Oomycetes, or water moulds, are fungal-like organisms phylogenetically related to algae. They cause devastating diseases in both plants and animals. Here, we describe seven oomycete species that are emerging or re-emerging threats to agriculture, horticulture, aquaculture and natural ecosystems. They include the plant pathogens Phytophthora infestans, Phytophthora palmivora, Phytophthora ramorum, Plasmopara obducens, and the animal pathogens Aphanomyces invadans, Saprolegnia parasitica and Halioticida noduliformans For each species, we describe its pathology, importance and impact, discuss why it is an emerging threat and briefly review current research activities.This article is part of the themed issue 'Tackling emerging fungal threats to animal health, food security and ecosystem resilience'.

To identify the pathogens causing saprolegniosis among farmed fish in Nova Scotia, 172 infected tissues and 23 water samples were collected from six species of teleosts: Atlantic salmon (Salmo salar), brown trout (Salmo trutta), Arctic charr (Salvelinus alpinus), brook trout (Salvelinus fontinalis), striped bass (Morone saxatilis) and rainbow trout (Oncorhynchus mykiss) at nine facilities over a 600 km range. Following laboratory culture, 132 isolates were recovered. Six species of oomycetes were identified from analysis of the internal transcribed spacer (ITS) sequence of the nrDNA: Saprolegnia parasitica, Saprolegnia ferax, Saprolegnia diclina, Saprolegnia aenigmatica, Saprolegnia torulosa, Saprolegnia sp. and Pythiopsis cymosa. Further phylogenetic analyses of the ITS and cytochrome c oxidase subunit 1 (Cox1) regions revealed four strains of Saprolegnia parasitica (named here as S1, S2, S3 and S4), of which S1 and S2 were common (37% and 42% of the isolates), and two strains of S. ferax. Among S. parasitica, S2 and S3 are more closely related to each other than to S1 based on the phylogenetic analyses and predicted RNA secondary structure of the ITS region. Sexual structures with a similar morphology were formed by S1 and S3 in vitro, but were not formed by S2.

The paper reports the first incidence of successful induction of breeding in the riverine catfish Rita rita using carp pituitary gland (PG) extract. A breeding trial using four PG doses viz. 80, 100, 120 and 140mg/kg body weight of fish was conducted to optimize the dose of pituitary gland (PG) extract in terms of induction of ovulation in female. The male received a dose of 40mg PG/kg body weight in all cases and was sacrificed for collection of milt. The best performance was shown by the fish treated with 100mg PG/kg body weight in respect of inducing ovulation in females and fertilization and hatching rates of eggs.

No abstract

Two experiments were conducted with a view to assessing the effects of stocking densities on the growth and breeding performance and larval growth and survival of Channa striatus. The first one was carried out with a view to find a suitable stocking density of Channa striatus at which they could spawn while the second experiment dealt with the effect of stocking density on growth and survival of the produced larvae. The first experiment was carried out in nine equal-sized chambers of raceway separated by netted wooden frame with an area of 1.83 ×1.12m 2 each. The effective water depth in each of the raceway chambers was maintained at 1.2m. The broodfish were stocked at 4 (357.22g/m 2 ), 6 (541.78g/m 2 ) and 8 (719.18g/m 2 ) at 1:1 male-female ratio and fed with washed and chopped poultry viscera twice a day and with live silver carp fry twice a week. No significant (P>0.05) difference in weight and specific growth rate was observed among the broodfish reared under different stocking densities during the experimental period of 80 days. The broodfish stocked at a density of 357.22g/m 2 and 541.78g/m 2 spawned naturally in their respective chambers. The fish stocked at 541.78g/m 2 showed the best breeding performance. The second experiment was carried out with 4-5 days old larvae in 9 bowls of 10l capacity divided into three treatments having 20, 40 and 60 larvae each i.e. the stocking density were of 2, 4 and 6 larvae/l respectively. Larvae of treatment I which was stocked with 2 larvae/l showed significantly higher growth rate from the 7 th day of the experiment and maintained the same trend up to the end of the experiment (i.e. 21 st day) compared to other treatments. Larvae of treatment I also showed significantly higher health condition (13.31±0.69mg/mm) and survival rate (80.00±3.00) compared to those of the other two.

Since the ban of malachite green in the fish farming industry, finding alternative ways of controlling Saprolegnia infections has become of utmost importance. Much effort has been made to elucidate the mechanisms by which Saprolegnia invades fish eggs. Little is known about the defence mechanisms of the hosts, making some eggs more prone to infection than others. One clue might lie in the composition of the eggs. As the immune system in the embryos is not developed yet, the difference in infection levels could be explained by factors influenced by the mother herself, by either transferring passive immunity, influencing the physical aspects of the eggs or both. One of the physical aspects that could be influenced by the female is the chorion, the extracellular coat surrounding the fish egg, which is in fact the first major barrier to be overcome by Saprolegnia spp. Our results suggest that a thicker chorion in eggs from Atlantic salmon gives a better protection against Saprolegnia spp. In addition to the identification of differences in sensitivity of eggs in a fish farm set-up, we were able to confirm these results in a laboratory-controlled challenge experiment.

No abstract

No abstract