A new blight disease on Buxus in the UK caused by the fungus Cylindrocladium

Since 1998, a serious foliar disease has affected box (Buxus) plants in the UK. The symptoms are dark brown spots on the leaves, which eventually coalesce to cover the whole leaves, and black streaks on the stems and defoliation. The black streaks appear to progress from the bottom to the top of plants. In severe cases, complete leaf loss has been observed but until now no tree death has been recorded.

At the Royal Horticultural Society, we have consistently isolated a species of the fungus Cylindrocladium from 30 Buxus samples showing the blight symptoms, mainly on B. sempervirens 'Suffruticosa'. Samples were obtained from Central Scotland and the Borders region, the north of England, the Midlands, East Anglia, the southwest and southeast regions. We observed the same symptoms and isolated the same species of Cylindrocladium associated with the same symptoms on native B. sempervirens at Box Hill, Surrey; cultivated B. sempervirens, several varieties of B. sempervirens and B. sinica. We have reports of the fungus and the disease on different varieties of B. microphylla (R. Cook, Central Science Laboratory, York, UK, personal communication). The disease is widespread throughout the UK. The Cylindrocladium species we found on Buxus is very often associated with a second pathogen, Volutella buxi (teleomorph Pseudonectria rousseliana), the cause of volutella blight (Strouts & Winter, 1994). Although very little has been published on volutella blight, the causal organism is usually regarded as a wound pathogen, causing dieback on clipped box (Moore, 1959; S. Archer, Imperial College, London, UK, personal communication). The two diseases also occur independently.

Inoculation assays on detached shoots and on 2.5 year old plants of B. sempervirens 'Suffruticosa', with a conidial suspension (1-3 x 10⁶ spores/ml) of Cylindrocladium, isolated from diseased B. sempervirens 'Suffruticosa' from Kent and Lincolnshire, produced symptoms typical of box blight. Humidity was kept high until the development of the symptoms (7-10 days after inoculation) and was necessary for successful infection. The pathogen was successfully reisolated from the infected leaves fulfilling Koch's postulates.

We attempted to identify the species through morphological characterisation of the anamorph stage following the keys of Crous & Wingfield (1994) and Schoch et al., (1999) as well as sequencing of the ITS region using the primers ITS1 and ITS4 (White et al., 1990). Sporulating cultures on carnation leaf agar (Crous & Wingfield, 1994) had coniodiophores with stipe extensions terminating in ellipsoidal vesicles with pointed apices closely similar to C. mexicana (Schoch et al., 1999). The width of the vesicle and conidia characteristics also fit the description of C. mexicana. However, comparison of the sequence of the ITS region with other sequences available in the GenBank database did not reveal a 6 bp deletion in the ITS2 region found in C. mexicana, a trait found so far only in C. floridanum. These sequencing and morphological data suggest that the Cylindrocladium sp. isolated from Buxus does not conform fully to the characteristics of Cylindrocladium species already published.