Fungi of the genus Aspergillus have a long history of associations with humankind. The genus was first described by Micheli in 1729 [1] to include those fungi with long stalks and spore heads that radiated in long chains from a central structure resembling an aspergillum, the brush-like structure used in religious ceremonies for sprinkling holy water. Species of this genus are common, originally applied to the first species of this genus, A. glaucus by Link [2]. They are extremely diverse in their habitats, and extraordinarily versatile in terms of their ability to produce secondary metabolites. Several are important human and plant pathogens, while others are useful in the production of several fermented food products highly regarded by humankind. Due to their metabolic versatility, species of this genus have great biotechnological potential and several are used for the production of numerous food and nonfood by-products. The plant pathogens are of concern not only for their ability to destroy several agronomically important food crops, but also due to their ability to produce several mycotoxins. These mycotoxins are associated with specific species or subgenera of the Aspergillus and are in general toxic to livestock, poultry, fish, and humans.

Early attention was devoted to species of Aspergillus as causes of animal toxicities, mainly poultry [3]. Immediately after this report, it was established that this toxicity, described as the Turkey-X disease of peanuts, was caused by A. flavus and A. parasiticus [4]. Subsequent studies resulted in identifying the aflatoxins as the toxicological agent [5], which initiated mycotoxicology as a serious and complex problem of food safety. Continued studies resulted in distinguishing another class of mycotoxins produced by species of aspergilli, the ochratoxins, which along with the aflatoxins, were established as important carcinogenic mycotoxins. Ochratoxin A was discovered in maize by Van der Merwe et al. [6] as the mycotoxin produced by strains of Aspergillus ochraceus Whilh, and Penicillium verrucosum. One of the first reports for the natural occurrences of ochratoxicosis was in poultry, which consisted of five independent episodes including about 970,000 turkeys, two episodes of about 70,000 laying hens, and two episodes in about 12,000,000 broilers [7]. These observations served to establish ochratoxins as important toxins in agriculture, and in food safety. Recently the ochratoxins have been reported from several other species of Aspergillus sections Circumdati (A. ochraceus group), and by Eurotium herbariorum, a member of the Aspergillus section (A. glaucus group). Recently, the black species of Aspergillus have been shown able to produce the ochratoxins [8], which extends the geographic area of concern from the temperate zone to the tropical and sub-temperate zone due to the distribution of these black species.

The black aspergilli are commonly found as soil organisms decomposing dead plant residues [9] and they are pathogenic on several crops. As discussed below, the majority of the black Aspergillus species are associated with grapes, onions, maize, and peanuts, where they are cited as pathogens causing such diseases as peanut and maize seedling blight, and maize kernel rot. However, there are numerous examples of fungi associated with plants as symptomless endophytes, and there is evidence that this life habit may be practiced by the Aspergillus species as well. In association with several hosts, these symptomless endophytes have the capacity to either develop as pathogens or as saprophytes, and in either state become producers of mycotoxins. Symptomless expressions of several black aspergilli are indicated in the literature but nothing is indicated about their ability to produce mycotoxins and any associated pathology. Further, the black species of Aspergillus associated with any plant pathological problem was indicated in early publications as A. niger, the black species. In this review, we refer to A. niger sensu strictu, i.e., A. niger var. niger, to designate or distinguish the present day discription from the older published accounts, which in most instances were applied sensu lato and will be referred to here as simply A. niger. While several cryptic species within this subgenus have been delineated, this recent taxonomic revision creates a large gap of knowledge of black Aspergillus species that are presently defined, and which is important to food safety and plant pathology. This review focuses on identifying the species of the subgenera Circumdati, the section Nigri (=A. niger group) (Table 1) encountered on cereals and other plants, with an emphasis on maize and peanuts in particular. The nature of the association with their hosts and their potential to produce the ochratoxins and other toxins relative to specific species are also reviewed.

There are well over 190 number of Aspergillus species, and these can be conveniently separated into several distinct morphospecies, and several of these are based on colors according to the earlier classification of Raper and Fennell [10]. However, phylogenetic analyses of sequence data resulted in separating the Aspergillus genus into eight subgenera [11]. Following these analyses, the economically important species that produce the ochratoxins were divided to include those species of the subgenera Circumdati, the sections Circumdati (=Aspergillus ochraceus group) and Nigri (A. niger group). There are no known teleomorphic species of group Nigri. In recent years, members of the Aspergillus section Nigri have undergone an extensive taxonomic revision resulting in several new taxa, such as A. niger var. niger, A. melleus, A. sulphureus, A. brasilensis, A. ostianus, A. petrakii, A. scletotium, A. carbonarius, A. aculeatus, A. japonicus, A. tubingensis, A. ibericus and Eurotium herbariorum [12,13,14] (Table 1).

