Field of Science

Reference Review: Messing about with Mildews

Before I start, a reminder that I'll be putting up the Boneyard tomorrow evening, so get any posts for it in quick. Don't forget that Saturday comes earlier for us antipodeans than it does for you European and North American sorts!

Hosagoudar, V. B. 2003. Armatellaceae, a new family segregated from the Meliolaceae. Sydowia 55: 162-167.

It seems that this is Fungal Week here at Catalogue of Organisms - I've barely mentioned them in the past, and suddenly two posts on fungi in rapid succession. Not that I'm complaining - fungi are one of my favourite groups of organisms, and few things are more exciting than coming across some bizarre-looking fungus growing from a rotting log in some damp patch of forest. But as with Wednesday's post, today's subjects come from the less obvious but far more numerous sector of fungal diversity.

Black or dark mildews are parasitic fungi found on plants, particularly the leaves. There are a number of largely unrelated families of ascomycetous fungi that cause black mildew (the picture above from here shows a leaf infected by Apiosporum salicinum - I haven't been able to establish if Apiosporium is closely related to the specific family I'm dealing with today, but the general appearance is probably similar). Though parasitic on a number of food species, none of the black mildews is significant enough to have attracted a huge amount of research attention (reading between the lines, I suspect that they are also somewhat overlooked because they are more significant in the tropics than in temperate developed countries). According to Hosagoudar (2003), their growth on leaves raises the temperature in the affected area, increasing respiration and transpiration rates and reducing photosynthetic efficiency and therefore growth.

The greater part of Hosagoudar (2003) is taken up by a whirlwind tour of the taxonomic history of the Meliolaceae, one of the families of black mildews. At the time of Hosagoudar's writing, Meliolaceae was the only family in the order Meliolales, distinguished by the unique combination of features of an ectophytic (living on the surface of leaves) mycelium with lateral appresoria (swollen points on the hyphae that press against the leaf and give rive to hyphae piercing the leaf surface) and phialides (hyphal cells producing successive spherical asexual spores in chains). At the end of the paper, almost as an afterthought, Hosagoudar establishes the family Armatellaceae for a single genus, Armatella, previously included in Meliolaceae, that lacks phialides and also differs from Meliolaceae proper in having 1-septate ascospores as opposed to 3- to 4-septate ascospores.

I have rather a problem with this sort of setup. Armatella is separated from the other Meliolaceae solely on typological grounds, without any sort of detailed analysis to establish whether the remaining Meliolaceae are truly more closely related to each other than to Armatella. The most recent Outline of Ascomycota (Eriksson, 2006) accepts Armatellaceae in Meliolales, but the Notes on ascomycete systematics that first recorded Hosagoudar's publication (Eriksson, 2005) had a much more cautious reaction, noting that another genus, Diporotheca, had previously been isolated in its own family from Meliolaceae on the basis of lacking phialides. While Hosagoudar (2003) did mention Diporotheca in his taxonomic overview, no comparison of Armatella to Diporotheca was recorded. It is worth noting that in a later paper that Hosagoudar himself is an author on, Armatella has managed to quietly reinsert itself back into Meliolaceae (Biju et al., 2005)*.

*Two other possibilities must be acknowledged here, though: (A) Hosagoudar is not primary author on the latter paper, and it may be that the chosen classification represents the views of the primary author and not those of Hosagoudar, and (B) the time difference between 2003 and 2005 is small enough that Hosagoudar's contribution to the 2005 paper may have actually occurred before he wrote the 2003 paper, with a delay in the appearance of the 2005 paper at either the collation or publication stage.

My even bigger issue, however, is to ask what exactly is the point of establishing a monogeneric family. The concept of 'ranking' is, in my opinion, one of the biggest issues in classification today, and I currently have something of a hate-hate relationship with ranks. It is a widely-known secret that all taxonomic ranks (with the probable, but controversial, exception of the 'species') are essentially arbitrary concepts, and there is no real reason why a given taxon should be recognised as an order or a family or whatever beyond how it sits in relation to other related taxa that have previously been recognised as orders or families or whatever. Different historical factors in research on different groups of organisms mean that a family of insects is in no way a comparable unit to a family of birds or plants or fungi. I personally try to avoid referring a taxon to a specific rank, at least in the privacy of my own head. The problem that really makes me grit my teeth, however, is that when it comes to trying to discuss biodiversity to other people, ranks prove so irritatingly convenient! Most people who don't have to deal with the details of classification every day find it relatively easy to grasp the concept that each rank corresponds to a certain level of superficial distinction (at least from our own human-centric viewpoint), and that a genus represents a smaller degree of distinction than a family, which is in turn less distinct than an order. Also, try as I might, there's only so many times I can use a variation on "clade" or "group" without becoming repetitive, confusing or both (and besides, I usually end up having to refer to "clade A" and "subclade B", invoking an even more arbitrary sort of ranking to indicate that B is a section of A, even though there's no actual difference between "clade" and "subclade" and, were I to change my focus slightly, I might end up referring to "clade B" and "subclade C").

