Identification Of The Pathogen Causing Black Ring Disease In Radish

A novel disease infecting radish (Raphanus sativus L. cv weixian) in Weifang City, Shandong Province was observed and becoming more and more economically important. The disease was not easily identified by simply examining the surface of radish roots and can only be confirmed by cutting the radish roots. The primary symptoms on the radish root crosscut section were dark or black dots that mainly occurred in the cortex of fleshy root and form a"dark ring". Local farmers designated it as black-ring disease. Dark or rotten vascular bundle will be observed is the radish roots straightly cut. If the disease is serious the whole radish will rot from the top to the bottom. The whole vascular bundle will become dark and abscission layer will be observed.In this article we isolated pathogen of this disease and identified from morphological, physiological and biochemical properties and 16S rRNA sequences. The main results read as follows:1. Totally sixteen bacterial strains were isolated and five of them were selected randomly for pathogenecity tests with different methods. The inoculated radish'incidence of the"black-ring"disease was ranged from about 61.1% to 94.1% which depends on different inoculation methods, of inoculated radishes plants showed symptom of black ring. The results proved that the disease was caused by bacteria infection.2. The typical bacterial strain W7 was further identified. It's morphological, physiological and biochemical properties were similar to P. fluorescens and P. putida.3. The 16S rRNA sequence of W7 shared highest homology of 99.80% with those of P. putida (accession numbers ef529517). They formed a common branch in the phylogenetic tree constructed according to 16S rRNA sequences.4. The results of host range test show that the pathogen causing black ring in Weixian radish can result in same symptom on white radish and green radish, but it can't infect the tested cultivars of Chinese cabbage, cabbage, mustard and other vegetable.5. The pathogen can cause black-ring disease through damage on roots (with incidence of 85.0%), rhizosphere irrigation (61.1%), and leaf spraying (11.2%).So far, P. Putida strains are as a biocontrol agent or biodegradable bacteria worldwide. This is the first report that P. Putida can be plant pathogen, which might have great significance to research on plant pathology, and biological control. The isolate W7 showed some inhibitory effects against R. cerealis and F. graminearum, which might be served as indirect evidence on the taxonomic status of the pathogen.A bacterial strain, Lyc2, was isolated from tobacco rhizosphere and showed remarkable inhibition to the mycelial growth of multiple pathogenic fungi in dual culture. In greenhouse, strain Lyc2 showed 48.8% efficiency to control cotton seedling damping-off. In solution culture experiment, compared with control, Lyc2 significantly increased the fresh weight and dry weight of cotton seedlings. The morphological, physiological and biochemical characteristics of Lyc2 were similar to those of Burkholderia cepacia. Homology analysis of 16S rRNA and ITS sequences showed that Lyc2 was closely related to B. cepacia. Further results of recA gene sequence comparis- on showed that Lyc2 belonged to B. pyrrocinia, genomovar IX of B. cepacia complex.