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First Report of Leaf and Stem Downy Blight of Longan Seedlings Caused by Peronophythora litchii in Taiwan

Fruit downy blight caused by Peronophythora litchii Chen ex Ko et al. is an important disease of litchi (Litchi chinensis Sonn.) in Taiwan, especially in rainy seasons. Previous records indicate litchi as the only natural host of P. litchii, but this pathogen was found on seedlings of longan (Euphoria longana Lamarck) in 2000. Young seedlings of longan that had emerged in a litchi orchard near Caotun, Nantou County, showed symptoms of droopy leaves and leaf blight. Water-soaked lesions appeared on young leaves, which turned into brown, round or irregular lesions (about 3 to 5 cm long). Diseased leaves withered and collapsed eventually. Dark brown lesions were found on stems of some infected seedlings but none of the infected seedlings were killed. Also, no symptoms were found on mature leaves. The pathogen produced numerous sporangia on sporangiophores on diseased leaves under humid conditions. The disease on young seedlings was observed again in another litchi orchard at Caotun, Nantou County, in 2003. P. litchii was consistently isolated from diseased leaves. Two isolates from colonized longan seedlings, Tari 20250 collected in 2000 and Tari 23301 collected in 2003, were used for further studies. Both isolates produced large numbers of sporangia on long sporangiophores when cultured on 5% V8 agar (5% V8 juice, 0.02% CaCO₃, and 1.5% agar). Sporangia produced on the same sporangiophores matured almost simultaneously. Sporangiophores 240 to 1,600 μm (mean 623 μm) branched dichotomously two to eight times. Sporangia were oval or lemon-shaped with semispherical papilla and deciduous with very short pedicels (2 to 5 μm). The dimension was 25 to 55 (35.25) × 15 to 27.5 (21.2) μm for sporangia and 0.5 to 1 (0.55) μm for pedicels. The length/breadth (L/B) ratio of sporangia was 1.3 to 2.14 (1.67). Both isolates produced numerous oospores on 5% V8 agar cultures in darkness. Artificial inoculation tests were done by spraying 5 mL of sporangial suspension (1,000 sporangia/mL) on each longan seedling without wounding. Results showed that both longan isolates of P. litchii were pathogenic on young longan seedlings, causing symptoms similar to those observed on leaves and stems of naturally infected longan seedlings in litchi orchards. Also, both longan isolates of P. litchii caused downy blight on fruits of litchi (L. chinensis var black leaf) by artificial inoculation tests. Moreover, a P. litchi isolate from litchi caused symptoms of leaf blight on young longan seedlings. P. litchii was reisolated from the infected longan tissues. The ribosomal internal transcribed spacer (ITS) sequence confirmed that the longan isolate Tari 20250 (GenBank Accession No. JQ814693) was 100% identical to other P. litchii isolates (GenBank Accession Nos. Gu111613 to Gu111615). To our knowledge, this is the first report of longan as a natural host of P. litchii. The study also suggests that P. litchii on volunteer longan seedlings in litchi orchards may be a potential source of inoculum for fruit downy blight of litchi.

References: (1) C. C. Chen. Special Publ. Coll. Agric., Natl. Taiwan Univ. 10:1, 1961. (2) W. H. Ko et al. Mycologia 70:380, 1978.