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horticultural science Recently introduced diseases of ornamental plants Gardens are being increasingly exposed to new diseases, principally due to globalization of plant trade. Béatrice Henricot surveys the most significant introductions he main reasons for One key question is whether new and infamous examples include increased risks of introducing we are recording more new diseases potato blight in the 19th century and Tpathogens into any country than in the past as a result of these Dutch elm disease in the late 1960s. are globalization of trade and travel, factors. A study carried out by Jones More recently, gardeners have had evolutionary change of plant patho­ & Baker (2007) indicated that the to deal with box blight and new gens, and climate change and in part­ numbers of new or important species of Phytophthora such as icular its effect on insects as disease pathogens estab lishing in the UK P. ramorum and P. kernoviae. vectors (Waage et al. 2007). Global­ during the period 1970–2004 are This article will look at diseases ization is probably the main driver of not increasing. A total of 234 new which have recently been found increased risk of intro ducing plant pathogens were recorded during that in gardens in the UK. Cases mainly pathogens in the future and this has period and were mainly associated come from the RHS advisory been well documented (Brasier 2005, with ornamental plants (as opposed records since 1998 with some Henricot & Gorton 2005, Ingram to horticultural crops, wild native additions taken from the online 2005, Brasier 2008). Pathogen evol­ species, agricult ural crops, pasture journal New Disease Reports (www. ution is a continuous process but has plants, and exotic forestry tree bspp.org.uk/publications/new­ been accelerated by modern plant species). Although the rate of disease­reports). The RHS records trade. Nurseries are now prime establishment appears to be stable, are based on samples received from grounds for species mixing and the pressure from newly intro duced UK gardens and can be considered exposure to new hosts. On the other pathogens is increasing. as good barometers of successful hand, climate change may have mixed The problem of epidemics caused establishment of ornamental effects on disease establishment. by introduced plant pathogens is not plant diseases. Photographs: Horticultural Science, RHS Science, Horticultural Photographs: Blight and leaf spot Cylindrocladium buxicola on Buxus (left) and Elsinoë mattiroloanum on Arbutus (right) 216 December 2009 PlantsmanThe cases were recorded in 2007 and the disease is on the increase. It has been recorded on established Escallonia in gardens but also on nursery stock. Two fungi are associated with the leaf spots, a Septoria species and a Cercospora species, which have not yet been identified to species. It is not clear where the diseases came from, although fungi in both genera have been recorded on Escallonia in South America. Both fungi cause purplish brown leaf spots and the young shoots suffer from dieback. Powdery mildews Leaf spot An unidentified Septoria species on Escallonia, the first case in the UK was recorded in 2007 Powdery mildews are ubiquitous plant pathogens and can be recogniz­ Leaf spot and blight fungi Although affecting a much wider ed as a white coating on leaf surfaces Leaf spotting fungi are the most range of host of plants so far, it has and other aerial parts. Some species numerous introduced pathogens caused little problem for gardeners. are regarded as invasive because they (Jones & Baker 2007). Most cause Symptoms caused by Elsinoë spread rapidly from their point of minor damage on plants but a few mattiroloanum on Arbutus were first introduction. Early examples of diseases caused by these fungi have recorded in 1998 through RHS invasive powdery mildews are recently caused concern among samples, but the first case in the UK Erysiphe necator, grape powdery gardeners including box blight, and was recorded in 1978 (Jones & Baker mildew, and Podosphaera mors­uvae, Arbutus and Escallonia leaf spots. 2007). The symptoms are extremely American gooseberry mildew. Both The box blight pathogen Cylindro­ disfiguring, causing purplish­black were introduced to Europe in the cladium buxicola was first reported in spots on the leaves and infected 19th and early 20th centuries from the UK in 1994 (Henricot & Culham leaves drop read ily. Young stems are North America (Kiss 2005). 