Bacterial leaf spots in greenhouse crops can spell rapid losses for growers
Tom Creswell, PPDL Director, Purdue University
These photos illustrate the problem greenhouse producers of ornamentals sometimes face when bacterial diseases show up. These verbena plants, grown from cuttings, have bacterial leaf spot caused by Acidovorax konjaci. Characteristic symptoms include wet-looking dark brown or black lesions that are partially delimited by the major leaf veins, giving the spots a somewhat angular appearance (Fig 1). We have also found Acidovorax konjaci leaf spot on Petunia and Calibrachoa in past samples (Fig. 2).
We diagnose bacterial diseases by first looking for bacterial ooze coming from the affected area using phase contrast lighting on a compound microscope. The cloud of bacteria appear brightly lit against the dark background (Fig. 3). After the bacteria are isolated a single colony is transferred and used for further biochemical testing and DNA sequence analysis to confirm the identity of the pathogen.
The warm, wet conditions following rooting of cuttings and during production of seedlings are ideal to allow spread of bacteria and growers must be vigilant the catch problems early before they cause widespread damage (Fig. 4).
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Fig 1: Dark, angular wet-looking spots are typical of bacterial leaf spots in many ornamentals such as this Verbena with spots caused by Acidovorax konjaci.
Fig 2: Bacterial leaf spot of petunia caused by Acidovorax konjaci.
Fig 3: Bacterial streaming from an infected leaf, shown using phase contrast lighting.
Fig 4: Trays of verbena cuttings with widespread bacterial infection.