First Report of Pseudostem Black Spot Caused by Pestalotiopsis microspora on Tsao-ko in Yunnan, China

Tsao-ko (Amomum tsao-ko Crevost et Lemaire) is a Zingiberaceous plant, widely cultivated in southwest China, including Yunnan, Guangxi, and Guizhou, as well as northern Laos and Vietnam bordering China (Wu 1981). As of 2010, production on >56,900 ha in Yunnan accounted for >90% of the production in China. In August 2013, 30% of the plants surveyed on 100 ha of tsao-ko production in Pingbian County, Yunnan, showed symptoms of a pseudostem disease. Initially, oblong, water-soaked spots appeared on the green pseudostem, then enlarged and turned brown, then turned black in the center and brown on the margin. Infected pseudostem tissues were surface-sterilized with 5% sodium hypochlorite for 3 min, and rinsed 5 times sterilized water, then transferred to humid, sterilized filter paper in petri dishes at 28[degrees]C for 3 to 5 days. Individual fungal spores were picked from infected tissues using a stereo microscope, and inoculated onto potato dextrose agar (PDA). White, circular colonies were [< or =]70 mm in diameter after 5 days at 25[degrees]C, with black, oil-droplet-shaped conidiomata produced over mycelial mats, and many conidiomata formed after 15 days. Conidia were fusiform, straight or slightly curved, 4-septate, 18.6 to 24.4 (22.8) [mu]m x 5.2 to 6.4 (5.8) [mu]m (n= 50). The apical, colorless cell had 2 to 2 appendages (mostly 2) ranging from 12.3 to 33.8 [mu]m long (n= 50 conidia); the basal colorless cell had 1 appendage from 3.2 to 5.6 [mu]m long; the 3 median cells were light brown to dark brown, and the middle cell was darker than the other 2 cells. A spore suspension (1 x 10 super(6) conidia/ml) was prepared from each of 4 isolates by harvesting conidia from 4 two-week-old agar cultures grown in the dark at 25[degrees]C. A 30-[mu]l drop of spore suspension of each isolate was placed on each of 3 healthy, annual pseudostems of a 5-year-old plant (5 plants inoculated/fungal isolate) in the field at a relative humidity of 80 to 95% in Pingbian County. For the control treatment, each of 3 healthy pseudostems on each of 5 plants were treated similarly with sterilized water. At 15 days after inoculation, symptoms similar to those observed in the field had developed on all inoculated pseudostems but not on controls. Fungal cultures reisolated from the symptomatic pseudostems as described above were identical to the original isolates, completing Koch's postulates. The fungus was not reisolated from control pseudostems. The internal transcribed spacer (