species. There are two types of conidiophores: (a) densely branched conidiophores forming slimy pustules (sporodochia); (b) sparely branched or unbranched conidiophores. In both cases, phialides are slender and tapering, variable in length, colorless, and smooth. Several species of the genus may occur indoors under chronically wet or damp conditions (such as drain pans, around water faucets). However spores of Fusarium are often considered of outdoor origin.
14. Ganoderma (Basidiomycota). A wood decay fungus that produces bracket or conk on trees. It has never been reported growing in the indoor environment. Basidiospores of this genus are very common in the air outdoors and occasionally present in indoor air. Basidiospores of Ganoderma are thick-walled, rough, with a truncated end (Fig. A.14). The morphology of the basidiospores are very characteristic and are very reliable and useful for classification of this fungal genus.10 The spores of Ganoderma are easy to identify from spore trap samples. This fungus is absolutely from outdoor sources.
15. Memnoniella (Hyphomycetes). Conidia, in chains, rough to warted, dark olivaceous to brownish black, globose to subglobose, 3.5-6 mm (Fig. A.15).11 Phialides obovate, or ellipsoidal, colorless first, then turn to olivaceous, smooth, 7-10 x 3-4 mm, in clusters of 4-8 phialides. Conidiophores simple, erect, smooth to rough, colorless to olivaceous, slightly enlarged api-cally. The genus Memnoniella is differentiated from Stachybotrys by their different spore shapes and whether spores are in slimy conidial masses (Sta-chybotrys) or in dry chains (Memnoniella).11 However, more recent studies indicate that Memnoniella may develop Stachybotrys--like spores.12 M. echi-nata is the most common species and is an excellent indicator of water-damaged environments.
16. Mucor (Zygomycota). Sporangiophores are unbranched or irregularly branched, with basal rhizoids, ending with a sporangium, not originated
from stolons. Sporangia vary in size, are spherical without an apophysis, have large columellae (versicle in a sporangium), rupture or persist on maturity, and develop many sporangiospores in it. Sporangiospores are single-celled, vary in shape, and are smooth or slightly ornamented. Mucor plumbeus is often present in indoor environments (Fig. A.16). For spore tfa rkl w^d
Fig. A.15. Memnoniella echinata.
count it is not practical to differentiate sporangiospores of Mucor from other spores with similar morphology.
17. Nigrospora (Hyphomycetes). Conidia spherical, large, black, smooth, 14-20 mm, and an equatorial furrow frequently develops (Fig. A.17). Conidiogenous cells develop on superficial hyphae terminally or laterally, swollen, ampulliform, colorless, develop conidium singly. Its conidia are very characteristic and easy to identify. With other fungal structures, it is not difficult to identify members of the genus. Species of Nigrospora grow on a variety of plants. Common species is N. sphaerica. They may occasionally grow on water-damaged materials.
18. Penicillium (Hyphomycetes). Conidia in dry chains, one-celled, colorless or very light colored in a shade of green, variable in size and shape, smooth to finely rough, 2.5-8 mm in length, globose, ellipsoidal, fusiform, or short cylindrical. Phialides acerose to flask-shaped, and have colorless to pale
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pigmentation (Fig. A.18). Conidiophores, simple or aggregating into synne-mata, erect, colorless or pale-colored, terminally bearing one or several layers of metulae that develop into a group of phialides. Stipes smooth or rough, colorless or pale-colored. A conidiophore with conidia resembles a broom shape. Penicillium is a large genus and is easy to identify to the genus, but very challenging to identify to species. Some species are common in indoor environments with water damage or under damp conditions. For common species identification, please refer to the laboratory guide to common Penicillium species by Pitt.13 It is very difficult to differentiate the conidia of this genus from the conidia of Aspergillus or other similar genera. The conidia of these two genera are often grouped together as Aspergillus/Penicillium for spore count.
19. Pithomyces (Hyphomycetes). Conidia are dark brown, multicellular (phragmo- or dictyoconidia) develop on small peglike branches of the vegetative hyphae, broadly elliptical, pyriform, oblong, and commonly rough (Fig. A.19). Conidia of Pithomyces are cosmopolitan and are very characteristic, making them easy to identify. However, less experienced laboratory analysts may not be able to differentiate them from spores of Ulocladium with confidence. Pithomyces chartarum is the common airborne species. It is very common on dead leaves of a variety of plants and has also been found on paper.3 Ulocladium, on the other hand, is an excellent indicator of a water-damaged environment.
