Two Pathogenic Bacteria, Erwinia rhapontici (Millard 1924) Burkholder 1948 and Pseudomonas marginalis pv. marginalis (Brown 1918) Stevens 1925, Causing a Soft Rot of Onion

Abstract

During cooler seasons, a new bacterial disease of onion plants has recently be found in Tokai and Kinki areas in Japan. A small water-soaked lesions on the leaf blades appeared as an initial symptom, and the symptom developed into streak along the leaf veins, resulting from the enlargement and conjugation of the lesions. The streaks progressed to a part of leaf sheaths, and eventually leaf sheath and scale bulb tissues decayed completely with the disease development.
Thirty-one bacterial isolates obtained from the infected plants were divided into 2 genera from their diagnostic charaters. Twenty isolates of them were identified as Erwinia rhapontici (Millard 1924) Burkholder 1948 on the basis of their bacteriological properties. On the other hand, 11 isolates were also determined as Pseudomonas marginalis pv. marginalis (Brown 1918) Stevens 1925. These are the first description of bacteria which cause the diseases on onion plants.
These isolates possessed an ability to affect onion tissues by needle-prick and carborundum inoculations. The symptoms caused by E. rhapontici were hardly distinguished from those of P. marginalis pv. marginalis, although they were scarecely different in discoloration of soft-rotted host tissues. In addition, it was observed that most of naturally infected plants were attacked by these 2 pathogenic bacteria at the same time. From the reasons mentioned above, we propose to use a single common name “soft rot of onion” for the both bacterial diseases, hereafter.