The characters most often used in identification keys for filamentous fungi are based on arrangements, forms, sizes, and ornamentations of mature asexual and sexual structures. Another kind of latent characters used in identification keys are responses to abiotic factors such as macromorphology and colony growth rate or diameter at different temperatures, water activities, pH, redox potential, etc. (Pitt 1973). The problem with all these different tests is the potential large number needed (economy) and the need for standardization. Morphological features occasionally overlap or can be difficult to record precisely (Frisvad et al. 2000), as they may be dependent on the media used (for example different brands) and is particularly pronounced for colony diameters and colors. Standardization of incubation conditions (Pitt 1973) and the use of chemically defined media have been proposed to avoid these problems. Image analysis has also been explored in Penicillium (Dorge et al. 2000), here the information found in fungal colonies such a colony texture, diameter and color from very accurately recorded and calibrated images could identify terverticillate penicillia, and has recently been used for clones recognition (Hansen et al. 2003).
In order to identify filamentous fungi it is necessary to identify the fungi to genus level before using traditional keys or chemotaxonomic methods. Filamentous fungi can be identified to genus level by the use of keys [e.g., Samson et al. (2000)]. Once the genus is known references to keys and taxonomic treatments can be found, but as new species are described each year, it may be difficult to obtain good and up-to-date keys for large genera. Knowledge of the associated funga (mycobiota) of different habitats can be a major help in identifying the most common species (Filtenborg et al. 1996). For example the only Penicillia that can grow on citrus fruits are Penicillium italicum, P. ulaiense, and P. digitatum, so identification of greenish mould growth on citrus fruits will be relatively easy.
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