Bacterial Diseases on Geranium January 2004

Bacterial diseases on geraniums that growers should be on the lookout for are Xanthomonas campestris pv. pelargonii, the agent of bacterial blight disease of geraniums and Ralstonia solanacearum the causal agent of southern bacterial wilt. Southern bacterial wilt is caused by two specific races of R. solanacearum, Race 1 and Race 3. Race 1 is found in the southern United States whereas Race 3 is considered an exotic pathogen and is therefore regulated under federal quarantine. R. solanacearum Race 3, biovar 2 is also one of 10 pathogens on the list of "Select Agents and Toxins" as part of the Agricultural Bioterrorism Act of 2002. The concern is that Race 3, biovar 2 has the potential to carryover to potato and other solanaceous crops or weeds. (Geranium Bacterial Disease pictures)

Bacterial blight of geraniums, caused by Xanthomonas campestris pv. pelargonii can infect Zonal geraniums (Pelargonium x hortorum), ivy geraniums (Pelargonium peltatum), Regal or Martha Washington geraniums (Pelargonium domesticum), and cranesbill (Geranium sanguineum). Symptoms vary depending upon the species or cultivar of geranium affected and environmental conditions. Warm temperatures favor bacterial diseases such as Xanthomonas. Symptoms can develop in as little as seven days at 81o F, but may develop in three weeks at 60 o F.

Ivy geraniums are very susceptible to bacterial blight but infected plants do not develop distinctive symptoms. Infected plants may be off-color resembling a nutrient deficiency symptom or mite feeding damage. An important defense strategy is to avoid placing ivy geranium crops over bench-grown zonal geraniums to eliminate potential disease spread as water drips unto susceptible zonal geraniums below.

Xanthomonas can cause tiny, round, water-soaked, brown leaf spots that are from 1/16 to 1/8 inch in diameter. These spots develop as the bacteria are splashed from leaf to leaf (See figure 2). Sometimes, yellow to brown v-shaped wedges may develop on the leaves (See figure 3) that can be easily confused with Botrytis blight (Figure 4). The bacteria also enter the vascular system (xylem) of the plant causing the leaves to wilt while the roots remain healthy. However, water stress, root rot disease, or a fungal pathogen such as Verticillium wilt can also cause wilting. Prompt diagnosis is critical to reducing crop losses.

When geraniums are infected with Ralstonia, lower leaves may wilt, turn yellow and drop off the plant. Sometimes, plants may be infected and show few if any symptoms. Ralstonia is not spread as readily as Xanthomonas by splashing water, so no leaf spots occur. Ralstonia spread through the irrigation water, for example, in a subirrigation system, from one plant's root system to another plant's root system. Under cool conditions, infected plants may not show any visible symptoms. If latently infected plants are exposed to warmer temperatures, disease development is favored.

R. solanacearum (race 1) has a wide host range including impatiens, marigolds, zinnia, salvia, tomatoes, peppers, and petunia to name a few. R. solanacearum (race 2, biovar 3) can infect potatoes, tomatoes, eggplant and many different solanaceous weeds. Other reported hosts include portulaca, brassica, and tropaeolum.

 Laboratory diagnosis is critical to confirming the disease; one cannot rely on visual diagnosis alone. Samples may be submitted to:

Dr. Sharon Douglas
The Connecticut Agricultural Experiment Station
Plant Disease Information Office
123 Huntington Street
New Haven, CT 06511
Tel: 203-974-8601
Agdia, Inc.
30380 County Road 6
Elkhart, Indiana 46514
Tel: 1-800-662-4342

If bacterial diseases are detected in your operation, and confirmed after positive laboratory analysis, follow these steps:

Leanne S. Pundt, Extension Educator - Commercial Horticulture, University of Connecticut Cooperative Extension

References:
Daughtrey, M. 1999. Untreatable Plant Diseases. Northeast Greenhouse IPM Notes. 9(4): 1-2.
Daughtrey, M, R. Wick, and J. Peterson. 1995. Compendium of Flowering Potted Plant Diseases. APS Press.
Daughtery, M. 2003. Disease Alert: Ralstonia solanacearum.
Moorman, G. Geranium Diseases. Plant Disease Facts. 4pp.
Warfield, C. 2003. Science, Ralstonia and Your Geraniums. GrowerTalks. October 2003. 34-48.
White, J. W. (ed). 1993. Geraniums IV. Ball Publishing. 412 pp.
Ralstonia (Pseudomonas)solanacearum (E.F. Smith 1896) Yabuuchi et al. 1995 race 3 biovar 2 Outbreak of Ralstonia solanacearum race 3 biovar 2 on greenhouse geranium cuttings in the United States.

Information on our site was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

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