Sphaerobolus stellatus – The Tasmanian
by Ted Cutlan and Joy Stones October 2002
(Our Society is pleased to hear from our far flung antipodean members from Tasmania. Their theme is timely, given the current
threat of malevolent fungi moving up the Pacific coast)
Sphaerobolus stellatus from the Greek sphaero, meaning sphere, bolus – to throw and stellatus – star like.
Most people have never heard of it and we can honestly say that we wish we were still blissfully ignorant. Unfortunately that is not the case and things have had to change significantly in our nursery since we learned what we were dealing with.
Sphaerobolus stellatus is a fungus, which grows on decaying wood and is seen in the USA most commonly in mulched gardens where there is a high wood to bark ratio in garden mulches. It is spread through the recycling of organic material. It grows like many other fungi with no harmful effects to the plants around it, and in spring and autumn, (or when the conditions are right) it forms a fruiting body on the surface of the mulch or potting soil. This is a 2-3 mm diameter sphere, which when ready ejects a spore parcel (peridiole) with enough force to spit it 2-3 metres away. The peridiole itself is around 1 mm in diameter and sticks to whatever it lands on. This is of course how it is spread when it lands on a suitable surface and colonises the medium.
Since we first identified this fungus, checks around the state of Tasmania have found it to be widespread, and in many nurseries. Growers have mistaken the spores for a scale insect, and indeed, we at first thought that it was insect excrement. Nurseries were often unaware of the presence of the dots on their plants.
In our nursery a number of factors have combined to cause it to become more widespread, and some plants have so many dots on them that for purely aesthetic reasons we have decided that they are not of a suitable standard to sell.
The reasons for the rapid spread of the fungus in our nursery are many.
1. We have a lot of special hybrids and species and in our attempts to increase the numbers of these and the more popular varieties, we have taken cuttings off the plants as they grow in pots. These have themselves carried spores, and the resultant plants then go into the alphabetical arrangement of our stock along with every other ‘clean’plant and set about continuing the infection of the mix in which they are potted.
2. We recycle our pots as we move plants from a 3” to a 6” pot and then from a 6” to an 8” pot for eventual sale. These pots may have spores stuck to the inside lip if the previous plant in that pot was infected, and even if we were to sterilize them (a huge task for the 10,000 pots involved), we are not confident that the bleach we use would be effective. The peridiole is a hard, amber-like substance protecting the spore beneath.
3. The fungus enjoys light and moisture. We grow all our rhododendrons in full sun to get a compact, well-branched, budded plant. (Until now we have taken great pride in the quality of the plants we grow.).
4. We also have our watering system programmed to come on briefly three times in the heat of the afternoon during summer, to cool the leaves. (Trying to simulate the moisture and rains of the monsoonal areas from their native habitats). The rhodos love it, but unfortunately so does the fungus.
In short we realize now that we have done everything possible to aid the spread of the fungus. You get used to this sort of thing as a horticulturalist!
But all is not lost!!!! We should point out at this stage that we are not aware of any registered chemical controls, and the fungus is one of a group of fungi known as gasteromycetes, which do not respond to fungicides. Having said this we are confident that a satisfactory level of control using changed cultural practices will be reached in just two years, and total eradication within the nursery is possible.
To control the spread, all newly potted plants must be put in an area well away from infected plants, and those that are infected are mulched with an inorganic material like a coarse sand to exclude the light and smother the fruiting bodies. Heh! That’s only 16,000 plants.
When repotting the infected plants next spring, we are going to use pots with drainage holes only in the bottom. Most drainage holes extend up the sides of pots and we have seen the fruiting bodies on these areas of exposed potting soil. This of course means that we throw out 10,000 pots which we would otherwise have reused at no additional cost.
Now, having referred to the fungus thus far as Sphaerobolus stellatus we should point out that this is the name given by a visiting expert from the Royal Sydney Botanic Gardens and until this time no one we had contacted had any idea what the black dots were. We were extremely grateful to find someone who knew what we were dealing with.
There are however some significant differences between the fungus we have seen and the information we have gained from research on Sphaerobolus stellatus using the Internet. Firstly the fruiting body of Sphaerobolus stellatus is supposed to be 2-3 mm in diameter and, the fruiting bodies we have seen are much smaller. Secondly when we spoke to an expert on Sphaerobolus stellatus in the US we were told that despite it being widespread in mulches in the US it had never been seen in nurseries there. The other observation that we have made is that although we have planted many affected plants into our own large garden mulched with pine bark, we have not seen any evidence of it colonizing the mulch as in the US. Is it possible then that this ‘cannon’ fungus is closely related to, but not actually Sphaerobolus stellatus?
The fungus would seem to be most active in our nursery in freshly potted stock which suggests that maybe there needs to be adequate levels of fertilizer to break down the wood particles. After a year or so in the same pot the activity of the fungus seems to slow down. Is this because the wood particles in the potting mix are now composted, or is it that there is not sufficient fertilizer to continue the composting and decomposition of the wood? There are still many questions to be answered.
As an aside, while we have found this to be a particularly troublesome problem in our nursery it would seem there are strange people out there who collect stamps featuring fungi. We have just ordered a stamp from the African country of Sierra Leone featuring the fungus Sphaerobolus stellatus. We know absolutely nothing of Sierra Leone, but this is one fact we will not forget in a hurry.