Highly variable fungal diversity and the potential occurrence of plant pathogenic fungi in potting media, organic fertilizers and composts originating from 14 countries

Potting media, organic fertilizers and composts are widely used by growers in different parts of the world. This study was conducted to investigate the diversity of fungi in 46 potting media products, 17 organic fertilizer products and 9 compost products originating from 14 countries. Fungal diversity was evaluated using direct plating and the identification of fungal isolates was based on morphology and sequences of the internal transcribed spacer region of the ribosomal RNA (ITS rRNA). Analysis showed that Sordariomycetes and Eurotiomycetes (Ascomy-cota) were the most dominant classes in potting media, organic fertilizers and composts. Evaluation of products from the USA and European and Asian countries revealed that they have high fungal diversity, especially products from The Netherlands. Trichoderma, Aspergillus and Penicillium predominated in most samples. Simplicillium, Cladosporium, Bionectria, Alternaria, and Fusarium were recovered at different frequencies from samples of potting media. Fusarium spp. were recovered at high frequencies from samples of organic fertilizers, and organic and non-organic composts. Findings show that most of the products are dominated by saprophytic fungi and fungi with potential biocontrol characteristics. The findings are discussed in terms of factors affecting the diversity of fungi in these products and the intercontinental spread of fungal isolates, including potentially pathogenic ones.