First Report of Smut of Saccharum arundinaceum Caused by Sporisorium sorghi in Yunnan, China.

Abstract

Saccharum arundinaceum (Retz.) is a wild species of S. officinarum with good resistance to pests, diseases, drought, cold, and infertile soils, and has favorable tillering potential (4). Hybridization of S. arundinaceum with sugarcane may enable favorable characteristics of S. arundinaceum to be exploited, broadening the genetic base of sugarcane breeding. In May 2009, symptoms of a disease were observed on plants of S. arundinaceum at the National Nursery of Sugarcane Germplasm Resources in Yunnan Province, China, on approximately 10% of 120 plants within three germplasm collections of S. arundinaceum. The initial symptom was dwarfing, resulting in the spike heading 1 to 2 months earlier than on healthy plants. As infection progressed, the spikelet of each diseased plant became full of the black, powdery mass of fungal spores enclosed in an off-white membrane. Finally, the entire spike became severely infected. A Sporisorium sp. was isolated from diseased spike tissues that were surface-sterilized with 70% alcohol, then with 0.1% mercury chloride, dipped in sterilized water three times for 3 min each time, dried on sterilized absorbent paper, and placed onto potato dextrose agar (PDA) at 27 ± 2°C in the dark. Nine fungal isolates were identified as Sporisorium sorghi Ehrenberg ex. Link based on sorus morphology as well as spore morphology and color (1). Sori were cylindrical or oval and 2.5 to 12.0 mm in diameter. Spores were spherical, light olive-brown or black, and 5.0 to 9.0 × 4.0 to 8.5 mm. Preliminary morphological identification of the fungus was confirmed by PCR assay using genomic DNA extracted from the mycelia of pure cultures of each of nine isolates, which generated a 750-bp amplified region of the internal transcribed spacer (ITS) region of rDNA using ITS1/ITS4 universal primers (3). The ITS region was then sequenced (GenBank Accession No. JX183795), and displayed 98% similarity with the ITS sequence of an isolate of S. sorghi from Sorghum bicolor from each of France (AF038828.1) and Germany (AY740021.1). A pathogenicity test was completed with the S. arundinaceum isolate of S. sorghi by spraying 20 ml of a spore suspension (104 conidia/ml) onto each of 10 stems (2) of S. arundinaceum in a field in February 2010. The spore suspension was prepared from 30-day-old cultures growing on PDA. Stems were inoculated approximately 4 cm above the ground, where each stem was cut with sterilized shears. For the control treatment, each of 10 cut stems was inoculated with 20 ml of sterilized water. Three replications of 10 stems were used for each treatment. Approximately 3.5 months after inoculation, symptoms began to appear on 18 of the 30 stems (three replicates of 10 stems) inoculated with the S. sorghi spore suspension, when heading occurred on the stems. Five months later, each entire spike exhibited symptoms. No symptoms were observed on control spikes of stems inoculated with water. S. sorghi was reisolated onto PDA from the tissues of diseased spikes on inoculated stems, and the reisolates were identified by PCR assay using ITS1/ITS4 universal primers, as described above. S. sorghi was not isolated from the spike tissues of the control plants. To our knowledge, this is the first report of smut on S. arundinaceus caused by S. sorghi in Yunnan Province, China.