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Saprolegnia and Phytophthora: Water Molds or Oomycetes

Linda Crampton is a writer and experienced science teacher with an honors degree in biology. She enjoys writing about science and nature.

Water mold growing on a dead mayfly larva

Water mold growing on a dead mayfly larva

The Oomycetes or Water Molds

Oomycetes or water molds are interesting organisms that share some features with fungi. They often grow in aquatic and damp environments but are also found in drier areas. Saprolegnia and Phytophthora are important examples of the group. Saprolegnia is a common cause of the so-called fungal infections experienced by freshwater fish. Phytophthora was responsible for the devastating Irish potato famine of the nineteenth century and is also a pathogen of other plants.

Oomycetes (pronounced oh-oh-my-see-tees) were once classified as fungi because their body and behaviour have similarities to these organisms. They grow as branching filaments known as hyphae, as fungi do. They also absorb nutrients through the walls of the hyphae and reproduce by spores. Biologists have discovered that there are some important differences between oomycetes and fungi, however. Despite being called “water molds,” oomycetes are not molds. True molds are fungi.

Though biologists agree that oomycetes shouldn't be classified in the same group as fungi, there isn't yet a consensus about how they should be classified taxonomically. They are thought to be related to the Chromista, a group that contains several types of algae. These algae share certain features with oomycetes that fungi lack.

Saprolegnia Hyphae and Nutrition

Like Phytophthora, Saprolegnia is a genus containing multiple species. The body of Saprolegnia species consists of branching hyphae that extend through its food source. The walls of these structures are made of cellulose. The hyphae generally lack cross-walls, except at the base of the reproductive structures, and contain multiple nuclei.

Fungi very often (but not always) have cross-walls known as septa in their hyphae. These divide the structures into cells, each with its own nucleus or nuclei. The walls of fungal hyphae are made primarily of chitin and don't contain cellulose.

The various species of Saprolegnia are either saprophytes or parasites. Saprophytes feed on dead bodies or decaying material that was once living. Saprolegnia hyphae release digestive enzymes into their environment in order to convert dead or decaying material into a suitable form for absorption.

Parasitic forms of Saprolegnia are found in living organisms. They obtain their food by digesting materials, cells, and tissues in their environment and then absorbing the products. They are sometimes classified as necrotrophs because they kill living cells and extract nutrients from them.

The Saprolegnia photographs in this article show real organisms or structures made by them as viewed under a light microscope. The colour of the photos has been digitally adjusted to make the parts of the organisms easier to see, however. The photo above shows a hypha (item C) and reproductive structures. The structures are identified and described below.

Asexual Reproduction in the Genus

Some of the hyphal branches of Saprolegnia develop a zoosporangium at their tip, as shown in item B in the photo above. Item D is the upper wall of the zoosporangium, or the septum. The zoosporangium produces spores by asexual reproduction. Each spore is known as a zoospore and is motile. When a zoospore is released from the zoosporangium and germinates, it produces the first hypha of a new individual.

Each zoospore has two flagella, which are of different types. Flagella are long and thin extensions that are often found on motile cells. As flagella move, they propel a cell through a liquid. One of Saprolegnia's two flagella is known as a whiplash flagellum and the other as a tinsel flagellum. Each points in a different direction. Hair-like extensions surround the tinsel flagellum.

The two types of flagella possessed by a zoospore can be seen in the illustration of the Phytophthora infestans life cycle shown later in this article. The nature of the flagella supports a link to the Chromista. Fungal flagella are of the whiplash type while the flagella of the Chromista are the same as those of oomycetes. The video below shows zoospores being released from a Saprolegnia zoosporangium and then swimming away. Their thin flagella can't be seen, however.

Sexual Reproduction in Saprolegnia


Saprolegnia also reproduces sexually. The female organ is called an oogonium. It's item F in the photo above and is shown in an enlarged form in the photo below. The oogonium produces large oospheres or eggs. These are said to be haploid (n) because their nucleus has half the number of chromosomes present in the nuclei of the hyphae. The hyphal nuclei have double the number of chromosomes found in the oospheres—or a double set—and are said to be diploid (2n). The situation is somewhat similar to a woman's eggs (haploid) having half the number of chromosomes as her body cells (diploid).