However, given that where and what an individual author chooses to recognise as a given rank is essentially subjective, what does separating a genus into its own family really tell us? The prior establishment of the genus already tells us that it is a distinctive unit. There is a certain virtue to establishing a different concept of the taxon "Meliolaceae" from the taxon "Meliolales", rather than the previous set-up where there were two names for the exact same thing, but in establishing the taxon "Armatellaceae" to contain only "Armatella", we again have two names for the exact same thing, and that's just cluttering up the nomenclature.

Postscript: Unless I head them off at the pass now, it is entirely likely that someone will weigh in on the comments with the PhyloCode argument (I'm looking at you, Mike)*. Someday I'm going to be forced to actually say something on the whole PhyloCode question, on which I am an inveterate fence-sitter. For now, let it suffice to say that I'm not convinced that introduction of the PhyloCode principles would particularly improve matters in corners of phylospace such as this one where the vast majority of taxa still have not been phylogenetically investigated to a significant degree, and while yes, PhyloCode may stabilise taxon definition, taxon content here would probably continue to leap about like a drunken grasshopper.

*Some of you may know Mike Keesey as the author of the Dinosauricon, which was one of the first major web resources on Dinosauria, and I came to know his by-line well back in my DML days. The link above takes you to his brand-spanking new blog, so take a look!


Biju, C. K., V. B. Hosagoudar & T. K. Abraham. 2005. Meliolaceae of Kerala, India - XV. Nova Hedwigia 80 (3-4): 465-502.

Eriksson, O. E. (ed.) 2005. Notes on ascomycete systematics. Nos 3912-4298. Myconet 11: 115-170.

Eriksson, O. E. (ed.) 2006. Outline of Ascomycota - 2006. Myconet 12: 1-82.


  1. Thanks for the plug! Now I'll have to oblige with that PhyloCode comment (seeing as I am the moderator on its forum as well as its website's developer).

    The PhyloCode does favor stability of definition over stability of content. But, as others have pointed out, what's so great about stability of content? Would we really want to be saddled with a Reptilia that includes sharks and frogs, a Primates that includes bats, or a Homo that includes chimpanzees? These were all Linnaeus's original usages, after all. Content changecan be good--if it reflects changes in our understanding of relationships, not changes in subjective opinions.

    But I'm not completely dismissing your concern. There are examples of phylogenetic definitions out there that, under some hypotheses, would change things in an unproductive way. Fortunately, the latest draft of the PhyloCode features some fairly substantial additions to Art. 15 with this kind of stability in mind. In short, it specifies how, under certain circumstances, authors may emend preexisting definitions that have turned out to have unfortunate consequences.

    Also worth mentioning: the rank-based codes favor stability of definition over stability of content, too! They have very little to say about content, really. Include the type and whatever else you want. That hasn't exactly made for a lot of stability over the past few centuries. With the PhyloCode, though, content is dictated by phylogeny, so as our understanding of that stabilizes, the taxonomy will follow.

    And it seems to me that ranks (again, possibly excluding species) are a terrible indicator of biodiversity. Look at the orders Struthioniformes and Ornithischia, or your average vertebrate genus and your average insect genus. Surely telling laypeople that these are equivalent units is not doing them any service.

    Well, at any rate, hopefully I'll be able to provide a stronger argument when my Names on NEXUS utility (mentioned here) is available.

  2. Whoops, one other comment:

    I usually end up having to refer to "clade A" and "subclade B", invoking an even more arbitrary sort of ranking

    Those aren't ranks, any more than "set" and "subset" are ranks (or "group" and "subgroup", "taxon" and "subtaxon", etc.).

  3. Those aren't ranks, any more than "set" and "subset" are ranks (or "group" and "subgroup", "taxon" and "subtaxon", etc.).

    Technically not, yes, but in my experience the way words like "subclade" are structured tends to lead people who aren't familiar with the terms to mistake them for some sort of formal ranking. I have seen comments like "I'm not sure how to tell if this is a clade or a subclade". Unfortunately, there's no way around this misunderstanding except further education.

  4. Ha, that's kind of funny. (Or maybe it's just me.)


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