Pseudomonas syringae Leaf Blight, a New Disease of Kalmia latifolia

In spring and fall of 1997, and in February 1998, Kalmia latifolia cv. Olympic Fire plants with severe leaf blight symptoms were submitted to the Oregon State University Plant Clinic from a commercial nursery. The primary symptom was a dark purple leaf blight, often associated with the leaf mid-rib. Disease progressed down the petioles and into twigs, causing blackening of affected tissues and leaf drop. Abundant bacterial streaming was observed oozing from the margins of affected tissue when examined at ×100. Isolations from affected tissues were made onto King's medium B (KB). A fluorescent bacterium was recovered and identified as Pseudomonas syringae by the Biolog system of identification. Identity was confirmed by fatty acid methyl ester analysis performed by Larry Barnes (Texas A&M University, College Station). Attempts to determine the pathovar were unsuccessful. A single colony isolate of the bacterium was raised on KB. Koch's postulates were completed by the following procedures. A bacterial suspension was made from a 24-h-old agar culture of this isolate with phosphate buffer with 0.2% gelatin (PBG). The concentration of the suspension was adjusted to 8 × 10⁷ cells per ml by direct enumeration. Five milliliters of the suspension was atomized onto young leaves on six twigs of Kalmia latifolia. Controls consisted of young leaves on four twigs atomized with 5 ml of PBG. Twigs receiving the inoculum or the PBG were enclosed in plastic bags and maintained at room temperature near a north-facing window. Symptoms appeared 6 days later: dark purple spots on the margins of inoculated leaves and blight symptoms near the leaf mid-rib. Symptoms did not appear on PBG-sprayed leaves. Pseudomonas syringae was successfully reisolated from surface-disinfested inoculated leaves but not from leaves sprayed with PBG. This is the first report of Pseudomonas syringae causing a leaf blight of Kalmia.