P&PDL Picture of the Week for
October 21, 2013

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). 

Click image to enlarge

Acidovorax on verbena - close

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.

Acidovorax on petunia

Fig 2: Bacterial leaf spot of petunia caused by Acidovorax konjaci.

Acidovorax bacterial ooze

Fig 3: Bacterial streaming from an infected leaf, shown using phase contrast lighting.

Acidovorax on verbena tray

Fig 4: Trays of verbena cuttings with widespread bacterial infection.

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service