Names of Diseases and Pathogens

Diseases

Diseases do not have “scientific” names like organisms, and there is no formal mechanism for assigning or sanctioning disease names. However, societies or working groups may prepare lists of recommended disease names, such as the useful list produced by the Western International Forest Disease Work Conference ​[2]​ or these lists more or less maintained by the American Phytopathological Society (the conifer list seems to be more up-to-date than other tree lists).

Disease names may be taken from the host, symptoms, pathogen, and/or the kind of disease. Sometimes, part of the latin name of the pathogen is included in the disease name. If so, it is never italicized, but if the genus is used, it is capitalized (one journal does not follow this convention). Disease names derived from country names are capitalized as usual in English. Examples:

The most serious disease of southern pines is fusiform rust.

The greatest tragedy in American forest history was the devastation caused by chestnut blight.

A major disease of conifers in the northern hemisphere is annosus root rot, caused by Heterobasidion annosum sensu lato. The disease is also often called Heterobasidion root (and butt) rot.

Disease names like Swiss needle cast and Dutch elm disease are not much appreciated by the Swiss and the Dutch!

Both conifers and hardwoods are often infected by Armillaria species, which cause Armillaria root rot. The same disease is also called shoestring root rot because the fungus produces black, root-like structures.

In addition to diseases, there are names for types of diseases that are similar. They differ in the particular host or pathogen and often in details of infection, etc., but follow a general pattern that can be used in thinking about the diseases as a group. Here are some examples:

Stem decays are often the most serious diseases in hardwood stands.

In conifers of western North America generally, it is considered that dwarf mistletoes are the most serious diseases, but root and butt rots are probably a close second.

Let’s not name diseases after pathogens!

For better or for worse (mostly for worse), many diseases are named in part after pathogens, such as Cytospora canker, Hypoxylon canker, Armillaria root disease, annosus root disease, Elytroderma needle cast, etc.  Why is this a bad idea?  Oh, let us count the ways!

1. They have always been hard for nonspecialists to remember.  Anyone who has taught forest pathology or deals with foresters knows this.  Our other audiences include arborists, the gardening/landscape community, invasive pest advocates, regulators, and more.  What is the point?  If we feel the need to dazzle someone with our pedantic brilliance we can always throw the pathogen name at them.
2. Also for nonspecialists, these names are completely meaningless.  Wouldn’t it be better to make the name from some important and memorable aspect of symptoms, signs, associated environmental factors, etc.?  There are many such disease names already, and they are great!
3. The pathogen names can change due to improved phylogenetic understanding, and this is very common in modern times.  Of course this is a good thing in itself, but now we have to either:
   1. change the disease name to keep up (chasing pathogen names with disease names), which means the disease name is unstable, and the nonspecialists have to continually update things every time a pathogen name changes, or;
   2. we can leave it the same, meaning the disease name is now completely meaningless, even if your native language is Latin.
4. Now that we are in the “one fungus, one name” era, we are going to lose one of the names for fungi with names for both the sexual and asexual states.  Which one is kept may be argued about in some cases for many years, and could even go back and forth.  See point 3.
5. It tends to reinforce the mistaken notion that a pathogen and disease are the same thing, and that these are common and latin names of the same thing (see below).

Take as an example “annosus root disease”.  That was okay when the pathogen was called Fomes annosus. When the name Heterobasidion annosum came into common use, some felt the need to change the disease name to “annosum root disease”. When multiple species were carved out of H. annosum sensu lato, the feeling was to give up that approach and use the genus, or “Heterobasidion root disease(s)”.  And how long will it be until a new genus is used for these species? 

Or take all the diseases named “Cytospora canker of [host]”.  Maybe we’ll have to start using a genus other than Cytospora for these fungi, and Cytospora will mean exactly nothing in the future.  Instead, we might have half a dozen or more genera typified by the sexual state.  Do we really want to go down that road, renaming diseases, and changing when those genera change?  NO!  Pimple canker,  sandpaper canker, ANYTHING would be better.

Pathogens are not diseases!

A disease and the pathogen that causes it are two distinct entities. Always be careful to distinguish them. Unfortunately it is easy to find tables of tree diseases with columns like “Scientific Name” (actually the pathogen name) and “Common Name” (actually the disease name). Being sloppy in language leads to sloppy thinking. Equating them leads you into the trap of focussing too much on the pathogen and ignoring the role of environment and host in development of disease.   (I’m sure Yoda would have a good way to express this. If you can channel him and imagine how he might say it, please enter it in a comment below!)

Exceptions are rusts, powdery mildews, and parasitic plants, where terms are used for both the pathogen groups and the type diseases they cause.  But even there, they are not the same thing.

