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.