Florist's Geranium ( Pelargonium x hortorum L. H. Bailey) can be infected by three bacterial diseases that cause leaf spots, wilt, or both these symptoms. Xanthomonas campestris pv . pelargoni is the most destructive disease of florist's geranium; ivy and seed geraniums are also susceptible. This disease has been referred to as bacterial stem rot, bacterial wilt, bacterial leaf spot, and bacterial blight. In spite of widespread use of culture-indexing techniques, this disease remains a frequent and serious problem. Pseudomonas species, including P. cichorii and P. syringae pv.syringae, cause leaf spot diseases on a wide range of flowering, potted plants. Southern bacterial wilt caused by Ralstonia solanacearum Race 3, Biovar 2 is a serious disease of both geraniums and vegetables in the Solanaceae and is under federal quarantine due to the threat to agricultural crops (See: Southern bacterial wilt of Geraniums Fact Sheet). Symptoms of this disease are nearly identical to those caused by Xanthomonas; the main differences being a lack of leaf spots and host range.
Xanthomonas campestris pv . pelargoni
Symptoms of bacterial blight can vary depending on cultivar, species, and environmental conditions. Small, water-soaked lesions develop first on leaf undersides and in time become visible on the upper surface as round, tan to brown, slightly sunken, 2-3 mm spots with a defined margin. Initial lesions are often followed by wedge-shaped chlorotic to necrotic areas. The bacterium moves from leaf spots into the water conducting tissue of the plants and plants wilt. Wilt is often followed by stem rot and plant collapse. Infected cuttings may not root and develop rot at the base. Ivy geraniums fail to exhibit wilt due to the nature of their leaves; infected foliage loses its luster and develops symptoms of nutrient deficiency or mite infestation. Temperatures below 50° F or above 90° F may prevent symptom development and older plants are less susceptible to systemic infection; infected stock plants may not show symptoms resulting in infected daughter plants. Xanthomonas can infect through the root system, although it does not survive in growth media in the absence of undecomposed, infected host debris. It can also persist on the foliage of nonhost plants, epiphytically on geranium leaves, and on wild Geranium species. The bacterium spreads within the greenhouse on infected tools, through splashing irrigation water, removal of infected leaves, dripping of water from hanging baskets of ivy geraniums, and by the greenhouse whitefly. Photo of Xanthomonas
Pseudomonas cichorii and P. syringae cause leaf spots that are indistinguishable from each other and may vary with environmental conditions. Plants subjected to excessive leaf wetting develop large, irregularly shaped, dark brown to black lesions. In the absence of leaf wetting, lesions are smaller with tan centers and dark margins. Yellowing always occurs. The optimum temperature for P. syringae (60°- 70° F) is lower than that of P . cichorii (75°-85 ° F); otherwise, their life cycles are similar. The bacteria may be borne on seed, on cuttings, and epiphytically on other hosts. Chrysanthemums are known to carry populations of P . cichorii and should be kept separate from geraniums and other known hosts. The pathogens are favored by periods of high humidity and extended periods of leaf wetness. Photo of Pseudomonas
R. solanacearum, the cause of southern bacterial wilt, exists in two races: Race 1 which is commonly found in the southern United States and Race 3, which is under federal quarantine and is a threat to agricultural crops as well as geranium. R. solanacearum is a soil-borne pathogen that enters the plant through the root system and causes a vascular wilt. The disease almost always results in plant death. High temperatures (80°-90° F) and high soil moisture are conducive to disease development. This disease, unlike Xanthomonas Blight, causes root necrosis. Because R. solanacearum Race 3, Biovar 2 is under federal quarantine, suspect plants should not be discarded; but separated from the rest of the crop and submitted to a Diagnostic Lab for confirmation. Photo of Ralstonia
Contact M. Bess Dicklow, UMass Extension Plant Diagnostic Lab at (413)545-3209, firstname.lastname@example.org .
Management of Bacterial Diseases
Strict adherence to good sanitation practices is essential to the management of bacterial diseases. There are no pesticide sprays or drenches that will cure plants or provide adequate protection from bacterial diseases.
- Symptomatic plants should be discarded.
- Diseased plant debris should be promptly removed from the growing area.
- Workers should wash their hands frequently and immediately after handling infected plants or soil.
- Splashing from irrigation water should be minimized.
- Minimize leaf wetness by watering early in the day or sub-irrigating.
- Avoid handling plants when they are wet.
- Nutrition may affect disease susceptibility; avoid excess or insufficient fertilization.
- Culture index stock plants or purchase of plants from a reputable source.
- Cutting knives should be frequently disinfected.
- Do not re-use growing medium.
- Do not hang ivy baskets over seed or zonal geraniums.
- As much as possible, separate seed geraniums from vegetative cuttings and plants from different propagators should be separated.
- Bactericides such as copper hydroxide and copper sulfate pentahydrate are only marginally effective in controlling bacterial diseases.
- Photo Library: Diseases
- Field Guide to Diseases of Greenhouse Ornamentals. 1993. Daughtrey, M. L. and Chase, A. R. Ball Publishing, Chicago , IL 224 pp.
- Compendium of Flowering Potted Plant Diseases. 1995. Daughtrey, M. L., Wick, R. L. and Peterson, J. L. APS Press, St. Paul, MN. 90 pp.
- Diseases and Pests of Ornamental Plants, 5th ed. 1978. Pirone, P. P. Wiley-Interscience Publication, New York , NY 566 pp.
- Diseases of Annuals and Perennials . 1995. Chase, A. R., Daughtrey, M. and Simone, G. W. Ball Publishing Co., Batavia , IL . 202 pp.
- Diseases of Floral Crops, Volume 1 and 2. 1985. Strider, D. L. Praeger, New York , NY 638 and 579 pp.
- Geraniums IV. 1993. White, J. W. Ball Publishing, Geneva , Illinois . 412 pp.
Video - Diagnosing Bacterial Wilt Diseases
This movie was made by Dr. Rob Wick, Plant Pathologist at UMass Amherst during a diagnostic plant pathology workshop in Bangladesh. To demonstrate bacterial streaming, Rob used a tomato plant that had a vascular bacterial infection (Ralstonia solanacearum). A stem about 3 inches long is suspended in a glass of water using a paper clip. The paper clip is straightened and pierced through the stem and perched on the top of the glass. To see streaming, a dark background is placed behind the glass and illuminated with side lighting. Once set up, do not disturb; the bacterial will come out in seconds. The bacteria produce a sticky matrix which helps protect them, and allows the bacterial strands to remain intact as they fall through the water. Note the "pools" of bacteria forming in the bottom of the glass. This same sticky mess clogs up the water conducting cells of the plant resulting in wilt. This technique will work with Clavibacter, Xanthomonas and Ralstonia vascular infections. The type of plant does not matter, so will also work with a geranium leaf.