The following are examples of what NOT ( 🙁 ) to do.  These kinds of formulations are never seen in the older literature, but are all too common today:

🙁 Coniferiporia sulphurascens is a serious disease of Douglas-fir.

🙁 An important disease of aspen is sooty-bark canker (Encoelia pruinosa).

🙁 Leptographium wageneri is a root disease spread by root-to-root contact.

🙁 Stem decays produce basidiospores in conks.

🙁 Make a list of diseases, with the disease name column labeled “common name” and the pathogen column labeled “scientific name.”

Basics of fungal names

Names of fungal species, as with other organisms, have two parts, together termed a “latin binomial”: the genus followed by the “specific epithet”. Together they make the name of a species. The genus is capitalized, the specific epithet is not. Both should be underlined or italicized. Names of higher taxa (family, order, etc.) are always capitalized but not underlined or italicized (although there is a move to do that ​[4]​). For example:

Nectria galligena is in the phylum Ascomycota.

Some fungi have common English names, but not many.  Names are better established in European languages, but even there microfungi don’t have common names.  Common names are not capitalized, and are never underlined or italicized. Examples:

The common name of Fomitopsis pinicola is “red-belt fungus” because it usually has a reddish margin.

Phaeolus schweinitzii has a velvety surface and looks like a brownish lump on the forest floor. It has common names like “velvet-top fungus” and “cowpie fungus.”

One Fungus, One Name

Traditionally (in fact until 2011), fungi that had both sexual and asexual fruiting structures usually had two names, one for each form. Fungi for which we knew only asexual fruiting got only a name for that stage. The name and classification of the sexual stage, if known, was always regarded as reflecting phylogeny as we understood it. The asexual taxa were form-taxa, not reflecting phylogeny, necessarily, but just a form of a fungus.

For example, the pathogen of coral-spot Nectria canker has both sexual and asexual fruiting structures that occur separately. The sexual structure is in the perithecial ascomycetes, characteristic of the genus Nectria, and the fungus is called Nectria cinnabarina.

The asexual structure is a sporodochium. Traditionally, fungi with sporodochia are classified in the Deuteromycota. This particular sporodochium and conidia it produces placed it in the genus Tubercularia, and this one is Tubercularia vulgaris. So the fungus had two names!

Of course this is confusing to people, especially students. How can we have two names for one fungus? But it is easy to see how this developed historically. When we found these various fruitings, why, there’s a fungus, let’s name and classify it. It may have been many years later that it was discovered they were the same fungus. And in many cases, even today, when we find an asexual fruiting, we may not know if there is a sexual stage or which one it is phylogenetically related to.

Today mycologists are trying to reduce this confusion by not having separate classifications for asexual stages, and not having separate names for them. The meme for this is “One Fungus, One Name”.  Dual nomenclature was ended after the International Botanical Congress in Melbourne in July 2011 ​[1]​.

OK, one name, but which one?

So we must consistently use one name. Now the question becomes, One Fungus, Which Name? The answer is not simple.  Amy Rossman wrote the clearest explanation of the process that I have seen ​[3]​. To determine the one name to rule them all:

1. Choose the genus. Determine if the two genera are based on type species that are congeneric, i.e., phylogenetically in the same genus.
   1. If so, the two genera are synonyms. Choose the genus first described (priority).
      1. If the genus with priority represents the asexual state, in order to use it, it must be protected or conserved against the name representing the sexual state.
   2. If the genera are not synonyms, determine in which genus the fungus belongs. Your species must be phylogenetically congeneric with the type species. That is the correct genus.
2. Choose the specific epithet. The specific epithet is based on priority: use the oldest one representing any state of the species. If necessary, it must be formally combined with the chosen genus to form a new legitimate binomial.

This may make sense to you, but it gets worse:

1. First, some of these genera are huge. No matter which genus you identify as the correct one, there may be scores of new combinations to be made based on (2) above. Who is going to publish such a paper? And the result will be lots of what the forest pathology community largely detests: name changes.
2. Second, there is a lot of wiggle-room for making exceptions to the priority rule in (1a) above.  Progress thus far suggests that such exceptions will be very common.  Criteria for exceptions include minimizing the number of new combinations (name changes) required, which genus is used more in the literature, whether there are recent monographs (thorough studies) of one genus or the other, preferences of user communities (like plant pathology), etc. Mycological wars are and will be fought over the selection of important genera.

Mycologists will spend many, many years working this through!

“You must follow the rules for naming fungi and diseases. I’ll be watching!”

“Fungus” is singular; “fungi” is plural. Don’t confuse them!