However, these new taxa have not been identified as to the responsible species in diseases of food crops, such as maize seedling blight, maize ear rot and seedling blight of peanuts. Further, any role they may play in the pathogenic expression of maize kernels and plants prior infected with Fusarium verticillioides, a common maize pathogen usually co-associated with black aspergilli as a symptomless infection, is unknown.

Klich [9] classic paper on the biography of the Aspergillus genus indicated that most of the species occurred in the tropical latitudes below 25 degree north and south, with greater than expected frequencies in the subtropical to warm temperate zones at latitudes between 26 and 35 degrees. This study suggested that species abundance peaked in the subtropics. This distribution is attributed to several biotic and abiotic interacting factors with the major factor temperature [9]. In general, the black species of aspergilli were found to occur more frequently in forest and cultivated soils and less frequency in desert soils. However, A. niger var. niger was uniformly distributed throughout the entire samples areas including forest, grassland, wetlands, deserts, and cultivated soils [9]. Thus, this documents this species as the most common species in both subgenera of Circumdati.

In a recent survey of maize and peanut using rep-PCR to distinguish morphotypic and molecularly derived species (Figure 1), several basic black Aspergilli were distinguished in peanuts and maize [15]. The survey was designed to analyze for endophytic species of these two plants using surface disinfection of kernels and plant parts. This survey indicated that several species were present in these two plants as seedborne systemic and endophytic infections (Table 2). This survey indicates that the A. niger var. niger was the major species isolated from these two plants, with A. foetidus and A. japonicus occurring as minor species. Peanuts accounted for three species, while only two were isolated from maize. The work is similar to that of Magnoli et al. [16] who found that of the black aspergilli, A. niger var. niger along with A. japonicus var. japonicus was isolated from surface disinfected maize kernels from Argentina.

The earliest report of toxicity from the black Aspergillus species was by Frischbier and Richtesteiger [17] who reported on the experimental poisoning of pigs fed bread that was inoculated with A. niger and that the toxic component was oxalic acid. Later, Wilson and Wilson [18] indirectly indicated A. niger as the toxic organisms since oxalic acid was isolated from moldy feedstuffs that was toxic to livestock. Presently, the type of the section, A. niger, has important industrial application and most strains of this specific species hold the Generally Recognized as Safe status issued by FDA. What is not clear, however, what species is intended when the term “A. niger” is used for all black spored aspergilli. Thus, correct taxonomic descriptions of species within this group are extremely important as this can serve to distinguish those that are phytopathogens and mycotoxic from those that have technological applications.

Black aspergilli are reported as pre- and post-harvest pathogens in maize, other cereal grain, bunch grapes, onions, garlic, soya beans, apples, mangoes, and peanuts [16,41,59,60,61], although the extent of damage on each host depends on unknown predisposing environmental factors. The inoculum source of most species is the soil and litter, particular the vineyards soils [9,62,63,64]. Other sources include the seed of most crops [15,62] or fruit [63]. Most black aspergilli are indicated as opportunistic pathogens of fruits such as grapes and some spices [62,63]. In a small survey [15], several species of black aspergilli were isolated from surface disinfected maize and peanuts, which indicated that these species were endophytes. Endophytic associations have also been reported in onions and garlic [62], although these were characterized as latent infections. The better term for these infections is symptomless, as endophytic infections have proved in all instances as not latent or dormant as implied, but metabolically active, colonizing the host producing several classes of secondary metabolites, some of which are toxic. The major difference is the absence of disease symptoms produced during these infections. These endophytic infections can become, however, pathogenic under some biotic and abiotic conditions either pre- or post-harvest. Symptomless infections pose grave problems from a food safety concern, as commodities contaminated by such infections are not obvious, appear normal, but can contain toxic metabolites.

An investigation of the endophytic nature of maize seedling was investigated. Aspergillus carbonarius SRRC 2131 and A. niger var. niger SRRC 13 were transformed with a yellow fluorescent protein vector that was used to measure colonization of maize seedlings [15,65]. The transformed black aspergilli did not affect the ability of these strains to colonize maize seedling (Figure 2). These transformed species were isolated from surface disinfected seedling roots and leaves of plants grown under ideal growth room conditions, and there were no significant differences in seedling height and stem thickness [15]. The infection remained symptomless and attempts at inducing seedling blight by inducing drought did not produce the disease. The endophytic nature of the black aspergilli was indicated from the recovery of A. niger from surface disinfected plant materials of onions by Hayden and Maude [62]. The infection in onion was also symptomless and infection was proved to develop from contaminated onion seed. Seedling onions were, as in the maize seedlings, similar to non-infected except in shoot length [62].

