Diagnosis and Control of Bacterial Leaf Spot on Poinsettia

Poinsettia (Euphorbia pulcherrima) is an important ornamental plant in the world as well as in Taiwan. It is mainly cultivated for the domestic flower market with the size of 3 and 5-inch pots in Taiwan, and approximately 1.5 million pots of poinsettia are produced every year. They were propagated by cutting from June to August, and their color-turning of red bracts initiates in mid-September ready for the sale during Christmas holidays. The newly surfaced disease, leaf spot of poinsettia, caused by Xanthomonas axonopodis pv. poinsettiicola (Xap) was first discovered in the Dongshih Township, Taichung in November 2005. It showed brown spots surrounded by yellow haloes on the leaves. The spots enlarged rapidly, coalesced into necrosis and finally lead to early defoliation. The disease often happened in the summer and retarded the vegetative growth period, which significantly reduced the poinsettia production and lost the best timing for selling. This study was dedicated to develop efficiently diagnostic and control measures for this disease to provide assistance for the poinsettia industry. The selective medium, Xan-D, was used for the culture and differentiation of Xanthomonas identified by the wet-shiny, convex and yellow-green colonies surround with milky and clear zones. For the further molecular detection, the specific primers of Xap were also designed for PCR based on the published Xap genomic sequences including estA, hrpF, gumD, gyrB and rpoD genes. The primer pair rpoDⅡ-b showed the better specificity to Xap. In the control trials, several chemicals such as penicillin, streptomycin + oxytetracycline, chitosan and nanoscale titanium dioxide were used for either prevention or therapy tests in the greenhouse using totally 7 important cultivars of poinsettia. The results of therapy tests showed that the streptomycin + oxytetracycline is most effective for this disease control especially in the initial stage of disease development. The results of prevention tests showed that the chitosan had good protective effect when it was used before Xap infection. Various cultivars of poinsettia showed different susceptibilities for Xap, and all of tried control measures seemed to be not effective in those cultivars categorized into high susceptibility. The application of adequate bactericides or protectants (ex. chitosan), wind-proof facilities and dipping irrigation system (instead of spraying) might be recommended for the growers to improve the control of this disease.