August 13 - Phytophthora infestans

Phytophthora infestans is a potato-infecting water mold (oomycete) that travelled with tubers from America to Europe and was the causal agent of the Irish Potato Famine in the 1840s that led to massive emigration to the New World. The potato late blight disease is difficult to control even today, and is usually attempted by repeated spraying of chemicals. Although P. infestans might look like a typical fungus, it isn't one, but rather is related to brown algae, and relies on water for efficient infection. Swimming flagellated zoospores start germinating when they come into contact with host plants like potatoes, tomatoes and a few other nightshade relatives. Quickly a hyphal network grows between the cells of leaves and tubers. Hyphae produce digit-like extensions called haustoria that stick into the cells to acquire nutrients. To suppress the plants immunity, P. infestans delivers dedicated effector proteins into host cells (learn more here). At later infection stages the parasite shifts into killing mode and destroys leaves to finally produce sporangial containers filled with an army of new zoospores to start a new infection cycle. Sequencing of the pathogen’s genome revealed a huge amount of repetitive regions which enable the parasite to exquisitely adapt to new conditions.

Image and post contributed by Sebastian Schornack.

June 26 - Botrytis cinerea

Have you ever enjoyed a glass of sweet dessert wine? It's all thanks to a fungal parasite called Botrytis cinerea, or "grey mould". It is a pathogen which can infect a number of different crop plants including beans, strawberries, cucumbers, tomatoes, and grapes. Unlike many parasites, Botrytis cinerea is not obligately parasitic - it can alternate between living as a parasite, or as detrivore which digests dead organic matter. While this fungi is a cause for headache for most farmers, in vitriculture, it is also known as "noble rot". A late infection by B. cinerea in white grapes lead to drying of the grape berries which increase the sugar concentration without increasing acidity. Maybe a good dessert wine brought to us thanks to the "noble rot" would be a good way to accompany the course of parasites you have been enjoying this week.

June 22 - Ustilago maydis

Ustilago maydis, also known as Corn Smut, is a fungus which infects corn. The fungal infection usually occurs in the plant's ovaries, replacing the corn kernels with enlarged, distorted "galls" which then produce infective spores that are carried away by wind or rain. Like any good parasite, Ustilago feeds off its host, resulting in decreased yield in the infected crop. From an evolutionary biology view point, it is interesting to note that functionally, Ustilago acts rather like a parasitic castrator which reduces or eliminate the host's reproductive capacity, a strategy that has independently evolved in many parasite lineages. Whereas corn smut is considered as a blight on corn in Europe and North Armerica, in Mexico it is considered as a delicacy. Known as huitlacoche, this 'blight' fetch a higher price on the Mexican market than the corn itself. Its taste has been described as being similar to that of truffle - which is perhaps not too surprising as they are both are fungi that grow in association with a plant host. In addition to being a culinary treat, Ustilago maydis is also a useful lab model organism, used in research into plant diseases, as well as other research into cell and molecular biology.

Contributed by Tommy Leung.

May 19 - Pythium insidiosum

Oh man - this is a nasty one. I had a really hard time finding a photo that wasn't completely disgusting and opted for just a nice image of the simple fungi themselves. The parasite is Pythium insidiosum, a fungal parasite that causes a disease known as pythiosis in dogs and horses. It's a common parasite in warmer parts of the world including the American South, eastern Australia, and south America and primarily exists in swampy, standing water. Dogs can become infected in their GI tract from drinking tainted water and it can cause thickening of the tissues or granulated lumps. Horses more often have subcutaneous lesions of these fungi, which they get from standing in the swampy water. The damage to their legs, bellies, and chests can be quite gruesome and often the lesions will have bits of dead tissue in them with the somewhat amusing name of "klunkers."

The disease is apparently on the rise in the U.S. and is troublesome because many vets are not familiar with it. I found this website with some rather sad tales of beloved pets who have been lost to the disease.

The image is from this site.