In grapes, another situation is described. Aspergillus rot in grapes is caused by the black species, A. niger, var. niger, A. carbonarius and A. aculeatus [63]. These black aspergilli are reported as opportunistic pathogens of damaged berries of grapes, since in the absence of damaged spores remain over the surface of grapes without causing visible pathology. Further, A. niger is known to cause kernel rot of maize (Figure 3), which is similar to ear/kernel rot produced by Fusarium verticillioides, and F. graminearum. However, black aspergilli are isolated from surface disinfected kernels of maize (Figure 4). Symptomless infections in onion by A. niger var. niger can develop into a postharvest disease, but perhaps due to injury to the bulbs or unsuited storage conditions [62,66]. In the case of maize, infection and the resulting kernel rot may occur from wounds produced by earworms and other damaging insects, similar to that which occurs from infections by A. flavus. Nevertheless, endophytic infections do occur as demonstrated from isolations from surface disinfected kernels, and these endophytic infections can colonize plant tissue. Therefore, some black aspergilli are capable of a biotrophic endophytic existence with maize and onion. Since these two plants are widely separated taxonomically, it is possible that endophytic infection by the black aspergilli exist in numerous plant taxa.

Since ochratoxin A is present in maize, and other cereal grain under field conditions [16], there is a suggestion that the black aspergilli species may form some relationship with maize during its growth under field conditions or that it is interactive with other field fungi. In a laboratory study, it was demonstrated that the fumonisin-producing fungi, F. verticillioides, can exclude the Aspergillus species such as A. niger var. niger, from colonizing maize, but this depended on optimum temperature and specific water activity levels [67,68]. The interaction of antagonistic substances was not examined in that study, but conceivable competition may also relate to inhibitory substances as well as the abiotic factors measured.

In the past most descriptions and concerns relative to the black aspergilli have been referred to as A. niger, resulting in a great deal of confusions concerning the actual species observed. Recent molecular analysis of the black aspergilli indicated several cryptic species, which now should be aligned with recent phytopathological and toxicological concerns. To prevent this confusion, the older A. niger has been replaced by A. niger, var. niger in order to exclude the prior confusion in discussions of both the earlier and recent discriptions. The subgenus Circumdati section Nigri consists of at least 19 species of black spore species of which A. niger var. niger is probably the most dominant species in the US where it is found in most soil types and on dozens of fruits and cereal grains. Of major concern is the relationship of the black aspergilli with maize and peanuts, two plants of major economic concern in the US Strains of A. niger var. niger, along with others, can produce the ochratoxins and the fumonisin B₁ mycotoxins. In addition to the effect of the mycotoxins on animals and humans, these toxins are interactive with other secondary metabolites to produce synergistic effects. Others are also phytotoxic [69], suggesting role for this mycotoxin in the pathology of seedling blight of peanuts and maize. The production of both the ochratoxins and the fumonisins by the black aspergilli extends the concern since food and feeds not inductive to one mycotoxin can be inductive to the other. Similarly, attempts at control of one might not control the other. Current disease descriptions are based on the casual identification of the black aspergilli as belonging to the older descriptions of A. niger. Certain species have been characterized as producing specific toxins and associated pathology. This indicates the need for reexamination of taxa associated with specific food crops. Additionally, detailed studies of their host relationship are also indicated, as some species are associated with crops as symptomless endophytes. There are also reports on the co-occurrence of both ochratoxin A with the fumonisins in maize [70,71], which suggests interactions between producing endophytic fungi on maize, such as the maize endophyte F. verticillioides. Further, evidence of co-infection of A. niger with A. ochraceus in maize is observed also a concern as this presents difficulty in identifying the correct offending mycotoxic species. The black aspergilli are known pathogens in peanut culture where major problems are indicated from the seedling stage or during peanut set late in matured plants. The result is poor quality nuts that are rated too low for human consumption. Thus, pathogenic expressions in maize, cotton, grapes, peanuts and other plants, the production of the ochratoxins and fumonisins on these products, and the unknown species associated with specific pathological expressions on these important crop plants underlines the need for further studies of the black aspergilli. Finally, the prediction of rising global temperatures should influence the population patterns and shifts of species within the A. niger group to more northern latitudes that should increase the number of additional crops, adding to the mycotoxigenic potential globally for all species of the subgenera Circumdata.