Pseudocercospora cladosporioides, the cause of leaf spot on olive, a pathogen new to the United Kingdom
Plant Pathology Department, Royal Horticultural Society Wisley, Wisley, Woking, Surrey, GU23 6QB, UK
Accepted: 24 Feb 2009
Olive (Olea europaea) is of great economic importance for its fruits and oil. In the UK, olive trees are popular ornamentals grown in frost free areas or in glasshouses. Olive trees are benefiting from climate change and are surviving further north in Europe than 30 years ago (Moore & White, 2003). Evidence of this includes the establishment of Britain’s first commercial olive grove of 120 trees, planted in Devon in 2006 (McCarthy, 2006).
In May2005, diseased samples of oliveoriginating from private gardens in Suffolk and Surrey were received at the Royal Horticultural Society, Wisley. The symptoms were diffuse spots on the lower leaf surfaces. Leaves subsequently turned yellow, reddish-brown and then dropped (Fig. 1). The fungus was dirty grey in appearance and growing on upper leaf surfaces. Conidia were subcylindrical, guttulate, very pale to medium olivaceous-brown, apex obtuse, straight or often curved, 3-8 septate, 27-55 x 3.5-4.5 μm (Fig. 2). Conidiophores were mostly unbranched, 110-125 x 3-4.5 μm. Conidiogenous cells were terminal bearing one or more scars. The morphological characteristics fit with Pseudocercospora cladosporioides (Ávila et al., 2005), the causal agent of cercosporiosis or leaf spot disease on olive.
Single spore cultures were made on potato carrot agar supplemented with ampicillin and streptomycin. The fungus was slow growing but produced abundant conidia when plated onto potato dextrose agar and incubated for four weeks at 20°C in the dark. To fulfil Koch’s postulates, one olive plant was inoculated with a spore suspension (1.5 x 105 spores/ml). Sterile distilled water was used on the control plant. The plants were kept in a glasshouse at 20°C, covered with a polythene bag for the first 48 h to increase humidity. After 15 days, the leaves of the inoculated plant became yellow and dropped. Pseudocercospora cladosporioides was reisolated from these infected leaves and confirmed by morphology. The control plant was not infected.
The cercosporiosis disease is common and widely distributed in most olive growing regions in the world including Algeria, Argentina, Australia, Brazil, Chile, China, Germany, Greece, India, Italy, Montenegro, Netherlands Antilles, New Zealand, Portugal, Spain, Tanzania, Tunisia, USA and the former Yugoslavia (Crous & Braun, 2003; David, 2004). This is, however, the first record of P. cladosporioides in the UK.
- Ávila A, Groenewald JZ, Trapero A, Crous PW, 2005. Characterisation and epitypification of Pseudocercospora cladosporioides, the causal organism of Cercosporaleaf spot of olives. Mycological Research 109, 881-888.
- Crous PW, Braun U, eds, 2003. Mycosphaerella and its anamorphs: 1. Names published in Cercospora and Passalora. CBS Biodiversity Series No. 1. Utrecht, Netherlands: Centrallbureau voor Schimmelcultures.
- David JC, 2004. Pseudocercospora cladosporioides. IMI Descriptions of Fungi and Bacteria No.1604.
- McCarthy M, 2006. Britain's first olive grove is a
sign of our hotter times. http://www.independent.co.uk/environment/britains-first-olive-grove-is-a-sign-of-our-hotter-times-405557.html
Moore D, White J, 2003. Cassell's Trees of Britain and Northern Europe. Jersey, UK: Domino Books.
This report was formally published in Plant Pathology
©2009 The Authors