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Saprolegnia (fungus)

By the koi.wiki editorial team · Updated

Key facts: Saprolegnia is a genus of water mold, often called “cotton mold,” that forms white or gray fibrous patches on fish skin and gills[1]. In koi it is generally a secondary, opportunistic pathogen that colonizes tissue already damaged by injury, parasites, or poor water quality[2][1].
Saprolegnia is a genus of oomycetes (water molds), taxonomically classified among the heterokonts in the order Saprolegniales[1]. Infections are sometimes called oomycosis, and in aquaculture literature the condition is referred to as saprolegniasis[1][7]. On koi and other fish, the organism appears as white or grey cotton-like growths on the skin, fins, or gills[3][1].According to the University of Florida IFAS Extension, fungal spores including water molds are common in aquatic environments and do not usually cause disease in healthy fish; they typically colonize tissue only after a fish has been injured, infected with an external parasite, or damaged by handling[2].

Biology of Saprolegnia

Saprolegnia is both a saprotroph (feeding on dead organic matter such as fish waste) and a necrotroph that can take advantage of already-injured tissue[1]. It tolerates a wide temperature range of 3–33 °C (37–91 °F) but is more prevalent at lower temperatures, and while most common in freshwater it can also tolerate brackish water and moist soil[1].The organism grows as long filaments called hyphae, which contain zoospores; when a mass of hyphae becomes large enough to see without a microscope it is called a mycelium[1]. Colonies are generally white but may appear grey when bacteria or debris become trapped in the fibrous mass[1]. Reproduction includes both an asexual phase, in which zoospores encyst and release further zoospores in a process called polyplanetism, and a sexual phase producing an oospore after fusion of male and female gametangia[1]. Some of the most pathogenic strains have tiny hooks on their hairs that improve their ability to attach to and infect a host during the dispersal stage[1].

Signs of infection in koi

Saprolegnia infection on koi typically presents as cotton-like growths on the skin and gills[3]. According to the Wikipedia entry on the genus, the mold spreads across the surface of its host as a cotton-like film through necrosis of the skin, usually remaining in the epidermal layers, though it does not appear to be strictly tissue-specific[1]. A related presentation sometimes described by koi dealers as “mouth fungus” involves white, cotton-like growth in or around the mouth[3].Infection is usually fatal if untreated, eventually causing hemodilution, though time to death depends on the site of infection, growth rate, and the host's ability to withstand the stress of infection[1]. Outbreaks of Saprolegnia have also been linked historically to mass mortalities in wild salmonid populations in western Europe, where the organism acted as a secondary infection following an underlying ulcerative condition[1].

A secondary, opportunistic pathogen

Saprolegnia is generally described as a secondary pathogen, though it can act as a primary pathogen under the right circumstances[1]. The UF/IFAS Extension notes that fungal spores are common in aquatic environments and do not typically infect healthy fish; colonization tends to follow damage from external parasites, bacterial infection, or handling injuries[2].Southern Regional Aquaculture Center publications similarly list Saprolegnia as a condition that can predispose fish to, or occur alongside, Aeromonas infections, with the two conditions often appearing together on compromised fish[4]. The fish louse Argulus is also noted to be commonly associated with secondary Saprolegnia and Aeromonas infections, as parasite damage to the skin creates entry points for the water mold[5]. For background on parasites that can predispose koi to secondary fungal infection, see parasites of koi and fish lice (Argulus).

Predisposing factors and stress

Because Saprolegnia is largely opportunistic, its appearance is closely tied to factors that compromise a fish's natural defenses. The Southern Regional Aquaculture Center explains that the surface mucus layer is a fish's first physical barrier against disease organisms, and that handling, poor water quality, or chemical treatments can damage this layer and reduce its effectiveness[9]. Damage to scales and skin from handling or crowding can likewise create entry points for pathogens[9].Stress hormones released during handling, transport, or poor water conditions can suppress both non-specific and specific immune responses, increasing susceptibility to infection[9]. In aquaculture more broadly, the risk of fungal infections such as Saprolegnia rises with higher population densities and the difficulty of maintaining water quality in intensive systems[10]. Relevant background on stocking and water parameters is covered in water quality and quarantine.

