Nettle pox

The caterpillar-like structure on the stem of this nettle plant is a gall caused by the nettle rust fungus Puccinia urticata.


According to Prof. Google, all rusts (there are in excess of 7000 species!) are obligate parasites, meaning that they require a living host to complete their life cycle. They generally do not kill the host plant but can severely reduce growth 

Rusts have exceedingly complex life cycles and can produce up to five spore types from corresponding fruiting body types during their life cycle, depending on the species. Roman numerals have traditionally been used to refer to these morphological types.

0-Pycniospores (Spermatia) from Pycnidia. These serve mainly as haploid gametes in heterothallic rusts.
I-Aeciospores from Aecia. These serve mainly as non-repeating, dikaryotic, asexual spores, and go on to infect the primary host.
II-Urediniospores from Uredia (Uredinia). These serve as repeating dikaryotic vegetative spores. These spores are referred to as the repeating stage because they can cause auto-infection on the primary host, re-infecting the same host from which the spores were produced. They are often profuse, red/orange, and a prominent sign of rust disease.
III-Teliospores from Telia. These dikaryotic spores are often the survival/overwintering stage of life cycle. Later they germinate to produce basidia.

IV-Basidiospores from Teliospores. These haploid spores often infect the alternate host in Spring.

The complexity of life never ceases to impress and to amaze.