Tip Dieback of Peaches Identified as Bacterial Canker
This spring, growers and USU Extension staff noticed a tip dieback, bud death, and flower blasting problem of peaches in various orchards. Certainly, spring frosts contributed to a portion of the problem, but the symptomology suggested that a pathogen was in play. Cultures of several specimens yielded the answer: a bacterium called Pseudomonas syringae pv. syringae, that causes bacterial canker. The disease is characterized by a complex interaction of plant host, pathogen, and environmental conditions that can make disease development difficult to predict.
Infections by the bacteria often start in the late fall just prior to winter, and symptoms appear the following spring. The bacteria persist in orchards, living as a non-pathogenic epiphyte on leaf surfaces of peaches and many other plants including most weeds. Late season rainfall spreads the bacteria from leaf surfaces to buds where the infections take place. Infections are inconspicuous in the fall and winter but become more obvious in the spring, with dead buds that often exhibit signs of gummosis. The bacterial pathogen can also infect blossoms at flowering if rainfall occurs. Infected flowers will die prematurely and symptoms of gummosis are often evident on dead or dying blossoms.
A bacterial infection can girdle the twig. Branch death caused by cankers usually spreads from the infection point to the tip (Figs. 1 and 2), but occasionally spreads downward. The disease does not cause death of the trees’ roots, like the fire blight bacteria can on apples. The disease can affect crop yield and can weaken the host tree (although 2008 losses due to this disease have not been determined for Utah). Infected trees are predisposed to more infection or infection from other diseases such as coryneum (shothole blight), powdery mildew, or other diseases.
Dead wood should be removed by pruning in late summer. (Care should be taken to disinfect pruning tools between cuts using isopropyl alcohol, 10% bleach solutions, or spraying tools with a surface disinfectant containing at least 70% alcohol.) Copper compounds, such as basic copper and/or copper sulfate, should be applied in the fall (two to three applications beginning at 10% leaf drop to just after full leaf drop) and early spring prior to bud break.
Isolates of the bacteria causing this disease have been found resistant to copper compounds in Michigan, California, and Oklahoma, and could potentially occur in Utah as well, although that has not been shown to date. We are presently isolating these pseudomonad bacteria, which are often fluorescent (Fig. 3), from infected peaches and will be testing them for resistance to copper compounds this fall.