We had some friends around for dinner last Saturday night to watch the election results. It was sad to watch as New Zealand wandered aimlessly to the centre right all because Auckland couldn’t buy a flat white during Covid lockdown. Or that too cynical.
Anyway, my friends asked me did I know about the artillery or cannonball fungus? I admitted that I had read about it but didn’t know it personally. It seems that their house was covered in it. Not the fungus itself but its projectile spore masses. I went and had a look and the outer wall of their white painted house, and the balustrade around their deck was densely polka-dotted with black spots. And about the highest spore mass we found was 3.2m above ground.
Only one side of the house was affected, and this abutted a well mulched garden and pathway. The garden had been mulched with the chipped and shredded leaves, twigs and small branches of native hardwood species. It would seem that this mulched material had been colonised by the wood decay fungus Sphaerobolus sp.
The genus name is derived from Greek and means sphere or ball thrower, hence the common names of artillery or cannonball fungus. According to Wikipedia the projectile spore masses are “sticky and have a tendency to strongly adhere to whatever surface they encounter, making them a nuisance to homeowners, pressure washing contractors, landscape mulch producers and insurance companies”. This is not a problem that I had ever heard of in Aotearoa but that might just be because it hasn’t been recognised.
As already noted, Sphaerobolus species colonise and reproduce on decaying wood. The fruitbodies are small and inconspicuous unless en masse. Each fruitbody is about 1-3mm in diameter and begins forming beneath the bark or epidermis of the branch or twig it is decaying. At first it just appears as a lump, but soon breaks through to the surface. The exposed ball-shaped fruitbody slits open forming a star-like mouth. At this stage if you could cut a section through the little fruitbody you would see it is composed of multiple layers. At its centre is where the spores are produced and when mature this mass of spores is called the peridiolum. Surrounding the peridiolum is a layer that will become a stick liquid.
The two important layers of the splitting fruitbody wall are quite remarkable. The outer wall is inflexible and does not absorb water. In contrast the inner wall undergoes an internal chemical change after splitting, developing internally a high concentration of glucose which is very water aborning. As water is absorb the internal pressure of this layer grows until suddenly it goes from concave to convex in shape. This sudden change in shape shoots the peridiolum up and out into the air. Peridiole have been measured travelling up to 6m from their projecting fruitbody. [See fruitbody development here.]
The Sphaerobolus fruitbody is also phototrophic in that it can sense light and shoots it’s peridiole towards the brightest source of light. Why do this? The fungus shoots its spores as far away from itself that it can to find new wood to decompose. Normally the fungus is growing on the ground under shrubs and trees so, just shooting the peridioles straight up into the air would result in most of them getting stuck on leaves immediately above the fruitbody. By aiming at the brightest source of light its more likely the peridiole will pass through a gap in the plant canopy and travel further. When it gets to its destination the sticky surface of the peridium anchors it down and the spores will be released as the outer layer breaks down.
This is absolutely amazing biology. However, it has a downside in the human world. If Sphaerobolus colonises wood mulch in a home garden the likeliest source of bright light will be sunlight reflected of a house particularly a white painted house. So, many of the peridiole that are produced and launched will end up sticking to the house’s walls. This is what has happened to my friends. I also understand that once the peridiolum has adhered and dried on a painted surface it can be very difficult to remove although elbow grease and sugar soap might be worth a try.
References
Smith WG, Gwynne-Vaughan HCI, Barnes B 1927. The Structure and Development of the Fungi. Cambridge at the University Press. p. 315, figure 282.
Sphaerobolus https://en.wikipedia.org/wiki/Sphaerobolus
Sphaerobolus stellatus fruitbody development. italianmyxomycetes https://www.youtube.com/watch?v=0_nQdKrL4dE
Spores, moulds, and fungi 