6.18
BACTERIAL CANE BLIGHT OF ROSES

SK MOHAN and VP BIJMAN

University of Idaho, Research and Extension Center, Parma, Idaho 83660, USA

Background and objectives
During the spring of 1996 and 1997, numerous rose (Rosa spp.) plants belonging to more than 50 cultivars of hybrid tea, grandiflora, floribunda and miniature types in south-western Idaho were found affected with a cane blight symptom, causing severe damage or death of entire plants. Symptoms appeared to initiate at any point on the canes, but commonly at the base of a vegetative bud, as brown, irregular, necrotic lesions on the bark, which turned dark purple to black with well defined margins. The lesions developed along and around the cane, often involving a major part or even the entire cane. Vegetative buds on the affected parts turned brown, wilted and died. Occasionally, the lesions girdled the cane and the distal part above dried up. The tissue beneath the blackened bark was reddish brown to dark brown, and moist in the early stages. Literature search revealed no report of a rose disease with symptoms similar to this cane blight. In the past, such symptoms have been considered by the local gardeners and nurserymen to be the result of winter/cold injury. However, observations on the pattern of incidence, symptom characteristics and progression suggested involvement of a biotic agent. The objective of this study was to determine the aetiology of this disease in roses and identify the pathogen involved.

Materials and methods
Isolations from symptomatic tissues were made on King's medium B (KB). Plates were incubated at 24C. Single colonies were selected, purified and subjected to Gram staining and standard LOPAT tests. Lemon (cv. unknown) and unripe cherry (cv. Bing) fruits were inoculated by pricking with a needle contaminated with 24-h-old culture of the bacterium, and were incubated in plastic boxes at 22-24C. Cultures were also characterized using the Biolog bacterial identification system (Biolog, Hayward, CA) and cellular fatty acid analysis. For pathogenicity tests, young shoots of rose (cvs Don Juan and King's Ransom) were injured by pin-pricks and sprayed with a bacterial suspension containing approximately 107 c.f.u./ml. Similar shoot-prick inoculations with the rose isolate were made on lilac cv. Ellen Wilmott. A known Pseudomonas syringae pv. syringae strain from lilac was similarly inoculated on lilac and the two rose cultivars mentioned before. All the inoculated plants were incubated at 20C for 48 h in a dew chamber and then transferred to greenhouse benches at 22-25C.

Results and conclusions
Microscopic examination of the symptomatic tissue revealed profuse bacterial streaming from cut edges of tissue sections mounted in water. Isolations on KB consistently yielded cream-coloured, glistening, translucent, circular colonies which produced a diffusible bluish-green fluorescent pigment. The bacterium was a Gram-negative rod, levan positive, oxidase negative, arginine dihydrolase negative, did not rot potato but caused hypersensitive reaction in tobacco and sunken lesions on lemon and cherry fruits. Results of Biolog and cellular fatty acid analysis indicated high similarity of the rose isolate to P. syringae. Inoculated rose shoots developed slightly sunken, black, necrotic areas within 5 days after inoculation. The rose isolate caused limited necrosis around inoculated sites on lilac. The lilac strain did not produce any symptoms on rose.

Based on the cultural, morphological and bacteriological characteristics and pathogenicity, the bacterium causing the cane blight of roses was identified as an undetermined pathovar of P. syringae. White [1] in 1932 described a bacterial disease on greenhouse roses causing spots on leaves, calyces and young stems, and a petal decay. Rosen [2] in 1935 reported P. syringaecausing rose blast with symptoms of blackish-brown, dead spots and streaks on flower receptacles, calyx lobes, flower stalks and only rarely on petioles. The symptoms of cane blight observed in this study are, however, distinctly different from the symptoms described in these earlier reports.

References
1. White RP, 1932. New Jersey Agricultural Experiment Station Nursery Disease Notes 5, 1-2.
2. Rosen HR, 1935. Journal of Agricultural Research 51, 235-43.