Bacteria blight and leaf spot
In 2008, bacterial leaf spot of Gerbera, caused by the bacterium Pseudomonas cichorii, was recorded for the first time on Gerbera in Victoria and bacterial blight of Lobelia, caused by the bacterium Pseudomonas viridiflava, was recorded for the first time on Lobelia in Australia.
Symptoms and management
Bacterial leaf spot of Gerbera
Bacterial leaf spot, caused by Pseudomonas cichorii, tends to be a problem not only during warm weather with periods of heavy rain, but also where overhead watering is practised.
Symptoms of this disease are large black spots concentrated at the base of the plant. The spots often begin at the leaf margin but may also occur randomly. The spots are soft when tissue is wet and sunken and brittle when leaves are dry. From the leaf, the bacterium can move through the petiole and into the stem, resulting in a canker. The sepals of infected flower buds will become brown to black and up to several inches of pedicel may be killed.
Key management practices include planting pathogen-free seed and cultivars or resistant varieties; good sanitation; and avoiding overhead irrigation or handling plants when they are wet.
Once plants become infected with bacteria, it is best to rogue infected plants.
The P.cichorii bacterium has been recorded as the cause of bacterial leaf spot of Gerbera overseas (Miller 1974) and of a range of other hosts including, hibiscus, cyclamen, vinca, chrysanthemum, impatiens, lettuce and chicory.
Bacterial blight of Lobelia
Symptoms of bacterial blight leaf spotting appear on the flowers and leaves of Lobelia plantlets, with lesions on leaf margins of older leaves. Initially lesions are water-soaked; later they become brown, dry, and papery. The leaf spots are surrounded by a distinct brown border.
Affected plants eventually die back to the ground. In the recent detection in Victoria, 100% of the crop was decimated by this disease.
The P.viridiflava bacterium has been found to be the cause of bacterial blight and leaf spot of a range of hosts including, viola, jasmine, chrysanthemum, grape, celery, tomato, beans, Allium spp. and black nightshade and can cause bud rot of kiwifruit.
Limited information is available on how these diseases are distributed across nurseries in Victoria. However, the organisms that cause the diseases are present in a wide range of hosts in many states.
Miller, J.W. (1974). Bacterial Blight of Gerbera Daisy. Plant Pathology Circular No.139.
To report suspected exotic or unusual pests or diseases, call the EXOTIC PLANT PEST HOTLINE 1800 084 881