Wednesday, June 12, 2013

Blame it on the rain (Part II): Reprise of the slime molds

An aerial view of a grove of strange trees? Read on.
With all this wet weather we can expect to see a lot of plant diseases in the upcoming weeks. Oddly enough, damage due to root rots won't be evident on trees and shrubs until the weather turns hot and dry. That's when the damaged root systems won't be able to keep up with the demand for water in the foliage. All this is a story for another day, however. Today I want to revisit a group that was a blog topic two years ago: the slime molds. (See our archived post.) These are sometimes called myxomycetes, but don't let the Greek suffix fool you; they are not related to true fungi.

Stemonitis sp. (Photo: Matt Bertone)
We had several inquiries about slime molds during the last week of May. Fortunately they do no harm to plants, animals, or people, though some folks have been shown to be allergic to the spores. One inquiry was accompanied by a spectacular photo - not reproduced here - of Diachea leucopodia on grass blades. Another was the specimen of Stemonitis - probably Stemonitis fusca - pictured at right. It had been picked up off of a living room baseboard. This particular slime mold is usually associated with wood, sometimes indoors. The photo at the top of this blog was taken on a stump in Schenck Forest in Raleigh several years ago. In both pictures you can clearly see the thin "stipe" or stalk that supports each cylindrical spore-bearing "sporangium".

Remnants of slime mold fruiting on Japanese holly
A third specimen from that same week was submitted by one of our campus landscapers. She was concerned about white material on the foliage of a Japanese holly. This turned out to be another slime mold. Most of the sporangia had broken off, but a the base (called the hypothallus) was still stuck to the leaves. This particular case is covered in the "Geeks Corner" below.

Then yesterday while out looking for leaf spots on maples, I noticed some whitish gray tufts in a mulched bed in the shade of several trees. It turned out to be yet another slime mold, Acyria cinerea. The tiny sporangia are only about a millimeter-and-a-half tall, not counting the stipe. If you look closely, you can see that they are often in clusters.Within these sporangia, the white spores are held by a network of spiny threads called a capillitium. The characteristics of the capillitium are important in slime mold identification.
Capillitium and spores of Acyria cinerea at 400x.
Fruiting of Acyria cinerea on mulch
What we are seeing in all these cases is the end-game of the slime mold's life cycle. If the wind-dispersed spores land and find the right conditions they'll germinate into ameoba-like cells that will eventually become a multicellular, macroscopic, living film called a plasmodium. This plasmodium will slowly "crawl" or ooze around on wood or bark, in leaf litter, on dung, or on other materials. They almost always escape notice at this stage, during which they "feed" by engulfing bacteria, fungal spores, and probably  bits organic matter. Eventually the plasmodium switches gears and changes over to spore production, treating us to one of nature's great works of art. The Georgia Museum of Natural History has posted a gallery of myxomycete images by photographer Ray Simons.

Fruiting of Fuligo septica
Not every member of this group is quite as lovely. Fuligo septica, the "dog-vomit" slime mold, is gross in both the German sense of "large" and the English sense of "repulsive". It is very common on hardwood mulch and was highlighted in our June 14, 2011 blog. When the plasmodium first comes to the top of the mulch and before becoming a crust-covered mass of spores, it's basically a bright yellow froth. A picture of a more mature fruiting is shown at left. Up close, they don't look so bad, actually.

Sporangia of what is probably Physarum leucopus.
The width of this photograph covers only a quarter inch.
Geeks Corner. The slime mold pictured at right baffled me for a while. As a non-expert in this group, I was having trouble deciding whether there was lime in the capillitium or not. There are clearly lime deposits on the peridium (outer covering), giving it a frostly look. Using Martin & Alexopoulos's 1969 tome The Myxomycetes, I eventually keyed it out to Physarum leucopus. The reference states, "Before the peridium breaks, this species is remarkably similar in appearance to Didymium squamulosum." The key distinctions: white nodes in the capillitium, the lack of a columella, and the fact that the lime on the peridium exists as granules rather than crystals.

For anyone interested in learning more, I highly recommend starting with Stephenson & Stempen's book Myxomycetes: A Handbook of Slime Molds (1994. Timber Press).