Disease Note.

A New Bacterial Disease of Arugula in California. S. T. Koike, University of California Cooperative Extension, Salinas 93901 . R. F. Smith, University of California Cooperative Extension, Salinas 93901; and A. M. Van Buren and D. A. Maddox, STA Laboratories, Longmont CO 80501. Plant Dis. 80:464. Accepted for publication 1 February 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0464E.

In January and February of 1995, a leaf spot disease became widespread on commercial arugula (Eruca saliva Mill.) grown in three coastal counties in California (Monterey, San Benito, and Santa Cruz). Extensive winter and spring rains encouraged the development of the disease, which resulted in significant loss in crop quality. Arugula leaves, intended for use in packaged salad mixes, were severely diseased in the fields and in many cases were not harvested. Initial symptoms were small (<2 mm diameter), water-soaked spots on both fully-expanded and younger leaves. As disease developed, lesions enlarged and became angular and tan in color. Older lesions were desiccated and had purple margins. A blue-fluorescent pseudomonad was consistently isolated when diseased tissues were macerated and streaked onto King's medium B. Representative strains were oxidase negative, arginine dihydrolase negative, and levan positive. Strains induced a hypersensitive reaction on tobacco (Nicotiana tabacum L. ‘T. R. Maddle') but did not rot potato slices. The bacterium associated with the disease was identified as Pseudomonas syringae. Fatty acid analysis of representative strains indicated P. syringae from arugula had a high similarity (index 0.923 or higher) to P. syringae pv. maculicola. Pathogenicity was demonstrated for each of 16 representative strains that were grown in nutrient broth shake cultures for 48 h and misted onto groups of 3-week-old arugula seedlings. Control plants were misted with nutrient broth only. After 8 to 10 days in a greenhouse, leaf spot symptoms developed on all plants inoculated with the 16 strains and P. syringae was reisolated from lesions. Reisolated strains were recharacterized as P. syringae. Control plants remained symptomless. The experiment was repeated with 16 isolates recovered from the first inoculated plants and results were similar. In another experiment, water suspensions were made of P. syringae from arugula, P. syringae pv. syringae from bean, and P. syringae pv. maculicola from cauliflower; these suspensions, along with a water control, were swab-inoculated onto arugula seedlings. After 10 days, symptoms developed only on the plants inoculated with arugula isolates and P. syringae was reisolated from these plants. Pseudomonas syringae could not be reisolated from the other inoculated plants. This is the first report of a bacterial disease of arugula.