Identification and characterization of Pseudomonas syringae pv. mori affecting white mulberry (Morus alba) in Poland

Abstract

The white mulberry (Morus alba L.) is particularly valued in Asia and Europe because its leaves serve as fodder for the mulberry silkworm (Bombyx mori L.) in the production of raw silk (sericulture). However, the health of silkworm caterpillars producing silk fibres, and the quality of their cocoons depend strongly on fodder quality. Moreover, this plant is also valuable in the food and pharmaceutical industries, where high-quality herbal material is required. In one of the Polish mulberry plantations, trees with chlorotic, angular leaf spots and blotches were observed. The disease multiplied in spring in humid conditions and spread to developing blossoms and leaves. Leaves were collected during two growing seasons (2017–2018) from 18-year-old trees cultivated at the Experimental Farm of the Institute of Natural Fibres and Medicinal Plants in Petkowo, Poland (52°12′40″N 17°15′31″E). Leaf lesions exhibited bacterial streaming, and bacterial colonies were readily isolated on tryptic soy agar. Biochemical (Biolog Gen III system v. 2.8.0), molecular (16S rDNA and gyrB genes), and pathogenicity testing in the greenhouse identified the causal agent as Pseudomonas syringae. This bacterial species was the most abundant in each of the sample tested. To our knowledge this is the first description and characterization of P. syringae affecting mulberry trees not only in Poland but throughout Europe. Our aim was to point out the importance of plant diseases caused by bacteria. This finding serves as a warning for the phytosanitary services in Europe and for the sericulture industry that a new disease threat has appeared for European mulberry crops.