First Report of Leaf Spot Disease Caused by Chaetomium sp. on Hevea brasiliensis in Yunnan, China

Abstract

In May 2016, some irregular pink spots with gray necrosis in the center and a yellow halo around the edge were found to develop on rubber (Hevea brasiliensis) leaves in a nursery in Jinghong, Yunnan. Some infected leaves were washed with tap water, dried and sterilized with 75% alcohol for 30 s, and subsequently dried and cut into pieces of 0.2 to 0.3 cm. Finally, the heathy pieces neighbor to symptomatic leaf tissues were placed on potato dextrose agar (PDA) medium at 25°C for 5 days. Then some pure cultures identical to the colony and spores could be obtained after transferring several times. After 15 days, healthy leaves of three plants were selected for pathogenic tests in the nursery. Spore suspension (1 × 10⁶ spores/ml) was made from a pure culture 10 days after inoculation on PDA. Spore suspension (10 ml per leaf) was sprayed on the surface of the selected 10 leaves per plant; in the meantime, an equal amount of sterilized water was sprayed on another 15 healthy leaves as the controls, and then all leaves were covered respectively with a fresh bag to remain moisturize for 3 days. All the bags were removed on the 4th day after inoculation. Some light yellow spots developed, turned into red on the 7th day, and then turned into pink on 10th day. The symptoms of inoculation were consistent with those of collection in the field but not on the controls. The fungus identical to the previous colony and conidia could be reisolated from the symptomatic leaves but not from the controls. The mycelium cultured at 25°C for 5 days is light and yellow on the back, and ascospores began to appear in the colonies after 2 days. Ascus, dark brown to black, superficial, elliptic or subglobose, approximately 151 to 316 × 142 to 312 μm, the ascomal hair, setaceous, apex acuminate, separated, unbranched, brown, with smooth surface; lateral hairs are shorter, setaceous, brown, unbranched, apically acuminate, with gradually shallow color; ascus wall dispels in early stage, ascospore is brown, oval shaped to elliptic, with the size of 4.8 to 5.7 × 3.2 to 5.5 μm, one end slightly pointed with a top bud hole, flat on both sides. The internal transcribed spacer (ITS) sequence was extracted and was amplified by the primers ITS1 and ITS4, and the 563-bp product of the ITS (GenBank accession no. KX869740) showed 100% sequence identity to Chaetomium funicola (GenBank accession no. KR909159.1). Therefore, both molecular and morphological observations indicated that the pathogenic fungus was mostly similar to C. funicola (Zhang 2013). C. funicola was reported to cause chromoblastomycosis on a man’s skin in western Panama (Piepenbring et al. 2007). Chaetomium globosum could cause leaf spot on pomegranate in Yunnan, China (Guo et al. 2016). As far as the authors know, this is the first report of rubber leaf spot caused by Chaetomium sp. in Yunnan, China, as well as in the world.