The efficacy of selected plant extracts and bioflavonoids in controlling infections of Saprolegnia australis (Saprolegniales; Oomycetes)

Saprolegnia infections can account for the significant loss of salmonid eggs and adults in breeding condition and although there are a number of chemical agents available to control infections in hatcheries, treatment is difficult and there are reservations regarding their efficacy, prompting a search for suitable replacements. Here, the efficacy of 24 crude plant extracts and 3 bioflavonoids on the growth of Saprolegnia australis were assessed using an in vitro agar diffusion method against a set of synthetic chemotherapeutants commonly used within the aquaculture industry. The 12 best extracts inhibiting growth at ≤ 100 ppm, were then added to cultures of S. australis in a GP broth and the daily growth was monitored spectrophotometrically over a 7-day period. All 12 plant extracts reduced growth, however, only Atractylodes macrocephala, Zingiber officinale seed, Chrysanthemum and Yucca were as effective as 10 ppm malachite green in slowing growth. A subsequent trial, where doses of the plant extract were given at 72 and 120 h post-inoculation, found that Rumex obtusifolius, Sophora flavescens, Echinacea and Z. officinale were the most effective compounds impacting growth. The study demonstrates that certain plant extracts impact on the growth dynamics of S. australis but further research is required to assess their performance in vivo