Powdery mildew on tomato caused by Oidium neolycopersici in Changchun in China

A powdery mildew was observed on tomato (Solanum lycopersicum formerly Lycopersicon esculentum) in greenhouses in Changchun in September 2006. Mildew first appeared as white circular patches on the adaxial leaf surface (Fig.1). As the disease progressed, abaxial leaf surface, stems and petioles were covered by cotton-like masses of mycelia and conidia. Yellow lesions appeared on the abaxial surfaces which turned necrotic and led to defoliation. Young plants (cotyledon period) through to flowering plants were affected, while the fruits remained free from infection.

Conidia were ellipsoid or cylindrical, produced singly, lacked distinct fibrosin bodies and measured 22- 51 x 13 -25 µm (average 34.2 x 19.1µm, n = 150). Conidiophores were unbranched and measured 60 - 125 µm (average 83.8 µm, n = 100) (Fig.2). Appressoria were nipple-shaped or slightly lobed. Foot cells were cylindric, sometimes slightly flexuous, followed by 1 or 2 cells and measured 27 - 53 x 6 - 10 µm (average 39.8 x 7.4 µm, n = 100). Germ tubes were produced from a side of the conidia ending in lobed appressoria. Chasmothecia were not observed. The latent period of this disease was 5 - 7 days. The fungus from tomato also infected tobacco (Nicotiana tabacum) by inoculation but the symptoms on tobacco were usually small and localized.

The morphological characteristics of the fungus described above fit Oidium neolycopersici (Kiss et al., 2001). In June 1988, tomato powdery mildew was found in Xinjiang, and the pathogen was identified as Oidiopsis taurica (anamorphic state of Leveillula taurica; Jia, 1990). Oidium neolycopersici clearly differs from Oidiopsis taurica by its ectophytic nature. In May 1991, Fang et al. (1992) found tomato powdery mildew in Shengyang and identified the pathogen as Erysiphe polygoni based on the anamorphic characteristics. In November 2003, Wang et al. (2004) found tomato powdery mildew in Shengyang and identified the pathogen as Oidiopsis lycopersici. Oidium neolycopersici sometimes forms conidiophores with pseudo-chains in humid conditions, and has been confused with Oidiopsis lycopersici with conidia produced in chains. This is the first report of tomato powdery mildew caused by Oidium neolycopersici in Changchun in China.