Phloeosporella ceanothi causing leaf spot and dieback on Ceanothus in the UK

Ceanothus is a genus of approximately 50-60 species native from North America but is now widely planted as an ornamental garden plant in the United Kingdom. Recent cases of leaf spotting followed by defoliation and dieback (Fig.1 A and B) on Ceanothus shrubs have been observed in several counties in the UK through the advisory services of the RHS and CSL. Since 2001, the disease has been confirmed on Ceanothus thyrsiflorus var. repens, C. arboreus 'Trewithen Blue', 'Concha' and 'Blue Sapphire' in Kent, Hampshire, Herefordshire and Surrey.

Microscopic examination revealed conidiomata producing thick, white tendrils of conidia (Fig. 2) that were hyaline, flexuous, straight or slightly curved , fusiform, tapered at one end, each with two septa (Fig.3 A and B) and measuring 54 µm x 2.4 µm (45.3 - 62.1 µm x 2 - 2.7 µm) consistent with Phloeosporella ceanothi (Sutton, 1980).

Single spore cultures were isolated on potato carrot agar supplemented with ampicillin and streptomycin. The fungus was slow growing but produced discrete colonies of branched mycelium with abundant sporulation when plated on potato dextrose agar (PDA). After 30 days growth on PDA, the colonies were removed and mixed with sterile distilled water to a concentration of 1 x 10⁵ spores/ml and used to perform Koch's postulates on healthy Ceanothus 'Blue Mound' plants.

Inoculation was facilitated by dipping whole branches into the spore suspension (3 repetitions). Sterile distilled water was used for control plants. The plants were covered with a polythene bag for 5 days, to maintain high humidity, and placed outside. After 23 days brown circular lesions on the leaves were observed. Phloeosporella ceanothi was re-isolated from the leaf lesions fulfilling Koch's postulates. No symptoms were observed on the control plants. To our knowledge the disease has only been recorded so far in the United States (Farr et al., 2007). The records mentioned above are the first confirmed cases of Phloeosporella ceanothi in the UK.