Factors affecting the local distribution of Polystigma rubrum stromata on Prunus spinosa

Hattie R Roberts,Gareth W Griffith,Sara E Pidcock,Sky C Redhead,Emily Richards,Kevin O’Shaughnessy,Brian Douglas

doi:10.5091/plecevo.2018.1442

Hattie R Roberts, Gareth W Griffith + Show 5 more

Open Access

https://doi.org/10.5091/plecevo.2018.1442

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Abstract

Background and aims – Polystigma rubrum forms orange-red stromata on the surface of living leaves of Prunus spinosa and P. domestica. Records suggests that this fungus now has a much more limited distribution in Britain than recorded in the 19th and early 20th century. Methods – We studied the local distribution of the fungus in the Burren Hills of western Ireland where it remains very common. Key results – Assessment of the local distribution of the fungus over two years found stromata to occur more frequently on P. spinosa leaves in hedgerows than woodlands. On individual trees in areas of open limestone pavement, the frequency of stromata was ten times higher in 2016 than 2015, possibly related to interannual rainfall differences. On hedgerow trees subjected to winter flooding, stromata were much less abundant, whereas stromata were more abundant on leaves also infected by the gall mite Eriophyes prunispinosae. The identity of Po. rubrum was confirmed by ITS sequencing.Conclusion – At a field location where Po. rubrum stromata are present in unusually high abundance, the distribution of stromata on trees in different habitats showed high levels of variation linked to both habitat and the presence of gall mites. Further work is required to determine whether variation in leaf surface and soil moisture are the key determinants of the observed distribution. Such investigations may reveal why Po. rubrum, once common in northern Europe is now restricted mainly to westerly, coastal locations.

Highlights

Polystigma rubrum (Pers.) DC., colloquially known as the “red spot disease” or “blackthorn dotty”, is a distinctive parasitic ascomycete found on living leaves of Prunus spinosa and Prunus domestica (Cannon 1996)
Rubrum was present on most surveyed P. spinosa trees in the Burren, with stromata abundant
We have shown that Po. rubrum stromata are very common in the high rainfall habitats of the Burren and that the localised distribution of these stromata varies according to the habitat of the host trees, which in turn may be linked to localised variation in the dynamics of ascospore formation on overwintered leaves below these trees

Summary

Introduction

Polystigma rubrum (Pers.) DC., colloquially known as the “red spot disease” or “blackthorn dotty”, is a distinctive parasitic ascomycete found on living leaves of Prunus spinosa and Prunus domestica (Cannon 1996). Infection does not cause necrosis but rather bright orange localised discolouration and swelling (Fig.). There is no evidence for systemic infection suggesting that reinfection each year is mediated via ascospores formed on dead fallen leaves (Blackman & Welsford 1912). The sexual stage of the fungus is characterised by the same stromata turning black on the fallen overwintering leaves and producing immersed ascomata and ascospores, Polystigma rubrum forms orange-red stromata on the surface of living leaves of Prunus spinosa and P. domestica. Records suggests that this fungus has a much more limited distribution in Britain than recorded in the 19th and early 20th Century

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