May 15 - Pseudoperonospora humuli

The weekend is here - and perhaps that means a nice cold beer or two. One of the main ingredients in beer, of course, is hops, which give brews their characteristic bitter flavor. Hops are dried flowers of the Humulus lupulus plant. Pseudoperonospora humuli is a parasite that could get in the way of your thirst-quenching IPA or ESB, however. This fungus, known as downy mildew infects hop plants and destroys their leaves and those wonderful flowers. A recent molecular phylogenetic study has suggested that P. humuli is not actually genetically distinct from Pseudoperonospora cubensis, another mildew fungus that was described from cucurbits (cucumbers, melons, etc.) So, if you're not a beer-drinker, but do like to enjoy a nice gherkin or melon ball as part your Saturday evening activities, well then, you can be annoyed at the very same parasite.



Image come from this site.

May 13 - Crinipellis perniciosa

Too bad it isn't Halloween yet - today's parasite is Crinipellis perniciosa, a fungus that causes a disease known as "Witches' Broom Disease" in cacao, the plant that we get chocolate from. The basidiospores of this fungus are spread by wind and if they invade younger, developing seed pods, can cause the plant to never produce any cocoa beans. The fungus can invade many other portions of the cacao plant as well, and if they get into the meristem, they will produce vegetative "brooms" that inspired their name. Its invasion into the Bahia region of Brazil caused the production of cocoa to plummet by more than half. Researchers are now searching for plants in other regions that might harbor genes that confer resistance genes, so that they can breed them to the Brazilian plants.

Image is from Invasive.org.

May 10 - Hemileia vastatrix

Monday mornings would be impossible without coffee, so I have a real issue with this parasite, known as Hemileia vastatrix, or coffee rust. This species of fungus attacks the leaves of the coffee plant (Coffea arabica), but not the fruit or other parts. It was first discovered in East Africa, but since then has spread to most of the other major coffee-growing regions of the world, such as Java, and Brazil. This parasite, in fact, might get some credit for the fact that Brits now drink so much tea. In the 1870's and 1880's, an epidemic of coffee rust essentially destroyed coffee production in many parts of Africa and Asia, including many countries that were British colonies. Some places, such as Sri Lanka (then known as Ceylong) upped their tea production instead.

April 25 - Cryptococcus gattii

The last parasite from "Whale Week" is Cryptococcus gattii, which both infects whales and dolphins, but also has been all over the news lately because it has been spreading across the Pacific Northwest and killing people as well. Thought to be native to the tropical regions of Australia, South America, and Africa the fungus invaded western North America (perhaps via a eucalyptus?) in the mid- to late-1990's and has worked its way as far south as Oregon now. Several whales and porpoises have been found to be infected, though how the fungal spores make their way to those marine creatures remains a mystery. Other animals such as dogs, cats, ferrets, llamas, and alpacas have also tested positive for the fungus. The recent deaths of 5 people in the Pacific Northwest have been creating a bit of a panic, but doctors and news agencies have been warning people to relax and not change their habits. Like other fungal infections, C. gattii cannot be spread from person to person.

A good paper on the recent outbreak in the U.S./Canada can be found here and if you'd rather listen than read, the CDC has a podcast here.

Photo is from this site.

April 9 - Pneumocystis jirovecii

Pneumocystis jirovecii is a fungus that can be an opportunistic pathogen and produce pneumonia in humans. This species was previously known as Pneumocystis carinii, thus these pneumonias are still usually called "PCP." These infections principally infect immunocompromised people and have become associated with AIDS. These fungi are geographically widespread and most people are exposed to them by a young age. In addition to the change in the species name, these pathogens were also previously classified as protozoa since they are single-celled, however genetic and biochemical evidence all places them in with fungi.