20. Rhizopus (Zygomycota). Sporangiophores develop directly from opposite side of rhizoids, single or in groups, usually unbranched (Fig. A.20). Stolons are present and are an important characteristic of this genus. Sporangia are big, light-colored when young, later turning blackish-brown with aging, and have many spores. Columella are spherical or half-globose, are brown, and have an apophysis. Sporangiospores are ellipsoidal, often
striate, and irregularly angled. Zygospores are occasionally produced in culture. Presence of rhizoids and apophysis will differentiate Rhizopus from Mucor. Rhizopus develops characteristic sporangiospores that are easily distinguishable by trained laboratory analysts. Rhizopus stolonifer is the most common species identified.
21. Scopulariopsis (Hyphomycetes). Conidia are in basipetal chains, one-celled, smooth or rough, spherical to ovate with a noticeable truncate end.
Conidiogenous cells are cylindrical or with a slightly swollen base (vase shape), annellate, single or in branched groups similar to the conidiophores of Penicillium. Several species may occur indoors. The most common species is probably S. brevicaulis (Fig. A.21). Conidia of Scopulariopsis are very characteristic with truncate ends.
22. Spegazzinia (Hyphomycetes). The species Spegazzinia tessarthra develops two types of spores. One is dark brown, spiny, four-celled, separated by a somewhat cross-septation. The other type is smooth, dark brown to black, four-celled, and with a well defined cross septation (Fig. A.22).
Conidiophores are slender, erect, solitary, pigmented, and develop apically. Spegazzinia conidia are very characteristic and easy to identify. There is no report of growth of this genus indoors. Its conidia are of outdoor origins, including plants, plant debris, and soils.
23. Stachybotrys (Hyphomycetes). Conidia are in slimy masses, smooth to coarsely rough, dark olivaceous to brownish black, obovoid, later becoming ellipsoid with age, 10-13 x 5-7 mm. Phialides are obovate or ellipsoidal, colorless early then turning to olivaceous with maturity, smooth, 12-14 x 5-7 mm, in clusters of 5-9 phialides (Fig. A.23). Conidiophores are simple, erect, smooth to rough, colorless to olivaceous, slightly enlarged apically, mostly unbranched but occasionally branched. Conidia of Stachybotrys are very characteristic and can be confidently identified in spore count samples. This genus is closely related to Memnoniella. Species of Memnoniella may occasionally develop Stachybotrys-like conidia, and vice versa.12,14 Stachybotrys species grow on drywall and paper products, and are hydrophilic.11 S. chartarum is probably one of the most common species of Stachybotrys found in buildings with long-term water damage.
However, other species of Stachybotrys have been found in indoor environments.14 Stachybotrys detected indoors is an excellent indicator of water damage. Precaution should be taken to interpret lab results with the presence of Stachybotrys.
24. Trichoderma (Hyphomycetes). Conidia appear colorless to green, smooth to rough, and are in moist conidial masses, variable in shape and size, small, 2.8-4.8 mm for common species. Phialides are flask-shaped, smooth, colorless. Conidiophores branch repeatedly, bearing clusters of phialides terminally in most cases. T. harzianum, T. koningii (Fig. A.24), and T. viride are reportedly found to grow in indoor environments. Recent research suggests that T. harzianum and T. asperellum are considered two true cosmopolitan species.15 Trichoderma is a saprobe, and is very common in soil and on decaying wood. Some Trichoderma species are mycoparasites. These species are fast-growing in media and on building materials.
25. Ulocladium (Hyphomycetes). Conidia are black, rough, with pointed base when young, with both transverse and longitudinal septae, single or in a short chain (only in U. chartarum) (Fig. A.25). Conidiophores are pale brown, erect, multicelled, and develop conidia in a sympodial mode. The two common species of indoor Ulocladium are U. botrytis and U. chartarum. This genus is closely related to Alternaria and Stemphyllium. Ulocladium conidia are characteristic and can be identified by properly trained laboratory analysts, although spores of Alternaria and Pithomyces may be confused with Ulocladium. Ulocladium is an excellent indicator of water damage. However, U. botrytis was found to grow on ceiling tiles under high humidity (unpublished data).
Important Fungal Structures
26. Amerospores (single-celled spores). Amerospores are defined as asexual spores that are nonseptate, single-celled spores with a length: width ratio not exceeding 15 : 1; if elongated, they have a single axis and are not curved through more than 180°; protuberances are less than one-fourth spore body length.16,17 According to this definition, amerospores include a large variety of one-celled spores. Some laboratories may report ameros-pores as individual, small, rounded single-celled spores. Aspergillus/Penicil-lium spores are those in chains. In such a case, the category should be specified or defined to avoid confusion.
