Bacterial leaf spot of ornamentals and vegetables

Bacterial leaf spot can be caused by the bacterium Pseudomonas cichorii. It was recorded for the first time in Victoria in 2008 on Gerbera.

P. cichorii also causes zonate leaf spot of cabbage and varnish spot of lettuce. Note that other bacterial genera and species can cause similar symptoms to P. cichorii.

Bacterial leaf spot caused by P. cichorii tends to be a problem for:

• gerbera during warm weather with periods of heavy rain or where overhead watering is practised
• lettuce during cool temperatures when grown under overhead irrigation
• cabbage during overhead irrigation or wet and windy weather

The bacterium P. cichorii can infect a wide range of hosts, including:

• basil
• cabbage
• celery
• chrysanthemum
• cyclamen
• endive
• hibiscus
• impatiens
• lettuce
• pepper
• vinca

Symptoms of bacterial leaf spot of Gerbera

The following symptoms can indicate leaf spot of Gerbera:

• large black, roughly circular, zonate spots concentrated on leaves at the base of the plant (the spots often begin at the leaf margin but can also occur randomly)
• spots that are soft when tissue is wet, but sunken and brittle when leaves are dry
• the bacterium can move from the leaf through the petiole and into the stem, resulting in canker
• the sepals of infected flower buds can turn brown to black in colour and up to several inches of pedicel may be killed

Symptoms of varnish spot of lettuce

The following symptoms may be found, usually on lettuce with enclosed heads:

• removal of wrapper leave reveals shiny, brown and firm spots
• spots may start several millimetres in size but can merge and expand and may eventually cover the entire leaf

Symptoms of zonate leaf spot on cabbage

The following symptoms can indicate zonate bacterial leaf spot of cabbage:

• brown, water-soaked, roughly circular, zonate spots develop on leaves.
• the zonate spots can occur on the inner leaves of the cabbage

Managing bacterial leaf spot

Key management practices include:

• planting pathogen-free seed and plants or resistant varieties
• employing good sanitation
• avoid handling plants when they are wet
• avoiding overhead head irrigation during the evening and night
• for hydroponic systems, use registered chemicals to control the bacterium in water
• rogue out infected plants

References

Miller JW, Knauss JF (1974) Bacterial blight of gerbera daisy. Plant Pathology Circular No. 139.

Koike ST, Gladders P, Paulus AO (2007) Vegetable Diseases, A colour handbook. Manson Publishing 448pp.