Antifungal activity of lichen extracts and usnic acid for controlling the saprolegniasis

Saprolegniasis is a major disease problem and of great concern for the aquaculture industry. It plays a serious threat to fish populations in natural habitats. It is usually recognized as a fungal disease caused by any of several species of water molds of the family Saprolegniacea e, Oomycota, however, “oomycetous fungi ” belong to the lineage of biflagellate “heterokont” organisms, commonly refe rred to as “stramenopiles” , within the kingdom Chromista [1,2] . They do not belong to the kingdom Fungi, although they are usually studied by mycologists. Saprolegnia parasitica and Achyla bisexalis are the two most important oomycete fish pathogens. Some species in Pythium can also cause the saprolegniasis. Natural recovery by infected fish is almost impossible [1,3] . Malachite green was quite an effective antifungal agent but it is mutagenic, teratogenic and carcinogenic [4,5,6] . It has been banned since 1991 in many countries [7]. Formalin is the drug currently permitted by FDA for controlling fungus on fish diseases in the United States . Hydrogen peroxide, sodium chloride and bronopol are also worth mentioning in preventing and treating saprolegniasis [8]. Recently, nikkomycin Z was employed to inhibit the growth of the mycelium of S. parasitica [9]. Due to the lack of efficient methods to control pathogenic Saprolegnia ceae, there is a dramatic reemergence of saprolegni asis in aquaculture. Thus, there is an important need for developing efficient and sustainable methods to stop the spread of these pathogens. 

Lichens are symbiotic organisms of fungi and algae or cyanobacteria that can produce unique secondary metabolites and have been used in folk medicin e since ancient times [10]. Studies in the lastt hree decades proved the antimicrobial (antibacterial and/or antifungal ), antiviral, antiprotozoal, antipyretic, antitumour, antiproliferative, anti-inflammatory, photoprotective, analgesic, as well as growth and enzyme inhibitory activities of some of the lichen extracts and compounds [10,11] . Various studies have demonstrated that many of lichen species contain usnic acid could play various biological roles and appear to function as allelopathic agents in na ture. Usnic acid was proved to have the inhibitory effect on the growth of mold, bacterium, and yeast [12]. It was also reported that the acetone, methanol and light petroleum extracts of Usnea were effective against Bacillus licheniformis, B. megatarium and Staphylococcus aureus [13]. The results of study on the antibacterial activities of different solvent extracts of Usnea florida showed that the extracts had certain inhibition effect on Staphylococcus aureus, Escherich coli, Pseudomonas aeruginosa, Micrococcus luteus, and Aspergillus flavus [14]. 

The efficacies of usnic acid and the acetone extracts of three lichens ( Cladonia amaurocraea, Cladonia rangiferina and Usnea longissima ) were evaluated for controlling the growth of Saprolegnia parasitica , Achy labisexalis and Pythium sp. in the present study. To the best of our knowledge, no information about the activities of lichen extracts and lichen acids against aquatic oomycetous fungi is available at present.