Puccinia libanotidis (moon carrot rust)

Underside of moon carrot leaves in early June showing elongated scab-like rusty-brown aecia, the first conspicuous spore stage of moon carrot rust of the year (Image: © Dr A. Martyn Ainsworth)

Puccinia libanotidis is restricted to the range of moon carrot, Seseli libanotis, and its closest relatives, and is widespread across northern and central Eurasia from England and Scandinavia to Siberia and Japan. In Britain, however, moon carrot is restricted to just a few sites on chalk soils on the northern Chilterns (including the Gog Magog Hills near Cambridge) and the South Downs. There are only two known historical records of its associated rust. One was from a plant gathered in Sussex in 1910 and subsequently examined in the RBG Edinburgh Herbarium, and the other was on moon carrot leaves collected near Cambridge in 1946 and is now preserved at Kew. The fungus was regarded as extinct in Britain until it was rediscovered after 63 years, following a few minutes' search in May 2009, at its former station near Cambridge. Subsequent searches revealed that a Chiltern moon carrot population was apparently rust-free whereas two South Downs coastal populations in the Seaford to Beachy Head Site of Special Scientific Interest area showed widespread rust presence.

Underside of yellowing and dying lower leaf of moon carrot in early August showing a few blackish telia (the third and final conspicuous spore stage of  moon carrot rust) amongst the browner, almost exhausted, uredinial stage. (Image: © Dr A. Martyn Ainsworth)

Of the five spore stages produced by moon carrot rust, the first to become conspicuous (the uredinioid aecial stage) is the most noticeable. This comprises clusters of brownish spores which burst through the surface of the host plant in an orange-brownish scab-like sorus. This can be elongated and extend for 0.5-3.0 cm along petioles and the underside of leaf veins causing some distortion of, and hence gall formation on, the host plant. The spores (aeciospores) are spiny (echinulate) and mostly within the range 25-35 x 20-25 µm. Smaller (around 0.1 mm across) submerged structures surrounding the aecia constitute the yellowish pycnial spore stage. This produces minute pycniospores, measuring around 3.5 x 2.5 µm, and a resinous fluid known as ‘pycnial nectar’.

Towards the end of May, these two early-season spore stages are joined by the uredinial stage. This spore stage then becomes dominant as the undersides of apparently healthy leaves become speckled with small cinnamon-tinted uredinia measuring about 1 mm in diameter. Under a microscope the uredospores exhibit a relatively thick (around 6.5 µm) wall and closely resemble the aeciospores.

In August the lower leaves turn yellow and uredinia are also found, albeit sparsely scattered, on the upper surfaces of leaves and petioles. That is the time to search amongst the denser uredinia beneath the leaves for the telial spore stage. Telia are distinguished from uredinia by their darker, almost black, colour. The associated dark brown teliospores are apically rounded, smooth-walled, slightly constricted at the cross-wall dividing them into two cells, and measure 30-50 x 15-25 µm. They overwinter and germinate to produce basidiospores, the fifth spore stage, which infect new leaves in the following year. 

Underside of moon carrot leaves photographed in situ in mid-July, speckled with cinnamon-brownish uredinia, the second conspicuous spore stage of moon carrot rust of the year (Image: © Dr A. Martyn Ainsworth)

Moon carrot rust is of conservation concern in the UK. It was Red-Listed (unofficially) as Extinct in Great Britain in 2006. Following its rediscovery in England in 2009, it now occupies three known sites (so would qualify for Endangered or Vulnerable status) but further field survey work is required to provide a clearer picture of its current British distribution. Conservation of this fungus is dependent on that of its host plant (assessed as Near Threatened) and its habitat. Further work is required to investigate how many and which moon carrot populations host the extant rust and how the numbers of infected plants might vary naturally from year to year.

Moon carrot rust was rediscovered in three out of four populations of its host plant in 2009 by Kew mycologist Dr A. Martyn Ainsworth (now part-funded at Kew by Natural England) on a Plantlife/Natural England contract working with volunteers and members of Buckinghamshire County Council’s Environment Group.

Moon carrot rust has not been recorded within the Gardens at Kew. However there are dried collections of Puccinia libanotidis from throughout its range preserved in the Kew Mycology Herbarium (Fungarium), which are made available to bona fide researchers by appointment.