First report of powdery mildew infection of Ligustrum caused by Erysiphe syringae-japonicae

Ligustrum can be infected by three different species of powdery mildew: Erysiphe katumotoi, E. ligustri, and E. syringae. The first two occur only in Japan, while the third is a cosmopolitan fungus. In Europe, E. syringae occurs mostly on Syringa spp. It occurs occasionally on Ligustrum vulgare, with records from Germany, Romania (Braun, 1995) and Poland (Wołczańska & Mułenko, 2002). In September 2002 a delicate, white mycelium with scattered cleistothecia was observed on L. vulgare at two localities in Kraków, southern Poland. Preliminary observations indicated that this pathogen was not E. syringae but there was insufficient material for a full identification. Fresh material was collected in October 2003.

The fungus formed small patches of whitish mycelium on both sides of older leaves, some joining up to cover larger areas. Localised parts of plants were affected and the current impact of the disease is low. Cleistothecia were brownish, 90-120 µm diameter, with polygonal to rounded cells. Appendages arose equatorially, and were aseptate, thick-walled throughout, up to 120 µm long, brown at the base, becoming lighter towards the middle. The apex of the appendages was 4-6 times dichotomously and regularly branched, with recurved tips. Asci 3-10, sessile or short-stalked, 50-70 ´ 30-45 µm, (6) 7-8-spored. Ascospores were ellipsoid, hyaline, 19-21 ´ 9-10 µm. The fungus was identified as E. syringae-japonicae and distinguished from the three existing Erysiphe spp. known to occur on Ligustrum. Prof. Uwe Braun made an independent confirmation. Specimens have been deposited in the herbariums of the W. Szafer Institute of Botany in Kraków (KRAM) and Martin-Luther-Universität in Halle/Saale (HAL).

E. syringae-japonicae was originally found in East Asia, but since 1998 it has been spreading quickly in Europe and has been recorded in Germany, Switzerland, Poland and Russia (Piątek, 2003) on Syringa spp. The present finding on Ligustrum and specifically on L. vulgare is novel. L. vulgare is a commonly grown ornamental shrub in Poland and beyond, found mostly in hedgerows. The disease poses a significant threat to existing widespread plantings and the commercial trade in Ligustrum.