The male organ is known as the antheridium. It's smaller than the oogonium. The antheridium of some organisms contains sperm cells, each containing a haploid nucleus. In Saprolegnia, the haploid nuclei are present in the antheridium, but sperm cells aren't made.


The stalk bearing the antheridium grows, causing the antheridium to come into contact with the side of the oogonium. The antheridium then creates a short tube that pierces the oogonium. A male nucleus travels along the tube and fuses with the female nucleus in an oosphere. The resulting diploid structure is known as an oospore or a zygote (item A in the illustration above). The oospore is released into the environment and produces a new Saprolegnia.

Items A and B in the photo above are immature oogonia of Saprolegnia. Item C is a mature oogonium, and item D is an oosphere or egg.

Saprolegniasis in Fish

Saprolegnia can cause disease in freshwater fish and their eggs. It can also infect amphibians and their eggs as well as crustaceans. Saprolegnia parasitica is the chief species that affects fish. It causes an infection known as saprolegniasis.

Saprolegniasis can be a problem in some fish farms. Wild and aquarium fish may also be infected by a species of Saprolegnia. Chemical treatments for the disease exist. These may be helpful, but this isn't always the case.

The parasite starts by infecting the outer layer of a fish. A mass of fine white threads may appear on the scales of the body and the fins. The mass may resemble cotton wool. The hyphae of the parasite may extend into the gills or muscles of the animal and also enter its blood vessels, which can cause serious effects.

Cysts are produced in some stages of the reproductive cycle of oomycetes. A cyst is a thick-walled and dormant structure that protects its internal contents from harmful environmental conditions. Researchers have found that a primary cyst in Saprolegnia parasitica has hook-like projections on its surface. These may help it to attach to fish that are passing by.

Saprolegniasis in pet fish may be treatable, but the most frequent treatment is potentially dangerous. A veterinarian should be consulted if fish are sick, and the instructions on a medication container should be followed carefully.

Phytophthora: The Plant Destroyer

The various species of Phytophthora can cause serious problems for plant growers. They can infect many different kinds of plants. Economic losses caused by the genus may be severe. The name "Phytophthora" is derived from two Greek words: phyto, meaning plant, and phthora, meaning destroyer.

As in Saprolegnia, the body of Phytophthora consists of branching hyphae. The hyphae have similar features to those of Saprolegnia and obtain nutrients in the same way. The life cycle of Phytophthora resembles that of Saprolegnia but has some slightly different features.

Life cycle of Phytophthora infestans on potato

Life cycle of Phytophthora infestans on potato

The hyphae of oomycetes have diploid nuclei, while the hyphae of fungi contain haploid nuclei. This is yet another significant difference between oomycetes and fungi.

Reproduction in Phytophthora infestans

Asexual Reproduction

Like Saprolegnia, Phytophthora reproduces asexually by producing a zoosporangium containing zoospores. Also like Saprolegnia, the zoospores have a whiplash flagellum and a tinsel one. The zoosporangium or its precursor may produce a new organism directly instead of producing zoospores that do this job, however, as shown in the illustration above. In this case, the sporangium may be called a conidium.

Sexual Reproduction

An immature Phytophthora oogonium contains multiple nuclei, but at maturity only one oosphere containing a single nucleus is present. Similarly, an immature antheridium contains multiple nuclei, but by the time it reaches maturity it has only one nucleus. During sexual reproduction, an oogonium grows into and through an antheridium, enabling the female nucleus and the male nucleus to meet.

Another difference between sexual reproduction in Saprolegnia and Phytophthora exists. In order for sexual reproduction to occur in Phytophthora infestans, two mating types must meet. These are known as A1 and A2 organisms.

Late Blight in Potatoes

Phytophthora infestans causes the disease known as late blight or potato blight. The organism infects the stems and leaves of the potato plant, producing dark lesions. White threads may be seen on the undersurface of the leaves. The infection may kill the plant.

The pathogen sometimes reaches the tubers of the potato plant, which are the part that we eat. The inside of the potatoes turns brown. The brown colour appears on the outer layer of a potato first and gradually moves inward, making the tuber inedible. The pathogen can spread through the potato even after it's gathered from the field. An additional problem is that the pathogen makes the potato plant susceptible to infection by other organisms. These may cause further damage to the tubers while they are in the field or while they are in storage.

