Pseudocercospora cladosporioides, the cause of leaf spot on olive, a pathogen new to the United Kingdom

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.