First report of taro black rot caused by Ceratocystis fimbriata in China

Colocasia esculenta is the most widely cultivated species of taro in China, because it does not require a large growing area and is relatively easy to obtain and maintain. In 2005, a hitherto unseen post-harvest disease of taro, a superficial black rot, was observed at a Chenglong vegetable company storage barn in Chenggong County in Kunming, Yunnan, China (Fig. 1). A fungus was consistently isolated from rotted corms when diseased tissue was incubated between slices of fresh carrot root (Moller & DeVay, 1968).

The fungus was identified as Ceratocystis fimbriata using morphology and molecular features. The ITS region of rDNA was sequenced using the procedures of Baker et al. (2003). Analysis of ITS sequence data (GenBank accessions AM293382-AM293383) showed that the isolates were 97% homologous to those from diseased taro in Brazil (GenBank accessions AY526286-AY526290) by BLAST analysis; and 96% homologous to isolates on Colocasia spp. from Hawaii and to Chinese herbarium material that was over 70-years-old (GenBank accessions AY526304-AY526307). Hawaiian and Chinese material was considered by Harrington et al. (2005) to represent a previously uncharacterized taxon in the Asian clade of C. fimbriata, while Brazilian isolates fall into the ‘Latin American’ clade.

Of thirteen isolates derived from single ascospores, five were tested for their ability to cause corm rot. In laboratory tests, taro corms were inoculated by wounding the corm surface and placing a 0.2 µl aliquot of a spore suspension of the fungus (2.0 x 10⁵ conidia per ml) over the wound; inoculated corms were then stored at 20-26ºC. All five isolates tested caused black rot symptoms identical to those originally observed; no rot was observed in controls inoculated with water alone. In greenhouse tests using 3 isolates, inoculation was done using the method of Harrington et al. (2005): a conidial suspension of each isolate was diluted to 2.0 × 10⁵ spores per ml, and 0.2 µl was injected into wounds on nine pseudopetioles on each of three two-month-old plants of C. esculenta. Control solutions consisted of water applied to a sterile MYEA plate. As disease progressed, inoculated leaves withered and dried; by 3 weeks, all 9 inoculated leaves had died (Figs 2 & 3), while control plants showed no symptoms. The fungus was successfully re-isolated from inoculated pseudopetioles.

Although this disease has been reported from Brazil and the United States (Hawaii) and the organism is known from China from one herbarium specimen from 1949 (Thorpe et al. 2005), this is the first report of the disease from China.