Blisters in the dry

As spring has indiced some rapid plant growth, I was doing some weeding in our small gravel/alpine garden, including pulling a few unwanted hairy bitter-cress Cardamine hirsuta that were bullying my less-aggressive plants. During this, I noticed some white blisterlike structures on the underside of many of the leaves - not leaf-mines, but something more fungal. A quick look in Ellis & Ellis (1997) told me it was, as I suspected, the 'brassica white rust' Albugo candida.

Albugo candida on Cardamine hirsuta

Close-up of A. candida on C. hirsuta - the widest part of the leaf here = 6.5mm. The leaf margin also shows some small bristles - despite the common and scientific names, C. hirsuta isn't very hairy, or hirsuite.

C. hirsuta is very similar to wavy bitter-cress C. flexuosa but the flowers only have 4 stamens (6 in C. flexuosa) and C. hirsuta tends to be found in drier locations. In this case the plant identification isn't hugely important as A. candida is found on a range of Brassicaceae, but many microfungi have a more specialised and narrow relationship with their host plant/s so (as with galls and leaf miners) it is usually essential to identify the plant.

C. hirsuta flower with 4 stamens.

Although A. candida is generally called a fungus or microfungus, technically it is a fungus-like 'water mould', one of a group of plant pathogens in the class Oomycetes of the phylum Heterokontophyta within the kingdom Chromalveolata (i.e. not the kingdom Fungi). The taxonomy is still a matter of debate (there are several competing versions and research is ongoing) but they are more closely related to photosynthetic organisms such as brown algae and diatoms. The class also includes some serious plant diseases such as late potato blight (Phytophthora infestans, cause - socio-economic factors aside - of the Irish potato famine of the 1840s) and sudden oak death (P. ramorum).

Reference

Ellis, M.B. & Ellis, J.P. (1997). Microfungi on Land Plants: An Identification Handbook (2nd ed.). Richmond, Slough.