2002). The fungus causes leaf spot also affected. It is not clear how the More recent cases are Erysiphe and stem dieback and has only been disease came to the UK but the azaleae infecting Rhododendron and recorded on Buxus species. It is a climate seems to be favourable for introduced to Europe in 1981, new fungal species whose origin is its estab lishment as a high number probably originating from North unknown. It was either introduced of trees with symptoms are now America or Asia (Inman et al. 2000); on imported nursery stock or on wild being recorded. E. symphoricarpi, a North American plant material brought in by plant Another species in the same genus, species infecting Symphor icarpos, collectors. Since its introduction to much more recently recorded and recorded in the UK in 1999 (Kiss the UK the fungus has spread rapidly potentially quite disfiguring on its et al. 2002); E. flexuosa on Aesculus through Europe where it causes only host (Quercus ilex), is Elsinoë recorded in the UK in 2001, epidemics in favourable environments quercus­ilicis. The fungus was first probably originating from North and also threatens native Buxus. reported at Royal Botanic Gardens, America (Ing & Spooner 2002); Outside Europe the fungus has only Kew, by Spooner (2009). It is E. elevata a North American species been recorded in New Zealand. widespread on the continent and has infecting Catalpa, recorded in the In the same genus, the fungus been recorded on a few occasions in UK and other European countries Cylindrocladium pauciramosum is also gardens. In Guernsey, Quercus ilex is in 2002 (Cook et al. 2004); and a new appearance in this country particularly affected by this disease. E. deutziae on Deutzia, introduced (Lane et al. 2006a) although unlike Trees are heavily defoliated as a from Asia to mainland Europe and C. buxicola, this species was probably result of infection. then UK in 2006 (Denton & in the country for a significantly Leaf spots on Escallonia are a Henricot 2006). longer period before it was recorded. recent problem in the UK. The first In some cases, new species of ➤ December 2009 217 horticultural science powdery mildew of unknown origin to overlap in gardens, or because the 2009). Early introductions include have arisen in the UK. Examples hosts overlap in the wild. A recent P. infestans and P. cinnamomi, the include two new Erysiphe, one on example of host jumping is the UK former being the cause of potato and various Crassulaceae (Henricot 2008) record of Neoerysiphe galeopsidis on tomato blight. More difficult to and one on Calluna (McQuilken et al. Catalpa in the Bignoniaceae (Cook et diagnose, and less familiar to garden­ 2002), and a new Podosphaera on al. 2006a) and Acanthus in the ers, is P. cinnamomi, a cause of root rot Cuphea (Beales & Cook 2008). Acanthaceae (Cook et al. 2006b). on many ornamentals. Nevertheless, Another process that leads to new Prior to these records, N. galeopsidis P. cinnamomi was also introduced to powdery mildew diseases is host was only found on Lamiaceae. Europe in the 19th century and jumping. This is part of the evolution­ Another example that illustrates this probably originated in the South ary process of powdery mildews and phenomenon is wisteria powdery Pacific. It caused epidemics in UK new hosts have been recently mildew, recorded in the UK for the nurseries in the 1960s and 1970s and recorded. The process could have first time in 1999. DNA analysis infects more than 3,000 hosts been accelerated by human indicated that it is almost identical to (Brasier 2008). According to a RHS intervention causing potential hosts Erysiphe alphitoides, the cause of oak survey it is the third most common powdery mildew (Henricot & Cook Phytophthora recorded in gardens and 2008). Further research involving has been found on at least 34 orna­ cross inoculation assays is currently mental genera, most commonly on supporting this hypothesis. Taxus baccata. Jones & Baker (2007) identified Oomycetes 14 species of Phytophthora introduced Oomycetes are related more closely to the UK between 1970 and 2004. to algae than fungi and include some Another three, P. niederhauserii, of the most important groups of P. quercina and P. tropicalis, can be plant pathogens that cause diseases. added to this list, making a total of Gardeners know them as phytoph­ 17. Among them, 12 species have thora blight, root rot, damping­off, been recorded more than once by downy mildew, and white blister. the RHS survey and may therefore In the last 20 years, numerous be considered as established in new Phytophthora species have been gardens. Examples of these include recorded in the UK, all probably P. ilicis, holly blight; P. alni, causing a imported on nursery stock (Brasier lethal disease of Alnus; P. citrophthora, Powdery mildews Neoerysiphe galeopsidis on Acanthus (top left), Erysiphe elevata on Catalpa (above left), wisteria powdery mildew on Wisteria (centre) and Erysiphe deutziae on Deutzia (right) 218 December 2009 PlantsmanThe causing root rot on many ornamentals; Oomycetes Phytophthora ramorum on and P. rubi and P. idaea both causing Rhododendron (above left), Phytophthora ilicis on Ilex aquifolium (above right) and Phytophthora root rot on raspberry. cinnamomi on Taxus baccata (right) Recently, the newly recognized species P. ramorum and P. kernoviae are causing much concern. Both are on 15 host genera in nine families quarantine organisms and are subject (Anon. 2008). The main shrub to eradication or containment.

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