First Report of Powdery Mildew Caused by Oidium lycopersici in Field-grown Tomatoes in California.

Powdery mildews are obligate biotrophic fungi that cause extensive diseases in crops. In tomato (Lycopersicum esculentum) powdery mildew is caused primarily by Leveillula taurica and Oidium neolycopersici, which in California cause infections mainly on field- (L. taurica) and greenhouse-grown (O. neolycopersici) tomatoes (Arredondo et al. 1996; Kiss et al. 2005). Oidium lycopersici (synonymEuoidium lycopersici) is known also to infect tomatoes, but this species has been reported only in Australia (Kiss et al. 2001). In 2015, mild to severe infections by powdery mildew were observed from July through October across most tomato fields in California. Diseased plants mostly showed yellow chlorotic lesions on the leaves that frequently became necrotic, symptoms that are typical of infection byL. taurica. Profuse fungal sporulation was visible also on the abaxial leaf surface, consisting of the 50- to 70-µm-long primary lanceolate and cylindrical secondary conidia of L. taurica (Correll et al. 1987). Additionally, on some plants small white lesions of epiphytic mycelium and conidiation were observed on the adaxial leaf surface and stem, which were reminiscent of infections by Oidium spp. Further examination of these specimens under a scanning electron microscope at 5 kV and 500 to 3000× indicated the presence of ovate to cylindrical single-celled conidia with slightly wrinkled surfaces measuring 15 to 25 × 12 to 15 µm (avg. 22 × 14 µm). Conidia were in chains of 3 to 5 spores emanating from unbranched conidiophores that measured 100 to 150 µm in length. Based on these morphological characteristics the fungus was identified as O. lycopersici (Kiss et al. 2001). To confirm the presence of this species, genomic DNA was extracted from 145 symptomatic tomato leaves and was used as a template for the PCR amplification of a 437-bp fragment that spanned part of the internal transcribed spacer 1 (ITS1), the 5.8S ribosomal DNA gene, and the internal transcribed spacer 2 (ITS2) of O. lycopersici. Amplification was performed using oligonucleotide primers Oidlyc-ITS1-F1 (5′-TGCACCGACCGGCTTC-3′) and Oidlyc-ITS2-R1 (5′-TACCTGATTCGAGGTCAAC-3′) designed in-house on the rDNA sequence of O. lycopersici (GenBank Accession No. AF229021). Using these primers, amplicons were obtained from 51 samples and subsequent sequencing and BLASTn analysis confirmed that the sequences matched 100% the partial rDNA sequence of O. lycopersici isolate Ol-1 clone 1 in GenBank (HQ286673). A representative sequence from one of our samples has been deposited in GenBank (KU182432). Similar ITS rDNA sequence analyses using species-specific primers for L. taurica and O. neolycopersici showed the presence of these two species in 145 and 4 of the samples, respectively. This indicates that although L. taurica is the primary causative agent of powdery mildew epidemics in California, the three species can coinfect plants. To our knowledge, this is the first report of O. lycopersici in California. As this state produces over 95% of the processing tomatoes in the United States, and that national production is growing with a record 14.6 million tons produced in 2014, the impact of this new disease on local and national economices may be significant. © 2016 The American Phytopathological Society.