Plant Pathology Science

Walnut is one of the most important fruit trees in Iran, which is always exposed to pathogens, especially bacteria. The three most important bacterial diseases of walnut in Iran are bacterial blight (Xanthomonas arboricola pv. juglandis), skin cankers (Brenneria nigrifluens, Brenneria rubrifacians) and crown gall (Agrobacterium tumefaciens). Bacteriological characteristics and pathogenicity of pathogenic bacteria, methods of managing these diseases, along with suggestions for identifying resistant genotypes of walnut trees by molecular methods are described in this article.

Agrobacterium tumefaciens is capable of causing distractive disease of crown gall in a wide range of dicotyledonous plants and causes great economic impact in its hosts. This soil-dwelling bacterium can survive as a saprophyte in soil and plant debris for a long time. When the host plant is present, the bacterium is absorbed into the plant through the root secretions from the wounds and enters the plant through it. The pathogenic bacteria introduce a part of its Ti plasmid, called T-DNA, into the plant cell. Integration of the T-DNA to plant cell genome results in expression of the encoded oncogenes and an increasing the production of phytohormones in cells. Overproduction of auxin and cytokinin leads to cells hyperplasia and hypertrophy, which results in the gall formation on the root, crown, and branches of infected plants. As regards the wide host range, high economic impact, and difficult control of this disease, in this article phenotypic, genetic and pathogenicity characteristics of bacteria have been studied, as well as biology and effective strategies of integrated disease management are presented.