The in vitro antifungal activity of 30 Chinese herb extracts to Saprolegnia sp.

Chinese traditional medicines have been used for several thousands of years in Asian countries, not only in humans but also in many animal species. These compounds prevent and control different types of diseases including internal diseases as well as some infectious diseases, where the aetiological agent is viral, bacterial, parasitic or mycotic. Rhizoma coptidis is believed to inhibit Shigella dysenteriae and that Radix isatidis can prevent flu caused by the influenza virus. It is thus hypothesized that some of these traditional herbal compounds will have anti‐fungal activity. Saprolegniosis is a disease common in fish and their eggs in both fresh and brackish water; a newer, safer medication against Saprolegnia is needed after the prohibition in many countries of the extremely effective fungicide, malachite green. In the present study an attempt is made to identify herbal compounds that have anti‐Saprolegnia activity. A strain of Saprolegnia, CCF1301, was isolated from the skin of infected grass carp (Ctenopharyngodon idella) and identified as Saprolegnia ferax by the 26S rDNA D1/D2 region and ITS region. This strain was used to evaluate the antifungal activity of thirty Chinese traditional herbal medicine extracts, and a modified dish dilution method was developed for the evaluation. Saprolegnia‐infected rapeseeds with visible hyphae were transplanted onto prepared PDA plates containing 2 g L^?1 herbal plant extracts and incubated at 20°C for 48 h. Each herbal plant species was tested in triplicate. Those herbal plant extracts that showed negative mycelium presence at 2 g L^?1 were further tested for minimum inhibitory concentration (MIC) evaluation. The results showed that Syzygium aromaticum, Magnolia officinalis, Melaphis chinensis, Euphorbia fischeriana Steud, and Sophora flavescentis exhibited enhanced growth inhibition at 2 g L^?1 and MIC values of 500, 62.5, 250, 62.5, 250 mg L^?1 concentrations, respectively. It was obvious that Magnolia officinalis and Euphorbia fischeriana Steud exhibited the best antifungal activity. Since there is a high natural toxicity in Euphorbia fischeriana Steud, its applicability as the main ingredient in an aquaculture therapeutic formulation requires further research. Thus, Magnolia officinalis would appear to be the more valuable antifungal herbal species with which to pursue further research.