Late blight was given its name because it appeared later in the year than early blight. Early blight is caused by a fungus and can also destroy potatoes. Despite their names, early and late blight may occur at the same time of year.

In one week during the summer of 1846, this disease wiped out almost the entire potato crop of Ireland, a crop which was the primary food of the poor at that time. Nearly a million Irish died in the Great Famine.

— University of California Museum of Paleontology (with reference to late blight of potato)

A potato infected by Phytophthora infestans

A potato infected by Phytophthora infestans

The Devastating Irish Potato Famine

Phytophthora infestans can be a serious problem for both potato and tomato plants today. Nevertheless, it no longer produces devastation resembling that of the Irish potato famine of the mid nineteenth century. The huge number of deaths (around a million) and the massive emigration (around one-and-a-half-million people) that occurred as a result of the famine affected both Ireland and the world.

As a result of studying herbarium specimens that were collected in the past, researchers have discovered that the famine was caused by a strain of Phytophthora infestans that (apparently) no longer exists. New strains appear to have arisen as new varieties of potatoes appeared and the famine strain seems to have disappeared at around the same time.

Researchers who have examined the genome of the famine strain say that it doesn't appear to be inherently more virulent than today's strains. They suspect that the two major conditions that caused the famine were the great importance of potatoes in the diet at the time and the fact that the potatoes grown then were very similar genetically. This low genetic diversity meant that the existence of a potato with genetic resistance to the pathogen was unlikely.

Important and Interesting Pathogens

Today Saprolegnia and Phytophthora are significant pathogens that can produce major effects in the environment. They are interesting organisms, despite the harm that they can cause. I think that studying them is a worthy pursuit. Preventing or treating fish disease and enabling crops to survive are important goals.

Exploring the nature and behaviour of oomycetes may help scientists to understand the living world better and might be beneficial for us in multiple ways. Managing and/or protecting those aspects of the living world that harm us or are useful for us is very important.


Questions & Answers

Question: We found some mold growing in our shallow well–which we checked because of the intestinal illness I have. Does it cause human illness?

Answer: Unlike the case for true molds, I haven’t read that water molds can make us sick (though this doesn’t mean they can’t), with one major exception. Pythium insidiosum can infect humans and even cause a fatal illness. It’s often found in tropical countries but has been found in parts of the United States and some other countries. It’s possible that other species of water molds can make us sick as well. You should consult your local public health department or your doctor to find out whether the specific species of water mold in your well is safe, especially since you have an intestinal illness.

© 2018 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on May 05, 2018:

Hi, Adrienne. I appreciate your comment. It's certainly a scary thought that the local potato crop was affected so badly. It must have been a terrible time for the people who relied on potatoes for nourishment.

Adrienne Farricelli on May 05, 2018:

So scary that potatoes were almost on the brink of extinction and that so many people perished because of that. This article was very interesting and I learned a lot of new things I wasn't aware of before.

Linda Crampton (author) from British Columbia, Canada on May 05, 2018:

Thanks, Devika. I agree about the smell. It's certainly not a good thought!

DDE on May 05, 2018:

Unique and well-informed. I learned something new here again. I can imagine a rotten potato odor not a good thought there.

Linda Crampton (author) from British Columbia, Canada on May 01, 2018:

Hi, Jackie. Yes, the ocean environment badly needs our help. The problems are depressing. I hope we can and do improve the situation.

Jackie Lynnley from the beautiful south on May 01, 2018:

I have sworn off farmed fish for awhile now. Yet seeing all the trash in the sea doesn't make me feel too good about any of it.

Maybe these new bucket containers that float in the water and collect all the waste will inspire an improved and larger version for helping to clean up our waters. Of course complete lack of filth added to our oceans and seas may be the only cure and with factories, oil spills, and careless people with no concerns for the environment growing so rapidly, I am not sure we have much hope for improvement.

Linda Crampton (author) from British Columbia, Canada on May 01, 2018:

That's an interesting thought, Heidi! I hope you have a very pleasant evening.

