First Report of Bacterial Foot Rot of Rice Caused by a Dickeya zeae in China

Abstract

A bacterial disease of rice, bacterial foot rot, was found in Guangdong Province, China in September 2011, with an incidence about 10%. The typical symptom was a dark brown decay of the tillers. In the early stages of the disease, a brown sheath rot seemed to spread from the ligulae regions. The lesions quickly extended down to the nodes, culms, and finally to the crowns. Neighboring tillers of the same crown were invaded systemically, causing foot rot symptoms. A soft rot with an unpleasant odor developed in young tissues of infected tillers. In the advanced stage, many tillers decayed, so that entire diseased plants could easily be pulled from the soil. Six diseased samples were collected and bacteria were isolated from the edge of symptomatic tissues, after samples were sterilized in 0.3% NaOCl for 10 min, rinsed in sterile water three times, and placed on nutrient agar (beef extract 3 g, yeast extract 1 g, peptone 5 g, glucose 10 g, agar 16 g, distilled water 1 L, pH 6.8 to 7.0). For identification, a total of 12 representative isolates were selected. All strains were Gram negative, grew at 37°C, were positive for indole production, and utilized malonate, glucose, and sucrose, but not glucopyranoside, trehalose, or palatinose. Biolog identification (Version 4.20.05, Hayward, CA) identified isolate EC1 as Pectobacterium chrysanthemi (SIM 0.827), which has since been transferred to genus Dickeya. PCR was used to amplify the 16S rDNA gene with primers 27f and 1492r, the dnaX gene with primers dnaXf and dnaXr (2), and the gyrB gene with primers gyrBf1 (5'-ATGTCGAATTCTTATGACTCCTC-3') and gyrB-r1 (5'-TCARATATCRATATTCGCYGCTTTC-3'), which were designed based on published gyrB gene sequences of genus Dickeya. A BLASTn search of all three loci [16S rDNA (JQ284040), dnaX (JQ284041), and gyrB (JQ284042)] revealed that EC1 had 100% sequence identify to Dickeya zeae [16S rDNA (AB713560), dnaX (AB713593), gyrB (AB713635)]. Pathogenicity tests were conducted by injecting 10 rice seedlings with 100 μl of the bacterial suspension (1 × 108 CFU/ml) in the stem base, and an additional 10 rice seedlings were injected with 100 μl of sterile water as negative controls. Inoculations were carried out in a greenhouse at 28 to 32°C and 90% relative humidity. Foot rot symptoms identical to those described above were observed after 7 days on inoculated plants, but not on the negative controls. The bacterium was reisolated from the lesions and had 100% sequence identity for all three loci to EC1. Previously, similar symptoms were reported on rice in Guangdong province of China, and the causal agent was identified as Erwinia chrysanthemi (1). To our knowledge, this is the first report of D. zeae causing foot rot disease on rice in China.