Disease Note.

Pseudomonas syringae, Causal Agent of a Leaf Spot on Ulmus sp.. G. W. Moorman, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. F. L. Lukezic, and R. Levine. Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Plant Dis. 72:801. Accepted for publication 10 June 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0801C.

During June 1987, small necrotic leaf spots surrounded by a yellow halo were observed on the upper surface of English elm (Ulmus procera Salisb.) leaves at University Park, PA. Lesions 0.1-3 mm long were on the underside of the leaf on veins. Bacteria isolated were identified as Pseudomonas syringae van Hall based on their production of fluorescent pigment; negative oxidase and negative arginine dihydrolase reactions; inability to hydrolyze starch, reduce nitrate to nitrite, or rot potato tissue; and ability to induce a hypersensitive response in tobacco and tomato leaves. Root suckers and twigs from English and American (U. americana L.) elms were collected and their leaves inoculated by: 1) puncture with a teasing needle dipped in a 24-hr-old bacteria culture, 2) sprayed until runoff with a suspension of washed cells (10 cfu/ml of tap water), and 3) a combination of procedures I and 2. Tap water was used in place of bacteria as controls. After inoculation, all of the plant tissues were placed in plastic bags for 24 hr. Symptoms identical to those on the original specimens developed within 6 days and reached their maximum within 10 days. P. syringae was consistently reisolated from the lesions. This is the first report of a bacterial leaf spot on elms.