Effect of Copper Treatment on Xanthomonas axonopodis pv.citri, the Causal Agent of Citrus Canker

摘要

At present, the strategy used to control citrus canker, caused by Xanthomonas axonopodis pv. citri (Xac) , is based on a management approach consisting mainly on the periodical spraying of copper based products. It is assumed that the bactericide action of copper on bacterial populations is responsible for the control of the disease. However, there are several problems associated with the use of copper products, including copper accumulation in soil and fruit, phytotoxic effects and selection of copper resistant Xac populations. The aim of this work was to evaluate the direct action of copper on Xac populations after exposure to copper oxide and copper sulfate for different periods of time. Capability of Xac to grow on culture medium and induce symptoms in grapefruit was analyzed. The role of free copper ions was estimated by using two copper products with different solubility and chemical forms. An initial inoculum of approximately 106 cfu/ml of Xac was exposed to copper contents equivalent to 1×, 3×and 5 X field applications and analyzed after 0,5, 10 and 30 minutes. Treatment for 10 min with copper level 3 x resulted in no bacterial growth on solid medium. The results show a significant effect of the exposure time and copper concentration on the reduction of Xac population and the appearance of pustules. The bactericide action of both products (copper oxide and cupper sulfate) on the same bacterial population was similar, suggesting that the bactericide effect is due to Cu2+. Surprisingly, some of the treatments which did not show Xac colonies on culture medium plates, did induce the development of symptoms, suggesting the existence of viable but non culturable bacteria.