Treatment approaches

UF/IFAS Extension notes that formalin or potassium permanganate can be effective against most fungal infections if they are not too severe, but since fungi are usually secondary, it emphasizes diagnosing and correcting the original underlying problem as well[2].Malachite green has historically been used against Saprolegnia, which infects fish eggs in commercial aquaculture, and is also used as an antibacterial and against Ichthyophthirius multifiliis in freshwater aquaria, according to its Wikipedia entry[6]. Hydrogen peroxide formulations are also referenced in fisheries literature: the product 35% PEROX-AID is FDA-approved in the United States for control of mortality in freshwater-reared finfish eggs and coldwater finfish due to saprolegniasis caused by fungi of the family Saprolegniaceae[7]. Ultraviolet sterilizers can also inactivate Saprolegnia, with published “zap dose” references in the aquaculture literature citing roughly 10,000 μW·sec/cm² for hyphae and about 39,600 μW·sec/cm² for zoospores[8]. Because these treatments concern dosing and regulatory approval for fish health, any decision about which treatment to use should be guided by qualified advice rather than general description; see diagnosis: scrapes, microscopes and fish vets and treatments overview.

Prevention

UF/IFAS Extension frames fish health management broadly as prevention-first: good water quality management, nutrition, and sanitation underlie the prevention of opportunistic diseases, since even sterilization technologies such as UV or ozone cannot eliminate all potential pathogens from the water[2]. Daily observation of appearance, behavior, and feeding activity allows early detection of problems, improving the chances of successful treatment[2].Minimizing stress is central to prevention, since stress compromises the mucus layer and skin barriers that keep pathogens out and suppresses immune function via cortisol and other stress hormones[9]. Recommended best management practices include using knitted rather than knotted nets to reduce injury and scale loss, minimizing handling time, maintaining stable temperatures during transport, and providing adequate aeration, all of which reduce the skin damage and immune suppression that allow Saprolegnia to take hold[9]. See also signs of illness and Aeromonas and ulcers for related opportunistic conditions, and pond design for background on stocking and crowding.

Common questions

Is Saprolegnia contagious between koi?
Saprolegnia spores are naturally present in most aquatic environments and are not usually a disease risk to healthy fish; infection typically follows injury, parasite damage, or handling stress rather than direct transmission between healthy fish[2][1].
What does Saprolegnia look like on koi?
It appears as white or grey cotton-like fibrous patches on the skin, fins, or gills, sometimes turning grey when bacteria or debris become trapped in the growth[1][3].
Can Saprolegnia kill koi?
According to the Wikipedia entry on the genus, infection is usually fatal if it progresses, eventually causing hemodilution, though time to death varies with the site and severity of infection[1].
What treatments are used against Saprolegnia in fish?
Formalin or potassium permanganate are cited as effective against most fungal infections if caught early[2]; malachite green has historically been used against Saprolegnia[6]; and hydrogen peroxide products such as 35% PEROX-AID are FDA-approved for saprolegniasis control in certain freshwater finfish and eggs[7].
Why do Saprolegnia and Aeromonas often appear together?
Both are described as opportunistic secondary pathogens that take hold after a predisposing stressor, handling event, or parasite infestation such as Argulus, so they are frequently observed on the same compromised fish[4][5].

References

  1. ^ Saprolegnia (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Saprolegnia. Retrieved November 2026.
  2. ^ UF/IFAS EDIS FA004: Introduction to Fish Health Management. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA004. Retrieved November 2026.
  3. ^ HELP, Is My Koi Sick?!? Diagnose Symptoms & Fish Diseases - FREE Health Checklist. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-sick-health-diagnosis-symptoms-diseases/. Retrieved November 2026.
  4. ^ SRAC Publication No. 478: Motile Aeromonas Septicemia (MAS) in Fish. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/478%20Motile%20Aeromonas%20Sept…. Retrieved November 2026.
  5. ^ UF/IFAS EDIS FA184: Argulus (Fish Louse) Infections in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA184. Retrieved November 2026.
  6. ^ Malachite green (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Malachite_green. Retrieved November 2026.
  7. ^ UF/IFAS EDIS FA157: Use of Hydrogen Peroxide in Finfish Aquaculture. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA157. Retrieved November 2026.
  8. ^ SRAC Publication No. 4707: Biosecurity in Aquaculture, Part 1: An Overview. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4707%20Biosecurity%20in%20Aquac…. Retrieved November 2026.
  9. ^ SRAC Publication No. 474: The Role of Stress in Fish Disease. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/474%20The%20Role%20of%20Stress%…. Retrieved November 2026.
  10. ^ Fish farming (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Fish_farming. Retrieved November 2026.