A frequently observed response to light is the rhythmic discharge of spores into the environment. Spores of some fungi can be confidently identified by microscopy following impact on a sticky microscopic slide. Sampling of spores in the air of agricultural fields in England at intervals showed maximum spores of Phytophthora infestans (Straminipila) in the early morning, of Ustilago nuda (Basidiomycotina) at forenoon, of Cladosporium (Fungi Anamorphici) around noon and of Sporobolomyces (Fungi Anamorphici) during the night (Figure 11.2). In an infected apricot orchard in California, the maximum conidia of Monilinia laxa (Ascomycotina) occurred at night. Air sampling close to the ground of rust-infected wheat fields in Kansas showed a diurnal pattern of liberation of urediospores of Puccinia recondita (Basidiomycotina). The ascospores of Sordaria fimicola (Ascomycotina) are discharged mainly during the day and of Daldinia concentrica (Ascomycotina) during the night. These observations suggest that light influences maturation and discharge of spores, although the influence of temperature, humidity and wind velocity cannot be overlooked. Fungi take advantage of the conditions of light, high humidity and wind velocity to maximize the dispersal and germination of their propagules. Other examples of seasonality and photope-riodism in fungi are given by Ingold (1971) and by Roenneberg and Merrow (2003). Some examples are considered below that show the intensity (brightness of illumination), the quality (wavelength of light) and the duration (relative lengths of the alternating periods of light and darkness) that influence fungal development.
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