In 2014, bacterial leaf spot was seen on commercial kale (Brassica oleracea var. acephala) grown in Monterey County, CA. Initial symptoms were small (2 to 4 mm in diameter), angular, water soaked lesions observable on both adaxial and abaxial surfaces. Lesions enlarged, became tan to light brown, and were often surrounded by chlorotic haloes. Incidence was as high as 10% and due to the impracticality of sorting healthy and diseased leaves, large portions of fields were unharvested. From macerated spots, blue-green fluorescent pseudomonads were consistently isolated on King’s Medium B agar. Six retained isolates were gram-negative, oxidase negative, arginine dihydrolase negative, produced levan, but did not nucleate ice. Isolates did not rot potato slices but induced a hypersensitive reaction on tobacco (Nicotiana tabacum cv. Samsun). These isolates therefore belong to Lelliott’s LOPAT group 1a Pseudomonas syringae (Lelliott 1966). Multilocus sequence analysis using the concatenated gene fragments of the loci gap1, gltA, gyrB, and rpoD placed the strains in P. syringae genomospecies 3, and sequences were identical to those of the pathotype of P. syringae pv. tomato. For Koch’s postulates, potted kale (cvs. Lacinato, Red Russian, Siberian) at the six-leaf stage were inoculated with bacteria grown on sucrose peptone agar for 48 h and suspended in sterile distilled water (SDW) to a concentration of 107 CFU/ml. Each of six isolates was sprayed until runoff onto 12 replicate plants per cultivar. Plants were enclosed in plastic bags for 24 h and then maintained in a greenhouse (23 to 25°C). After 4 to 5 days, water soaked lesions developed on all inoculated plants; lesions turned tan and were similar to symptoms seen in the field. Control plants sprayed with SDW developed no symptoms. Fluorescent pseudomonads were reisolated from all inoculated cultivars. The experiment was conducted twice and results were the same. The reisolates were identical to the original strains according to repetitive extragenic palindromic sequence PCR with the BOXA1R primer, confirming Koch’s postulates. Using methods described above, kale strain BS3736, P. syringae pv. tomato (CFBP2122PT), and P. syringae pv. maculicola (CFBP1657PT) were inoculated onto the three kales and tomato (cv. Super Beefsteak). In three experiments, all kale and tomato inoculated with the kale strain and the pathotype of P. syringae pv. tomato developed necrotic spots and yellow halos; distinctive pepper spot symptoms were only observed on kale cv. Lacinato inoculated with P. syringae pv. maculicola; pepper spot symptoms consist of small (3 to 4 mm in diameter), tan to brown spots visible from both sides of leaves. In this study, P. syringae pv. maculicola and P. syringae pv. tomato (including the kale strains) had overlapping but distinct experimental host ranges. P. syringae strains from kale were identified previously as P. syringae pv. maculicola though genetically identical to the pathotype strain of P. syringae pv. tomato (Zhao et al. 2000). While P. syringae has been reported on kale previously (Bull et al. 2003), this is the first report of bacterial leaf spot on kale caused by P. syringae pv. tomato in California and one of the few reports of P. syringae pv. tomato causing a natural disease outbreak on crucifers.
All Science Journal Classification (ASJC) codes
- Agronomy and Crop Science
- Plant Science