Rust

Alexanders Rust (Puccinia smyrnii)

In 1872, mathematician Augustus de Morgan wrote, in his poem "Siphonaptera":

Great fleas have little fleas upon their backs to bite 'em,  

And little fleas have lesser fleas, and so ad infinitum.

And so it is with plants too; even our robust Alexanders plants don't escape. Many carry patches of the fungus Alexanders Rust (Puccinia smyrnii), a species found only on Alexanders. This rust, which can vary from yellow to bright orange in colour, attacks both the upper and lower surfaces of leaves, as well as the plant's stem, creating large pustules and extensive swelling. As it's specific to Alexanders, it's restricted to those parts of Britain (plus Europe and North Africa) where the plant occurs. In the UK, that means it's most prevalent along the coasts of England and Wales, though it's spreading inland along with its host plant.