| Abstract |
Mango (Mangifera indica L.) is an economically important tropical fruit in southern Taiwan. In February 2019, new leaf blotches distinct from anthracnose lesions were noticed on mango leaves in Meinong, Kaohsiung (N22°54′43.7″ E120°32′59.3″). Symptoms were circular to irregular lesions with easily torn centers and were cream to light brown with a dark brown margin on both leaf surfaces. Similar symptoms were observed on mango leaves in Yujing, Tainan (N23°07′31.3″ E120°27′18.2″) in July of the same year. We surveyed the disease incidence on 60 mango trees consisting of three cultivars, Irwin, Yu-win No.6, and a native cultivar, in a commercial farm by randomly examining five shoots of each tree. The disease incidences of Irwin, the native cultivar, and Yu-win No.6 were 25, 37, and 73%, respectively. Diseased tissues from the two locations were surface sterilized and incubated on potato dextrose agar (PDA) for pathogen isolation. Seven isolates (Mgk3, TMg2-2.2, TMg3-1.2, TMg3-2.1, TMg4-1, TMg6-3, and TMg8-1.1) from different locations and cultivars were selected for further study. Pycnidia were produced on 7-day-old PDA cultures. Conidiogenous cells were hyaline and short cylindrical phialides. Conidia were hyaline, aseptate, thick walled, 20 to 24 × 11 to 16 μm, and subcircular to ellipsoid with one to two large oil droplets and a markedly flat protruding hilum at the base. These morphological features indicated that the seven isolates were a member of Pseudoplagiostoma (Cheewangkoon et al. 2010). Pathogenicity assays were conducted by the point inoculation method on 10 intact young leaves on Irwin mango plants in a greenhouse at 20 to 25°C. Each leaf was inoculated at six points on the abaxial surface with point inoculation. Each point was inoculated with a 10 μl conidial suspension (10⁶ conidia/ml) of isolate TMg 8-1.1. Sterilized water was used as a control. Shoots with inoculated leaves were covered with translucent plastic bags for 2 days. At 7 days postinoculation (dpi), 70% of conidia inoculated points (n = 60) displayed symptoms resembling the field symptoms, but sterilized water inoculated points (n = 60) did not. The six other isolates were inoculated on detached leaves by the point inoculation method. All inoculated leaves were maintained in humid containers at 25°C with 12 h light/dark regime, and displayed similar lesions at 7 dpi. Fungi reisolated from the symptomatic leaves showed the same morphological characteristics observed in the Pseudoplagiostoma originally isolated from diseased tissues. Internal transcribed spacer (ITS), TUB2, and LSU gene sequences of the seven isolates (ITS accession nos.: MN818659 to MN818665; TUB2 accession nos.: MW415921 to MW415927; LSU accession nos.: MN876849 to MN876855) were amplified with primer sets ITS4/V9G, T1/Bt-2b, and LSU1Fd/LR5, respectively, and used for molecular identification (Cheewangkoon et al. 2010). The seven isolates were 95.8, 99 to 100, and 99.8% identical to Pseudoplagiostoma mangiferae Dayarathne, Phookamsak & K. D. Hyde KUMCC 18-0179 (the ex-type strain) for the ITS gene (MK084824), TUB2 gene (MK084823), and LSU gene (MK084825), respectively. Phylogenetic analysis based on concatenated sequences of ITS and LSU genes was performed by the maximum likelihood method. All seven isolates were clustered in a well-supported clade with P. mangiferae KUMCC 18-0179 with a 100% bootstrap value. Based on the pathogenicity and morphological characteristics, the pathogen was identified as P. mangiferae, which was reported as a new species associated with mango leaf blight in Yunnan, China (Bezerra et al. 2019; Cheewangkoon et al. 2010; Crous et al. 2012, 2018; Phookamsak et al. 2019; Suwannarach et al. 2016). The newly emerging leaf blotch may become a prevalent disease of mango in future.
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