Botryosphaeria spp. are known to produce cankers and dieback in several woody hosts. Since 2001, a severe disease resulting in the dieback of branches was observed in olive (Olea europaea) orchards in Andalucía, southern Spain, where the only cultivar affected was Gordal de Sevilla. The affected trees showed abundant dead twigs, and wilted leaves remained attached. These symptoms were similar to those caused by Resseliella oleisuga, a bark borer insect common in the region. Closer inspection showed no evidence of insects, but revealed reddish brown bark lesions that girdled the affected branches. When the outer bark was removed, the affected tissue appeared dark brown, in contrast to the yellowish green of healthy inner bark. A Fusicoccum sp. was consistently isolated from the necrotic bark. Conidia produced in black pycnidia were hyaline, aseptate, fusoid, with a truncate base averaging 19.9 × 6.7 μm, becoming pale brown with all cells the same color, and developing one or two septa when germinated. These characteristics conform to Fusicoccum ribis (1), the anamorph of B. ribis. Pathogenicity of two isolates from symptomatic branches was determined by stem inoculations of 4-year-old cv. Gordal de Sevilla grafted on wild olive trees growing in plastic pots containing a sand/lime/peat soil mixture. Both isolates were inoculated by three techniques. In the first, 7-mm-diameter agar plugs bearing actively growing mycelium were applied to 7-mm-diameter bark wounds made with a cork borer on the middle point of stems (averaging 10 mm in diameter). In the second technique, the same type of inoculum was applied directly to the bark without any wounding. For the third technique, 100 μl of conidial suspension in water (2 × 105 conidia per ml) was applied to bark wounds. The inoculated stem sections were wrapped in Parafilm to retain moisture. All inoculated and control plants (only sterile agar medium or sterile water added) were kept in a greenhouse and watered as needed. There were three replicate plants per isolate and inoculation technique. After 3 months, all plants inoculated with mycelium applied to wounds showed girdling and apical death with browning and wilting of leaves above the inoculation point. In the treatments where mycelium was applied to unwounded bark or where conidial suspensions were applied to wounds, small bark cankers developed to 56 mm long, but neither girdling nor foliar wilting were observed. No symptoms developed in the control plants. There were no significant differences in pathogenicity between the two F. ribis isolates. F. ribis was recovered from necrotic bark from every inoculated plant. These results demonstrate that B. ribis is pathogenic on cv. Gordal de Sevilla olives, and they indicate that bark wounding favors lesion development but is not needed for infection.
Reference: (1) B. Slippers et al. Mycologia 96:83, 2004.
