We first reported Alternaria heveae (E.G. Simmons ) to be the pathogen that caused black leaf spot of rubber tree (Hevea brasiliensis Muell. Arg) in Heikou county in July 2014 (1). Black leaf spots that resembled the symptoms caused by A. heveae were observed on the leaves of rubber trees of the whole propagule collection nursery in Jingping County (22°68′ N and 103°05′ E) of Yunnan Province. Black foliar spots (0.1 to 2 mm in diameter) surrounded by a yellow halo with lesions slightly sunken on the leaf surface were observed. To confirm whether the disease was caused by the same pathogen, 5-mm2 sections were removed from the leading edge of the lesion and were surface-sterilized in 75% ethanol, air-dried, plated on potato carrot agar (PCA), and incubated at 28°C in the dark. Colonies of the fungus on PCA had round margins and little aerial mycelia with gray-black coloration after 6 days of growth on PCA (2). Medium brown conidia were found to be in short chains of two to eight spores, ovoid, obclavate, and obpyriform, with or without a short conical or cylindrical-shaped apical beak. Conidia ranged from 22.5 to 67.5 μm long (mean 39.9 μm) × 10 to 15 μm wide (mean 12.5 μm; 100 colodia were measured), with three to six transverse septa and zero to three longitudinal or oblique septa. Morphological characteristics matched the descriptions of A. alternata [(Fries) Keissler] (4).The ITS1-5.8S-ITS2 region of one single-spore isolate, Ah02JP1, was amplified with primers ITS1 and ITS4. The PCR product was sequenced directly and deposited in GenBank (Accession No. KM111289). A BLAST search of the GenBank database revealed 100% similarity with A. alternata isolates KJ829535.1, KJ677246.1, and KF813070.1. Therefore, the pathogen was identified as A. alternata on the basis of its morphological characteristics and ITS sequence. Pathogenicity of a representative isolate, Ah02JP1 was confirmed using a field rubber tree inoculation method. Three rubber plants (the clone of rubber tree Yunyan77-4) were grown to the copper-colored leaf stage. Leaves were spray-inoculated (104 conidia per milliliter spore suspension) until drops were equally distributed using a manual pressure sprayer. Three rubber plants sprayed with sterile distilled water were used as controls. After inoculation, the plants were covered with plastic bags to maintain high relative humidity. The plastic bags were removed 2 days post-inoculation (dpi), and the plants were monitored daily for symptom development. Five days post-inoculation, spots similar to the original ones seen on the field trees developed on all inoculated leaves, while control leaves remained symptomless. A. alternata was re-isolated from spray-inoculated leaves, confirming Koch's postulates. A. alternata has been reported as the causal agent of leaf blight of rubber tree in India, which initially appeared as minute spots on leaves and enlarged with the growth of the leaves (3). However, in the present study, the symptoms (black leaf spots) remained small over time after inoculation. To our knowledge, this is the first report of A. alternata on rubber tree in China. Correct identification of pathogens is essential for disease management strategies. This report will establish a foundation for the further study of Alternaria alternata to address the disease effectively.
References: (1) Z. Y. Cai et al. Plant Dis. 98:1011, 2014. (2) E. Mirkova. J. Phytopathol. 151:323, 2003. (3) C. B. Roy et al. J. Plantation Crops 34:499, 2006. (4) T. Y. Zhang. Page 32 in: Flora Fungorum Sinicorum, Vol. 16: Alternaria. Science Press, Beijing, 2003.