Infection of Arum Maculatum by the rust Puccinia sessilis at Carmel NNR, South Wales

Abstract

rum maculatum is probably one of the most familiar plants of our native flora. It adorns our hedgerows and base-rich woodlands in spring and summer, the characteristic fruits lasting well into the autumn. Its common name is Lordsand-Ladies, although Prime (1960) records over a hundred English names for this plant throughout Britain, together with detailed information on its folklore, variation and ecology. It is very common throughout England and Wales, and according to Prime is frost sensitive. As a consequence its frequency declines northwards and it is local in Scotland (www.bsbimaps.org.uk/atlas/main.php). It is an extremely variable plant with differences recorded between individuals and between populations. Prime records differences in the floral structure, leaf shape, venation and pigmentation, but the most striking visible difference is the presence in some plants of dark purple-black markings on the leaves (hence the specific epithet “maculatum”, which means “spotted”) whilst other plants are unspotted. The purple-spotted plants are very conspicuous (Fig. 1) and the dark blotches are usually irregular in shape and vary in size from small dots of a few millimetres to a few centimetres in diameter. This pigmentation is confined to the central layers of the leaf and was once thought to be the result of a virus infection but is now known to be due to anthocyanin production. Local and national floras have not generally formally distinguished these forms, though Sell & Murrell (1996), while not accepting the varieties as part of the formal description of taxa, do give varietal names for plants with spots (var. maculatum) and those without (var. immaculatum) in their discussion. Prime carried out a comprehensive study across Britain in the 1950s and he showed in general that spotted plants became less common towards the north and east of Britain. Puccinia sessilis is a rust fungus. Many rusts have life cycles which involve the production of up to five different spore stages and the participation of two different host plants. P. sessilis exhibits this type of life cycle. Two spore stages are produced on Arum and the other three spore stages are produced on the alternate host Phalaris arundinacea (Reed Canary-grass). Arum is infected by the rust in the early spring and this results in the formation of discrete orangeyellow pustules on the upper surface of the leaf (Fig. 2). Within these pustules minute spherical bodies called pycnia are produced which are the first spore stage of the rust. After about 10–14 days these develop into the next spore stage, which is produced on the underside of the leaf. These structures (called aecia) look like little coronets (Fig. 3) and produce aeciospores, which in turn infect another plant from a different family (the alternate host), which in this case is Phalaris arundinacea. Infection of Phalaris occurs during the summer months, and two different spore types are subsequently produced. The first of these (urediniospores) are produced continuously during the summer and are responsible for spreading the disease throughout the stand of Phalaris. During late summer/autumn yet another type of spore (teliospore) is produced which is the overwintering stage of the rust. The following spring these teliospores themselves germinate to produce the final spore stage in the life cycle (the basidiospore), and it is this spore stage that reinfects Arum in the spring thus completing the life cycle of the rust (Wilson & Henderson 1966). For the rust to survive, therefore, both host plants must be present to allow cross infection to take place to enable the rust to complete its life cycle.