Leaf Blight on Curcuma wenyujin Caused by Phoma matteucciicola in China

Curcuma wenyujin Y. H. Chen et C. Ling is a traditional valuable Chinese medicinal herb plant. In July 2011, symptoms of leaf blight were observed on nearly 80% of a C. wenyujin planting in Hongqi (Hainan Province). Initially, small yellowish-white water-soaked lesions surrounded by yellow halos were observed on the lower leaves of affected plants. Lesions later developed on the upper leaf layer, and leaves turned light brown as the tissues desiccated. Black spots appeared on the upper side of leaf lesions under hot and humid conditions. Finally, the foliage wilted and the plant died. To isolate the pathogen, leaves with typical symptoms were collected and washed with tap water. Small pieces of tissue (4 × 4 mm) were cut from lesion borders and were surfaced sterilized in 75% ethanol for 10 s followed by 1 min in 0.1% HgCl₂, rinsed three times with sterile water, plated on potato dextrose agar (PDA), and incubated at 28°C. Thirty-seven isolates were obtained from symptomatic leaf samples. All the isolates were similar in morphological characteristics, and two isolates (HQ-1 and HQ-6) were selected for further characterization. Seven days after incubation, the colonies (6.5 cm in diameter) with white aerial mycelia and a canary yellow color on the reverse side were observed. Globose ostiolate pycnidia (67.16 to 156.75 [103.94] µm) were glabrous, at first olivaceous, then olivaceous-black, usually with one nonpapillate ostiole. Conidia were unicellular, 7.52 to 10.24 (9.07) × 2.43 to 3.44 (3.16) μm, with two polar guttules. Chlamydospores (5.47 to 7.18 [6.31] × 7.77 to 13.53 [10.04] μm), single or in chains, brown, usually developed in aerial mycelium. Based on morphological characteristics, the fungus was identified as Phoma sp. (Boerema et al. 2004). Three gene regions were sequenced from the DNA extracted from the two chosen isolates. These included the ribosomal internal transcribed spacer (ITS) region amplified with the universal primers ITS1/ITS4, β-tubulin (tub2) gene amplified with primers Bt2a/Bt2b, and 28S large subunit (LSU) gene amplified with primers LROR and LR7. The ITS sequence (MG902891) had 95% identity to P. matteucciicola (GU237812). The β-tubulin sequence (MG902893) showed 90% identity to P. matteucciicola (GU237627). The LSU sequence (MG902892) showed 99% homology with P. matteucciicola (GU238100). Based on morphology and resequencing, the pathogen was identified as P. matteucciicola. Pathogenicity tests were conducted on 20 healthy plants by placing 5-mm-diameter mycelial plugs obtained from 8-day-old cultures on the leaves. The same number of plants treated with PDA plugs served as controls. All plants were incubated at 28°C and 100% relative humidity. After 5 to 7 days, symptoms identical to those on field samples developed on all the pathogen-inoculated plants, whereas the control plants remained asymptomatic. The pathogenicity tests were carried out three times. The same fungus was reisolated from the infected plants, and the identity was confirmed by cultural and morphological characteristics, confirming Koch’s postulates. Curvularia clavata has been reported on C. wenyujin in China (Chen et al. 2013), Colletotrichum curcumae in China (Li et al. 2016), and Athelia rolfsii in China (Ma et al. 2017). To our knowledge, this is the first report of P. matteucciicola causing leaf blight in the world. C. wenyujin is widely grown in Hainan, and the occurrence of this disease needs to be considered in C. wenyujin production.