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Diseases and Disorders of Photinia spp.

The wide use of new genera of plants, initially considered as disease-free, is not free from risks: following their massive plantations worldwide, some specific disorders increase in importance, while some others adapted themselves to use the new plants as a nutrient source. This phenomenon, called also host jump, is quite common when a wide and genetically uniform source of carbon become available.

For example,
 x Cupressocyparis leylandii, the natural hybrid between Cupressus macrocarpa and Chamaecyparis nootkatensis, widely planted for its fast growth and ornamental value, is now massively attacked by Seiridium cardinale, the agent of the cypress canker disease, and by some species of the genus Phytophthora commonly present on its parent species.

Photinia is a genus of several species of shrubs and small trees afferent to the Rosaceae family. Their natural range is restricted to the temperate regions of Asia. However, they are widely cultivated throughout the world as ornamentals. The most widely planted species are P. serrulata (originally from China), P. glabra (originally from Japan) and the varieties ‘Red Robin’ and ‘Birmingham’ of their hybrid P. x fraseri. Photinias are very popular ornamental shrubs, grown for their flowers and foliage.
Photinias are usually considered as trouble-free, but even this species has become the host of several previously unknown pathogens. A typical example is Podosphaera leucotricha, the agent of “Apple powdery mildew”, which has been observed since 2004 on P. x fraseri in Italy near Turin, and the year after in China on P. serrulata (Garibaldi et al., 2005; Liang and Xing, 2012). Powdery mildew produces symptoms on young shoots, leaves, blossoms. In general, symptoms are most noticeable on the leaves and flowers. The disease causes economic damage by reducing plant vigor, flower bud production, and its ornamental value.
Another disease affecting photinias’ foliage is ought to Entomosporium mespili, the agent of leaf spot. E. mespili is a fungal disease that spots the leaves of woody plants in rosaceae family. This disease is more than unsightly; heavy infections will cause defoliation, and death of the infected plant. This pathogen was first reported in Louisiana in 1957 (Jacobs et al., 1996), and now is also reported in Italy (Sicily and Apulia) on Raphiolepis, photinia and quince (Polizzi and Azzaro, 1998; Cariddi and Vovlas, 2009).
Leaf spots could also be caused by physiological problem (i.e. not caused by any pest or disease). The purple-brown spotting on the foliage is typical of a plant under stress. Photinias, since they are originated from temperate warm climates of Asia, are not fully hardy and can suffer after cold, wet winters. Cold winds and/or frosty conditions can also damage foliage, again causing leaf spotting if adverse conditions are prolonged.
Phytophthora cactorum has been reported as the agent of extended necrosis of leaf blade in Italian (Marche) ornamental nurseries (Vettraino et al., 2006). P. cactorum is particularly aggressive to members of the Rosaceae family, on which it may cause root, collar and crown rot. A taxonomically related species to P. x fraseriHeteromeles arbutifolia (syn. Photinia arbutifolia) has been reported to be a natural host of P. cactorum in the USA (Keim et al., 1976). 
P. ramorum, a lethal invasive pathogen, agent of the sudden oak death in the USA and more recently of sudden larch death in UK (Brasier and Webber, 2010), was isolated in Polish container nurseries from P. x fraseri ‘Red Robin’ (Orlikowski and Szkuta, 2004). The symptoms consist of petiole and leaf blade necrosis. Infected leaves easy fell down during wind weather. In addiction to the direct damage to photinia, this pathogen can use this species to hitchhike to other more susceptible hosts.

 A bacterial disease characterized by leaf spot and shoot blight has been reported occurring on P. glabra in Japan (Goto, 1983). The disease agent was classified as Pseudomonas syringae pv. photiniae, since its pathogenicity is restricted to photinia. This disease, which it is reported just in the photinia natural range so far, could spread by trade of living plants and its arrival in a new environment could bring to serious problems in the European ornamental plantations.
Fireblight is a bacterial lethal disease caused by Erwinia amylovora that has reported to occur on photinias. Erwinia amylovora is a quarantine pathogen native to North America and was introduced into northern Europe in the 1950s to 1960s. It attacks more than 130 plant species belonging to 40 genera, mainly from the sub-family Pomoideae of the family Rosaceae (Van der Zwet and Keil 1979). It is generally believed that E. amylovora is a homogeneous species and does not show pathogenic specialization. This means that each isolate of the pathogen is potentially able to infect any of the known host plants (Momol and Aldwinckle 2000). However, is unlikely to affect P. × fraseri cultivars such as 'Red Robin'. E. amylovora has been occasionally reported to affect P. villosa and P. davidiana, two photinias species of minor interest for ornamental aims.

Even some new insect pests have adapted to live on this plant. Peach tip moth, Cydia molesta, an Asiatic species established in Europe and North America, has been recently reported even on photinia and laurel (Trinci, 2009). Other minor problems can be caused by the cochineal Ceroplastes ceriferus, an Asiatic species, reported on many ornamental and fruit trees; and by Aphis spiraecola and A. gossypii, two aphids whose sucking activity can lead to growth abortion and shoot deformation (Trinci, 2009).
Even if Photinia is not incurring in lethal diseases, so far, nurserymen, gardeners and private owners should be aware by the possibility that their photinias could be attacked by some pest or pathogen described above, as well as by new invasive species. For this reason, it is worth to avoid extensive plantation of this species, but rather to alternate this shrub with other ornamental species not taxonomically related.

The wide use of new genera of plants, initially considered as disease-free, is not free from risks