Rusts are obligate biotrophic plant parasites with worldwide distribution. They can cause economically important diseases. Some of the rusts belonging to the Gymnosporangium genus have been declared quarantine fungi in various parts of the world. Quarantine fungi are often morphologically very similar to the already established ones, which can lead to covert introductions and, consequently, to their spread. Therefore, the detection of quarantine pathogenic fungi plays an important role in preventing economic and ecological consequences. Traditional detection and identification is difficult, so we wished to introduce a molecular approach and use the same approach to identify some other phytopathogenic fungi. We took a common hawthorn (Crataegus monogyna) with visible signs of infection as a starting material. We assumed that the infection was caused by European hawthorn rust (Gymnosporangium clavariiforme). From the infected tissue (leaf, fruit, branch) we isolated DNA and performed identification based on DNA fingerprinting on three genomic regions (28 S rDNA, ITS, COX3). Unexpectedly, we discovered that all the analysed tissues were infected with different fungal species. In addition to the expected Gymnosporangium, we identified members of 6 fungal genera (Fusarium, Filobasidium, Pseudomicrostroma, Aureobasidium, Cladosporium, Didymella) in the infected tissue. Similarly, different fungal species were discovered in infected common hazel (Corylus avellana), blue honeysuckle (Lonicera caerulea), olive (Olea europaea) and saffron (Crocus sativus) plant tissues.
Based on this observation, we suggest that plant tissue is infected by several different fung—pathogenic and ubiquitous non-pathogenic endophytic and epiphytic fungi, where fungi coexist, and that eventually infection by one fungus eases infection with other fungi.
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