Heidi Thorne from Chicago Area on May 01, 2018:

Ah, so that's the whole potato blight issue!

When I see or hear about creatures (?) like these, they seem so otherworldly. I wonder, when we eventually do find life on other planets, if these types of organisms are what we'll find.

Educated once again! Thanks for sharing your knowledge with us. Have a wonderful day!

Linda Crampton (author) from British Columbia, Canada on May 01, 2018:

It's interesting to hear that potato blight likely affected your family, Bede. Thank you for the comment.

Bede from Minnesota on May 01, 2018:

Linda, I hadn’t known the cause of the potato famine in Ireland. It’s likely the reason for my ancestors to immigrate to Canada in the 19th century. Let’s hope that scientists find something beneficial in the oomycetes. An interesting study.

Linda Crampton (author) from British Columbia, Canada on May 01, 2018:

Hi, Mary. Yes, because living things have so many features in common with respect to their biology, studying some of them may help us to understand others. Thanks for the visit.

Mary Norton from Ontario, Canada on May 01, 2018:

Linda, another interesting article on something I really don’t know about. You are right further study on these might bring more benefits. You have engaged me i this now as we try to eat more fish.

Linda Crampton (author) from British Columbia, Canada on May 01, 2018:

Hi, Peggy. I imagine that the people did eat the rotten potatoes when they could, but this wouldn’t have been possible in all cases. Blighted plants are often infected by secondary organisms that cause soft rot of the tubers. These organisms turn blighted potatoes into a dark, liquid mush that would be very hard to eat.

Peggy Woods from Houston, Texas on May 01, 2018:

This was a fascinating article as usual. I always look forward to learning something new from you. I noticed in the comments that supposedly humans are not affected by Saprolegnia if ingested by eating fish. What about the potatoes? I would imagine that during the potato famine if the tubers were infected with Phytophthora infestans starving people would have eaten them anyway.

Linda Crampton (author) from British Columbia, Canada on April 30, 2018:

Thanks for the visit, Manatita. Water molds are interesting organisms to investigate.

manatita44 from london on April 30, 2018:

These algae and fungi research are very deep. I did not do so much at school. Fascinating to see the sexual reproduction and to know of the effects they have on fish and potatoes.

Linda Crampton (author) from British Columbia, Canada on April 30, 2018:

Hi, Flourish. Connections in science are often very interesting!

FlourishAnyway from USA on April 30, 2018:

Who knew there was a connection between modern losses on salmon farms and the Irish Potato Famine that instigated not only massive human death and suffering but a huge migration pattern?

Linda Crampton (author) from British Columbia, Canada on April 30, 2018:

That's a good point, Larry! Thanks for the visit.

Larry Rankin from Oklahoma on April 30, 2018:

Sometimes it's the smallest things in the world the world that cause the biggest impact.

Great read!

Linda Crampton (author) from British Columbia, Canada on April 30, 2018:

Hi, Mary. Experts say that Saprolegnia doesn't infect humans. They also say that it's best to cook fish to destroy a range of parasites. I haven't yet found any information about whether chemicals made by the mold can hurt us.

I doubt whether we'd notice the mold after the cooking and canning process has finished. I hope a fish processor wouldn't use a fish heavily infested by Saprolegnia, though. Thanks for the comment and the interesting questions.

Mary Wickison from USA on April 30, 2018:

Although when affecting potatoes and tomatoes we would see a disease and it would never make it to market or in our own gardens we would discard it.

However, if this was processed into canned salmon, would we notice it? Would it be harmful to us?

Another fascinating article.

Linda Crampton (author) from British Columbia, Canada on April 30, 2018:

Hi, Bill. Thank you so much for the visit and the comment.

Bill Holland from Olympia, WA on April 30, 2018:

Another great biology lesson. I learn every single time from you, Linda!

Linda Crampton (author) from British Columbia, Canada on April 30, 2018:

Thank you very much for the comment, Peg. I appreciate your kindness.

Peg Cole from North Dallas, Texas on April 30, 2018:

I was always curious about the Irish potato famine and its causes. You've made the information interesting, educational and understandable. I wish you had been my biology teacher in high school and that I had paid more attention in class. Fascinating stuff.