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Colonies of Aspergillus flavus (yellow-green) and Penicillium cyclopium (blue-green) on a petri dish containing rose bengal agar (RBA, peptone dextrose medium with rose bengal).  In both species the colour is in the conidia.  Aspergillus is a much larger fungus and you can see the inividual pom pon (spherical) heads.  In Penicillium the conidiophores and conidial chains are tiny even by microscopic standards but they are closely packed and form a velevty mat over the surface of the agar.   We did an estimate of the number of conidia on a petri dish (9 cm diam) in the lab one day and came up with about 10 billion - give or take a billion or two!!

Rose Bengal Agar

In  dilution plate studies using soil,  host of different fungi can be recovered.  Some of these grow so rapidly that they will cover the plate in a few days and obscure or inhibit slower growing fungi.  Rose bengal is excellent for inhibiting colony growth of these fast growing fungi allowing us to recover the slow gowers that often prove to be the most interesting.  Some claim that RBA doesn't give the full spectrum of fungi, but no medium does and for my money RBA is the best single medium for soil studies.