27. Ascospores and ascus (pl. asci). Ascomycota produce ascospores (sexual spores) in a saclike structure called an ascus.17,18 In most cases eight ascospores are developed in an ascus. Only a few ascomycetes, are known to grow in indoor environments, due to water damage. Ascospores may be of indoor origins including those of Chaetomium, Emericella, Eurotium, and Peziza. Most ascospores are of outdoor origin. These ascospores include a large variety of different taxa of ascomycetes, which are saprophic or phytopathogens. Ascospores are highly variable and diverse in size and shape. They vary from single-celled to multicelled, from colorless to pig-mented, from spherical to needle-shaped. Ascospores are abundant during warm seasons. There is a significant increase in airborne ascospore concentration during and after a light shower or drizzling rain. Some members of the Ascomycetes class have well-defined diurnal patterns with early morning peaks.9
28. Conidiophores. Conidiophores are reproductive structures of anamorphic fungi and specialized hyphae on which asexual spores are developed. Morphological characters of conidiophores are very important to determine the mode of conidiogenesis so as to identify anamorphic fungi. Some ana-morphic fungi are very common indoors. Conidiophores of some genera are quite characteristic. The presence of conidiophores can be a good indication of fungal growth indoors. Conidiophores can become airborne and appear in the samples of spore traps.
29. Hyphal fragments. A hypha (pl. hyphae) is a vegetative, filamentous fungal structure.18 Its presence in air samples may indicate the presence of fungal infestation. It is unusual for hyphae or hyphal fragments to become airborne unless they are disturbed, such as in remediation. Elevated levels of hyphal fragments indoors may suggest fungal growth indoors and disturbance.
30. Mycelium (pl. mycelia) and mycelial fragments. Mycelium is a mass of hyphae.18 Presence of mycelial fragment or mycelia most likely indicates the presence and growth of fungi. Unless they are disturbed (i.e., by removal of moldy materials or mold remediation), it is not easy for mycelial fragments to become airborne. Elevated levels of airborne mycelial fragments indoors suggest fungal infestation indoors.
31. Aspergillus/Penicillium-like. For spore count, it is impossible to differentiate conidia of Aspergillus from Penicillium only on the basis of morphological characteristics of conidia. These conidia are very simple, mostly small (2.5-5 mm; up to 7 mm), rounded or subspherical, and smooth to ornamented, colorless to lightly pigmented. Many fungal genera and species produce spores that are very similar or difficult to differentiate from Aspergillus and Penicillium. Spores of the following genera may be identified as Aspergillus/Penicillium-like spores: Absidia, Acremonium, Aphanocladium, Beauveria, Chromelosporium, Phialophora, Gliocladium, Metarrhizium, Monocillium, Mortierella, Mucor, Paecilomyces, Thysano-phora, Torulomyces, Trichoderma, and Verticillium.
32. Basidiospores/basidiomycetes. Basidiospores are sexual spores developed on a basidium (pl. basidia) by basidiomycetes (including mushrooms, boletes, toadstools, bracket and conk fungi, polypores, jelly fungi, rusts, and smuts).17'18 Some basidiospores are characteristic and can be differentiated by the hilum on the basidiospores. These spores are common and abundance from spring to fall in the temperate climate. They originate mostly from outdoor sources. Under very unusual situations, basidiospores may be from fruiting bodies or basidiomata of a few mushrooms and wood decay fungi indoors due to long-term water damage. Some members of the basidiomycetes have well-defined diurnal patterns with night or
Fig. A.26. Clamp connections in basidiomycetous mycelium.
early-morning peaks.9 For microscopic analysis of badidiomycetes, clamp connection is the unique characteristic of some basidiomycetous mycelia (Fig. A.26).
33. Myxomycetes. Myxomycetes are commonly called "slime molds". Mycolo-gists study them but do not consider them to be true fungi. Spores of myxomycetes are variable, and their sources are mostly outdoors. Some fruiting bodies of myxomycetes have occasionally been identified from indoor samples. Without fruiting bodies, spores of myxomycetes are considered to be of outdoor origin. Their growth often occurs in hot, humid summer months.
34. Rusts. Rust spores are produced by a group of parasitic basidiomycetes. Rust fungi are obligate pathogens with a wide distribution.19 They infect trees and a wide variety of plants. These plant pathogens depend on living hosts to survive. Some rusts require two different hosts to complete their lifecycle. Five kinds of spores may be developed in lifecycles of certain species.19 They are called rusts because their spores (urediniospores and aeciospores) in mass appear rust in color. Their source is always outdoors.
35. Smuts. Smut spores are produced by a group of parasitic basidiomycetes.18 Smut fungi mainly infect angiosperms, especially monocots, and produce most black (other colors are possible) spore masses in mid to late growing season (such as summer).20 Members of the Gramineae family are the major hosts of smut. Cyperaceae and Compositae are important hosts also.20 The well-known corn smut is a species associated with corn and is
edible. Leaf smut is a disease on turfgrasses with a worldwide distribution and may become airborne during mowing and other human activities. Their sources are always outdoors. Differentiation of smut spores from slime mold (myxomycetes) spores can be very challenging. This is the reason why some laboratory reports combine these two groups.
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