A novel finding of powdery mildew caused by Oidiopsis haplophylli on spinach

Korean spinach growers reported leaf symptoms resembling a type of downy mildew infection or physiological degeneration. In November 2007, spinach plants (Spinacia oleracea) showing symptoms of powdery mildew were found in an organic farming garden in Busan, Korea. Initial symptoms were pale greenish, circular discolorations with a white fluffy covering on the upper and lower surfaces. Lesions expanded and coalesced to produce large areas of yellowing and chlorosis, occasionally with a necrotic centre (Fig. 1A, B). A sample was deposited in the Korea University Herbarium (KUS-F23217).

The conidiophores emerging through stomata were hyaline, cylindrical, 160-400 × 4.5-6 μm, simple or branched one or two times at random positions, producing conidia singly, followed by 2-3 straight cells (Fig. 2A). Conidia were dimorphic, with primary conidia navicular, 45-65 × 12-18 μm, and secondary conidia cylindrical, 45-70 × 12-20 μm; both hyaline, lacking distinct fibrosin bodies, with angular/reticulated wrinkling of outer wall (Fig. 2B-D). These features are in accordance with Oidiopsis haplophylli (Braun, 1987; Braun et al., 2002). No chasmothecia were found in our observations.

The occurrence of powdery mildew on spinach is rare. Only three records were indexed: Erysiphe communis from Pakistan and Jordan, and Oidium sp. from Venezuela (Farr et al., 2007). There has been no record of Oidiopsis infection on spinach, although O. haplophylli has been recorded from 53 plant families in most parts of the world (Amano, 1986; Braun, 1987). The teleomorph of O. haplophylli, Leveillula taurica, has not been reported previously on the Chenopodiaceae. However, the Chenopodiaceae are infected by L. cylindrospora and L. saxaouli (Braun, 1987). The powdery mildew isolated from spinach is distinct from the previously reported species based on conidial morphology. L. cylindrospora conidia are not pointed and not usually lanceolate, which is in contrast to the isolated conidia (Fig. 2A, B). L. saxaouli conidia are cylindrical with cingulum-like rings near the ends, which is unique to this species. Thus, this is the first record of O. haplophylli on spinach and on Chenopodiaceae.