The Highland Biological Recording Group

Species of the month, March 2010: Bramble Rust Fungi Phragmidium violaceum and P. bulbosum
Bramble Rubus fruticosus is abundant in Highland, and occurs throughout the area. It is affected by two species of rust fungus, the common and widespread Phragmidium violaceum (a serious pest of cultivated Blackberries) and the much rarer P. bulbosum. Indeed, it was only in February this year that we knew the latter occurred in Highland, when it was found at Cromarty. The 'big win' from this exercise will be finding more of the rarer species, but there are many gaps in the current map of the other and all records will be valuable. While it is easy to find the rusts, distinguishing the two species is a bit more difficult than our previous Species of the Month. Affected Bramble leaves are easily recognised by the red blotches on the upper surface and corresponding black fuzzy spore-making structures below. A quick check with a microscope is necessary to recognise the species. The difference is easily seen with a low-power microscope (x40) if a drop of water is placed on the underside of the leaf to make the black spores translucent. P. violaceum spores have mostly 4 cells, while those of P. bulbosum have 5 or 6 cells (see diagrams below). If you do not have the kit or the confidence to identify them yourself, leaves may be brought to the spring meeting on 27 March, when someone will be able to help, or passed to one of the committee. If you are totally confident of the species, please report it through the 'Submitting records' page where there is a link to submit records on line, but if you think you have the rarer species please keep a sample for confirmation. The two rusts have different hosts. P. bulbosum is associated with Dewberry (Rubus caesius), which is almost absent from Highland (BSBI map), and one section (Corylifolii) of the difficult complex of Bramble microspecies (BSBI map). P. violaceum is found only on the the other forms of Bramble. It is not necessary to explore the murky depths of Bramble taxonomy (if you have a strong constitution you can get a taster here, .pdf, 229kB), but for those who want to learn more about rust fungi the classic work by Grove is downloadable here (.pdf, 25Mb, so broadband only).
	Tell-tale red marks on the upper surface of the Bramble leaf.
	Corresponding black spores masses on the underside of the leaf. Photos © Murdo Macdonald.	By the end of the month the maps look like this. Many gaps in the distribution of the commoner P. violaceum gave been filled (LH map), but more importantly P. bulbosum, previously not known from Highland, has been found in several places on both E and W coasts (original sites blue, new ones green). There are many holes yet to fill, so please keep looking.
	Spores magnified: P. violaceum (left) is dumpy, usually with 4 cells; P. bulbosum (right) is more slender, usually with 5 or 6 cells. From Grove (1913) The British Rust Fungi. CUP.