First Report of Tomato Powdery Mildew Caused by Leveillula taurica in Taiwan

Tomato (Solanum lycopersicum L.) is one of the common and important economic crops in Taiwan. In July 2018, tomato leaves with unusual yellowing and lesions were observed in Taichung, Changhua, Nantou, and Yunlin counties in Taiwan. Symptomatic leaves initially showed chlorotic, irregularly shaped patches on adaxial side of leaves; and days later, necrosis appeared in the patches center. The disease incidence was 4 to 25% and occurred mostly on large type tomato in the field and greenhouse while rarely on cherry tomato. Since the pathogen could not be cultured in potato dextrose agar, for initial pathogen observation, symptomatic leaves were examined directly under light microscope. Conidiophores were found emerged through abaxial stomata of the lesion, and were erect, single and sometimes branched, with single-celled and dimorphic conidia, suggesting the lesions were associated with a powdery mildew. The white to hyaline pathogens collecting directly from fresh signs were also examined. Primary conidia were lanceolate, tip point, 50 to 78 × 14 to 22 μm, ratio of 2.3 to 5.9; secondary conidia were cylindrical to clavate, 47 to 72× 14 to 22 μm, ratio of 2.5 to 4.6. Conidial germ tubes were mainly terminal to subterminal and sometimes lateral. Conidial appressoria were alobate to multilobed, and hyphal appressoria were nipple-shaped, lobed to multilobed or even coralloid. No chasmothecia were found in the field samples. One representative specimens of isolates TARI_PM-3 was deposited in the National Museum of Natural Science in Taiwan (accession number F0034683), and DNA extracted from the fresh conidia was amplified respectively with primers PMITS1 and PMITS2 (Cunnington et al. 2003) for the sequences of internal transcribed spacer region (ITS1+5.8S+ITS2, partial sequence). The segment of sequence (accession numbers MT370494 in GeneBank) showed 99.8% identity with the sequence of AB045000, which was identified as Leveillula taurica (Khodaparast et al., 2001). Based on morphology and molecular analysis, the fungus was identified as L. taurica (Braun and Cook 2012; Choi et al. 2019). To confirm the pathogenicity, conidial suspensions (4 × 104 conidia/ml) of L. taurica (isolate TARI_PM-3) were used to inoculate by dropping (10 μl/site) on the abaxial side of leaves of 4-week-old potted tomato (cv. Golden Lucky). The plants were covered with transparent plastic bags for 1 day and then maintained at 16 to 26°C in a greenhouse. Lesion symptoms of leaves similar to those in the field were observed 4 weeks after inoculation, while the controlled plants inoculated with ddH2O showed symptomless. The same fungus was observed on the necrotic patches of the inoculated leaves. In Taiwan, the recorded causing pathogens of tomato powdery mildew are Erysiphe orontii and E. cichoracearum (Tzen et al. 2019). To our knowledge, this is the first report of tomato powdery mildew caused by L. taurica. Typical lesions appeared in the beginning of the disease progress in the field. Through inoculation, the same pathogen could infect sweet peppers (Capsicum annuum L.), which has been also reported as a host of L. taurica (Tzen et al. 2019), suggesting these two crops could be as alternate hosts of L. taurica in the field. Cross-species infection should be taken into consideration while managing the disease.

A Short-Read Genome Assembly Resource for Leveillula taurica Causing Powdery Mildew Disease of Sweet Pepper (Capsicum annuum)

Powdery mildew of sweet pepper (Capsicum annuum) is an economically important disease. It is caused by Leveillula taurica, an obligate biotrophic ascomycete with a partly endophytic mycelium and haustoria, i.e., feeding structures formed in the mesophyll cells of infected host plant tissues. The molecular basis of its pathogenesis is largely unknown because genomic resources only exist for epiphytically growing powdery mildew fungi with haustoria formed exclusively in epidermal cells of their plant hosts. Here, we present the first reference genome assembly for an isolate of L. taurica isolated from sweet pepper in Hungary. The short read–based assembly consists of 23,599 contigs with a total length of 187.2 Mbp; the scaffold N50 is 13,899 kbp and N90 is 3,522 kbp; and the average GC content is 39.2%. We detected at least 92,881 transposable elements covering 55.5 Mbp (30.4%). BRAKER predicted 19,751 protein-coding gene models in this assembly. Our reference genome assembly of L. taurica is the first resource to study the molecular pathogenesis and evolution of a powdery mildew fungus with a partly endophytic lifestyle.