First Report of Powdery Mildew (Pseudoidium neolycopersici) on Croton (Codiaeum variegatum var. pictum) in China.

Abstract

Croton (Codiaeum variegatum (Linn.) var. pictum (Lodd.)) is an ornamental plant commonly grown in southern China. In March 2014, severe powdery mildew infections were observed on crotons in gardens of Hainan University (20.1°N and 110.3°E), Haikou, Hainan province. Disease incidence was estimated in a random batch of 100 plants in three replicates, with the average value approaching 80%. Symptoms first appeared as white circular patches on the adaxial surface and expanded to the abaxial surface, petioles, and stems. The top leaves were the most affected. Upper surfaces of the infected leaves were covered by white, dense mycelia. As the disease progressed, infected leaves turned purple on the lower surfaces and curly before becoming necrotic and abscising from the plant. Powdery mildew was more severe in shaded environments, especially during rainy or foggy weather in early spring. Two hundred conidiophores and conidia were observed microscopically. The conidiophores were straight or occasionally flexuous, 62.3 to 127.6 × 6.2 to 10.2 μm, consisting of two to three straight cells. Conidia were born in solitary on the top of conidiophores. Conidia were hyaline, ellipsoidal, 26.4 to 42.2 × 11.7 to 23.4 μm (average 32.5 × 16.5 μm), contained no distinct fibrosin bodies, and produced a subterminal germ tube. The wrinkling pattern of the outer walls of older conidia was angular or reticulated. Appressoria were single and multilobed. Cleistothecia were not observed. Based on morphological characteristics, the fungus was identified as Oidium neolycopersici (2), which was recently renamed Pseudoidium neolycopersici (L. Kiss) (3). The identity was confirmed by sequence analysis. Genomic DNA was extracted from the foliar powdery mildew colonies using Chelex-100 (Bio-Rad, Shanghai, China). The rDNA internal transcribed spacer (ITS) region was amplified with primers ITS1 and ITS4 (5). The ITS sequence of the representative isolates C01 (GenBank Accession No. KJ890378.1) and four other powdery mildew samples collected from crotons in Hainan University was 100% identical to that of P. neolycopersici isolates from tomato plants such as JQ972700 and AB163927. Inoculations were made by gently pressing diseased leaves onto leaves of five healthy plants of croton and tomato ('Money maker'). Five non-inoculated croton and tomato plants served as controls. Inoculated and non-inoculated plants were maintained in an incubator at 25°C with a 12-h photoperiod. After eight days, typical powdery mildew symptoms developed on 93% of the inoculated plants, while no symptom developed on the non-inoculated plants. The pathogenicity tests were repeated three times. The same fungus was always re-isolated from the diseased tissue according to Koch's postulates. The pathogenicity tests further confirmed that the pathogen from crotons is P. neolycopersici (Basionym. Oidium neolycopersici (KJ890378.1)), which is commonly known as the tomato powdery mildew. P. neolycopersici is also a pathogen of Normania triphylla (1) and papaya (4). To our knowledge, this is the first report of P. neolycopersici infecting croton. The avenue of this pathogen entering gardens of Hainan University remains unknown. The gardens are located far away from tomato farms. Also no symptom of powdery mildew on croton was observed during surveys in other locations in Haikou. The origin of the pathogen warrants additional research.