Hidcote, Hypericum patulum Thunb. ex Murray, is a deciduous shrub that is cultivated as an ornamental in landscape gardens and courtyards in Japan. In early August 2008, severe leaf spotting was observed on plants growing in a courtyard in Nasushiobara, Tochigi, Japan. More than 30% of the leaves on five shrubs exhibited leaf spot symptoms. Small, round, pale brown lesions were initially observed. Later, they expanded to 5 to 12 mm in diameter, round to irregular-shaped with pale brown centers and dark brown margins. Under continuously wet or humid conditions, black acervuli developed on the leaf lesions. Conidia were straight or slightly curved, fusiform to clavate, and five-celled with constrictions at the septa. Conidia ranged from 17 to 21 × 5 to 8 μm with hyaline apical and basal cells. Fifteen percent of apical cells had two and the rest had three appendages (setulae) ranging from 10 to 21 μm long. The basal hyaline cell tapered into a 2 to 4 μm pedicel. The three median cells ranged from light or dark brown to olive green. These morphological characteristics matched those of Pestalotiopsis microspora (Speg.) G.C. Zhao & N. Li (1,2). The identity of the fungus was confirmed by DNA sequencing of the internal transcribed spacer (ITS) region (GenBank Accession No. GU908473) from single-spore isolates, which revealed 100% homology with those of other P. microspora isolates (e.g., GenBank Accession Nos. FJ459950 and DQ456865). Koch's postulates were confirmed using leaves of three detached branches of a field-grown asymptomatic plant of H. patulum. Thirty leaves of each branch were inoculated by placing mycelial plugs obtained from the periphery of 7-day-old single-spore cultures on the leaf surface. Potato dextrose agar plugs without mycelium served as controls. Leaves on branches were covered with plastic bags for 24 h to maintain high relative humidity in a greenhouse (approximately 24 to 28°C). After 5 days, all inoculated leaves showed symptoms identical to those described above, whereas control leaves remained symptom free. Reisolation of the fungus from lesions on inoculated leaves confirmed that the causal agent was P. microspora. To our knowledge, this is the first report of leaf spots on H. patulum caused by P. microspora in Japan. Management options may have to be developed and implemented to protect Hidcote plants in areas where leaf spot cannot be tolerated.
References: (1) P. A. Saccardo. Sylloge Fungorum III:789, 1884. (2) G. C. Zhao and N. Li. J. Northeast For. Univ. 23(4):21, 1995.