Bacterial blight and Southern bacterial wilt are two common bacterial wilt diseases that cause similar symptoms in geranium and may result in significant losses.
Bacterial blight is the more common of the two diseases. Bacterial blight, caused by the bacterium Xanthomonas campestris pv. pelargonii, is sometimes called bacterial wilt, bacterial stem rot, or bacterial leaf spot. Symptoms of this disease vary by plant cultivar and environmental conditions, and may include small watersoaked spots on the underside of leaves, followed by yellow and brown V-shaped lesions on the leaves. Lower leaves typically wilt while roots remain healthy. As the disease progresses, the stem may rot, with visible dark discoloration of the water-conducting tissue. Symptoms are suppressed at low temperatures.
Bacterial blight is most commonly introduced to greenhouses on propagative cuttings. It may be spread on infested cutting knives, via watersplash or plant-to-plant contact, or by the greenhouse whitefly. The Xanthomonas bacterium does not survive in soil, but may persist on plant debris in the soil.
Exclusion is the best way to prevent bacterial blight, and culture-indexing programs are available to ensure that propagative stock is free of the pathogen. Diseased plants should be removed promptly to prevent spread within a greenhouse. Keeping geraniums from different sources in separate areas of the greenhouse can also help to reduce risk of loss from this disease.
Southern bacterial wilt is caused by Ralstonia solanacearum, a bacterium that can infect many plants. Southern bacterial wilt is less common in Iowa than bacterial blight. Typical symptoms of Southern bacterial wilt on geranium include wilting of lower leaves and browning of the water-conducting tissue in the stem. Southern bacterial wilt may appear very similar to bacterial blight, but does not cause leaf spotting.
The Ralstonia bacterium is soilborne, and because of this, is uncommon in greenhouse crops growing in soil-less potting mixes. While bacterial blight typically appears in the greenhouse, Southern bacterial wilt usually appears after the geraniums are planted in the landscape.
Plants with Southern bacterial wilt should be removed. Often removing the infested soil and replacing it with pathogen-free soil can help to prevent further outbreaks.
One particular strain of Ralstonia solanacearum, race 3 biovar 2, has been accidentally introduced in Europe and North America on propagative material, causing significant disease on potatoes in Europe. In the US, this strain is under strict quarantine regulations to prevent spread to important food crops.
Laboratory tests are required to distinguish between bacterial blight and Southern bacterial wilt and to determine whether a strain of R. solanacearum is race 3 biovar 2.
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