February 21 - Tricophyton rubrum

Tricophyton rubrum is a dermatophyte - a fungus that is an obligate parasite of the skin or other keratinized tissues like nails. This species is the most frequent culprit behind the most common fungal infections of humans - ringworm, athlete's foot and jock itch. The fungi typically only colonize the outermost layers of skin and do not invade the true living tissues beneath. They make us itch when our immune systems respond to the metabolic waste they exude out. Although ringworm isn't usually a severe health threat to an infected person, and can be treated with aggressive topical medications, it's often horrifically hard it can be to get rid of in one's house. The fungi can remain in clothes, on sheets, even on furniture and other parts of the environment for over a year and transmission can occur between humans and pets. It's very common in showers and locker rooms so wearing flip-flops is always a good idea in these public places. The name "ringworm" comes from the tendency of the fungus to form a ring in the skin, though they do not always do this.

February 12 - Nosema bombycis

Busy little Bombyx mori caterpillers happily crunch up mulberry leaves and spin cocoons where they will pupate and transform into moths. These cocoons are made of silk fibers that are harvested and processed to produce silk - in fact, 70 million pounds of silk, every year. But, of course, like virtually all creatures on the planet, silkworms suffer from parasites, too. One of the best known is Nosema bombycis, a microsproridian parasite, that causes a disease known as pébrine - so called because it produces pepper-like spots on the caterpillars. The disease is highly infectious and can pass from mother moth to her eggs, so it has long been a major concern for silkworm breeders. Louis Pasteur, the famous French microbiologist discovered that infectious agents were causing the disease in silkworms and is credited for saving the industry at the time. Microsproridia are a very diverse group of spore-forming unicellular parasites, many of which infect insects. They were once thought to be primitive eukaryotes, but they are now thought to be more closely related to fungi.

Image from this website.

January 29 - Batrachochytrium dendrobatidis

This chytrid fungus (Chytriodiomycota: Chytridiales) is the causative agent of chytridiomycosis, an emerging infectious disease of amphibians. This fungus has been implicated as the cause of amphibian declines and extinctions of more than 250 species of frogs across six continents (the widespread distribution of this disease the likely consequence of anthropogenic effects). B. dendrobatidis can infect both larval and adult amphibians. Infections in larvae cause a reduction in grazing efficiency, food intake, and survival. Infections in adults cause thickening of the skin which might interfere with osmoregulation or ion balance. This fungus has two parts of its life cycle: one part in the host and one part outside of the host (a motile zoospore stage). Recent studies have shown that B. dendrobatidis can survive for long periods of time outside of the host, increasing its ability to drive host populations extinct. Although this fungus is believed to have originated in Africa, B. dendrobatidis was first reported in North and Central America and Australia in 1998, coinciding with massive amphibian declines. However, this fungus has been around since at least 1938 (documented in museum specimens), therefore researchers are trying to determine if B. dendrobatidis is truly a novel emerging disease or a long-term endemic pathogen (where population declines are the result of changes in pathogen virulence, host susceptibility, environmental change, or a combination of these factors). What kind of disease B. dentrobatidis actually is may help to determine how to stop the rampant spread of this devastating fungus, if possible. Findings currently point to B. dendrobatidis being a novel pathogen (laboratory experiments, wave-like declines of amphibians, etc), however much more work needs to be done to be sure.

Contributed by Jessica Light.
See this paper or this one (which just came out today) for more information.

January 17- Geomyces destructans

In 2006, a caver in upstate New York came across bats with an unusual white substance on their muzzles. By 2008, white nose syndrome (WNS), as it was now known, was reported in caves from Vermont to Virginia and thousands of bats were dying, with mortality rates of over 90% in some caves. The fungus growing on the bats has been indentified as Geomyces destructans, related to other soil fungi that are psychrophilic (=like cold temperatures). Bats with WNS often wake up from hibernation and begin to fly around, searching for food, as their fat reserves are low. This has caused a great deal of alarm as some endangered species of bats are at risk of extinction should they be exposed to the disease. But another worry are the trophic effects to the ecosystem if bats, potent insectivores, disappear in large numbers. One estimate, by the US Forest Service, is that there will be an extra 2.4 million pounds of bugs per year without the bats that have been killed by WNS. That will mean more crop pests and more mosquitoes and other disease vectors.

Read more about Geomyces destructans and white nose syndrome here and here.