Study on Root and Stem Rot of Sweet Potato

Severe root and stem rot of sweet potato was first found in Yong-He and Shu-Lin, Taipei county, in Taiwan in 2006. The disease symptoms include wilting shoot, pale leaf, black stem, root rot and death. The occurrence of this disease can be traced back to at least ten years ago as described by the farmers. The causal agent of root and stem rot of sweet potato is a pathogenic bacterium according to Koch’s postulates. The pathogen was identified as Erwinia chrysanthemi based on the morphology, Gram’s stain test, Bilolog identification, and NGM differential medium test. The pathogen mainly attacked the root and stem and caused black rot of stem base, brown rot of root, and death. The symptoms were similar to those of the bacterial stem and root rot reported in United States, although in United States it majorly attacked the roots. This is the first report of this disease in Taiwan. Four varieties including “Taoyuan#2”, “Tainong#71”, “YongHe# 1” and “YongHe#2” were screened for the disease resistance “YongHe#1” had the highest resistance, while “Taoyuan#2”, “Tainong#71” and “YongHe#2” were sensitive to E. chrysanthemi. Nine bactericides at three concentrations were tested for their control effects on E.chrysanthemi by paper disc diffusion method. Among them, tetracycline has been proven to be the most effective, while oxine-copper＋copper hydroxide, cupric hydroxide, streptomycin sulfate + tetracycline, kasugamycin +copper oxychloride, thiophanate-methyl + streptomycin, streptomycin and validanmycin could inhibit the growth of E. chrysanthemi. However, kasugamycin alone was no effective to control this bacterial disease.