Roses in several areas of the Treasure Valley have been found to be infected by a bacterial cane blight which has damaged or killed hundreds of bushes in home gardens. The first samples of diseased canes were brought to the attention of the U of I Extension Service in the spring of 1996, when they were diagnosed as a distinct bacterial disease and not winter kill or frost damage. Some growers claim to have seen this disease for at least the past three years, and many of them considered it to be winter damage. Infection appears to initiate at any point on the cane and progress both up and down with the infected area turning black. The tissue beneath the blackened bark is brown in color and initially moist. Isolations, followed by characterization of the organism and pathogenicity tests, have shown that this disease is caused by a pathovar (strain) of Pseudomonas syringae.

Although a bacterial disease causing blast and blight of rose buds, sepals, pedicels, and petioles was described in 1935 from Arkansas, this is the first time we have seen these aggressive cane blight symptoms in Idaho. Hundreds of rose varieties including floribunda, grandiflora, hybrid tea, and miniature type roses have been observed to be seriously affected. If the infected tissue is not too extensive, it may help to cut it out to limit the spread of the pathogen into the rest of the plant. Since it is a bacterial disease, most of the common rose fungicides in the market will not be effective. Although we do not have any information on their efficacy against this disease, copper-based fungicides may help in preventing infection and spread, especially if applied in the fall and in early spring. If you have a suspected case, send a sample to Krishna Mohan at Parma R & E Center for confirmation.

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