First Report of Spotted Knapweed (Centaurea maculosa) Stem Dieback Caused by Pseudomonas syringae.

In June and July of 1995, stems of spotted knapweed (Centaurea maculosa) displayed dieback at two field locations near Bozeman, MT. Dieback developed after plant bolting and during apical bud development. Seedlings and plants in the rosette stage were not affected. Symptoms included curling of stems at the tip, similar to a shepherd's crook, and brown, discolored stem tissues separated from healthy tissues by a constriction. Dissection of the stem showed disintegrated pith and blackened, infected vascular bundles. The disease was observed after a 3-week period of cool (0.5°C below normal of 15°C, 2 nights below 0°C) and wet (nine rainfall periods totaling 7.1 cm) weather and extensive bud wounding by the seed head fly, Urophora affinis Frnfd., an introduced biological control agent on spotted knapweed. Fluorescent pseudo-monads were isolated from six symptomatic stems by culturing the homogenate from surface-sterilized, macerated, symptomatic sections on King's medium B (KB) (1). A representative purified strain induced a hypersensitive reaction on tobacco (Nicotiana tabacum 'White Burley') leaves. The strain was identified as Pseudomonas syringae pv. syringae by GC-FAME analysis (TSBA [rev. 3.90]) with similarity index of 0.945; and as Pseudomonas syringae pv. aptata by Biolog (version 3.7) with similarity index of 0.840 by Microbe Inotech Laboratories, Inc. St. Louis, MO. New stems on five spotted knapweed plants were spray inoculated with bacteria from 48-h KB cultures suspended in 0.2% Silwet L-77 to a concentration of 10¹⁰ CFU per ml. Five plants were misted with 0.2% Silwet L-77 in sterile water as a check. All plants were placed in a humidity chamber for 48 h, then transferred to a greenhouse. After 30 days at 20 ± 3°C, approximately 30% relative humidity, and a photoperiod of 14 h, no symptoms were observed on inoculated or control plants. Pathogenicity was demonstrated by injecting each of 10 healthy, developing spotted knapweed buds with 0.1 ml of the strain suspended in sterile water at 10⁷ CFU/ml. Inoculated plants and controls, injected with sterile water only, were placed in a humidity chamber for 48 h, then in greenhouse conditions as before. After 14 days, only inoculated plants developed stem necrosis and dieback. Fluorescent pseudomonads were isolated from affected stem tissue 3 to 5 cm below the point of inoculation. The original strain and the strain from inoculated plants were identified as Pseudomonas syringae pv. syringae (GC-Fame analysis (TSBA [rev. 3.90]) with similarity indices of 0.941 and 0.938, respectively) by Ann Kennedy, USDA-ARS-LWMC, Pullman, WA. In the field, diseased plants had a sporadic distribution, but were more frequent in areas of high soil moisture. The combination of cold and wet conditions, coupled with bud wounds created by the seed head fly U. affinis, may be environmental requirements for spotted knapweed stem dieback caused by P. syringae in the field. This is the first report of a bacterial disease of spotted knapweed. Reference: (1) E. O. King et al. J. Lab. Clin. Med. 44:301, 1954.