Bacterial Blight of Geraniums
Gary W. Moorman, Professor of Plant Pathology
Bacterial blight of geranium is the single most important disease of geraniums. This disease is caused by a bacterium named Xanthomonas campestris pv. pelargonii which attacks only geraniums. When the disease is suspected of being bacterial blight, contact a plant pathologist immediately for diagnosis.
Symptoms
Leaves - small water-soaked spots are sometimes seen. These spots, usually 1/8 inch in diameter or smaller, become sunken, well defined, and eventually die. On some cultivars, spots may be up to 1/4 inch in diameter. V-shaped yellow areas in which the wide part of the V is on the leaf margin and the point is on a vein are frequently observed. The bacteria enter the leaf, eventually enter the water conducting tissue, and spread throughout the plant.
Stems
leaves of a branch wilt along the margin and slowly back toward the petiole, giving an umbrella-like appearance. These leaves yellow and die as will all the leaves on that branch. Cutting across the branch will reveal darkened vascular tissue. If this cut stub is examined hours later, a slimy ooze sometimes exudes from the darkened vascular tissue. This ooze contains millions of bacterial cells. As the disease progresses, the dead leaves fall, stems become blackened, and the entire plant dies.
Spread of the disease
Cuttings taken from infected stock plants are the most important means of spread. Stock plants may not exhibit symptoms but if infected, cuttings from such plants are probably infected. The bacteria can be spread from infected plants to healthy ones on cutting knives. Bacteria can be splashed from plant to plant during watering as well as spread to cuttings through infested propagation medium. The bacterium survives in plant debris in soil for up to a year. Thus, if geraniums are planted in outdoor beds where blight developed last year, blight may develop this year.
The disease
Xanthomonas campestris pv. pelargonii can cause disease in all cultivated geranium varieties. Ivy geraniums are particularly susceptible. Certain types of geraniums (Pelargonium X domesticum) appear resistant but have been shown to carry the disease without exhibiting symptoms.
The pathogen resides in the water conducting vessels. If it enters via the roots, disease development may be slow. Bacterial blight develops slowly when temperatures are below 60° F. If plants are held above 70° F but below 81° F, symptoms appear in 7-10 days, possibly 3 wks.
Verticillium wilt, caused by a fungus, causes very similar symptoms. Samples must be cultured in a special medium to be certain whether the plants are infected with Xanthomonas or Verticillium.
Control
- Purchase disease-free plants each year.
- Discard all geraniums at the end of the season and wash down bench surfaces with a disinfectant such as Clorox*, Physan*, Greenshield*, or ZeroTol*.
- Do not save outdoor-grown plants for use as stock plants.
- Discard all plants returned by customers and do not allow such plants to be brought into your growing area.
- Break out branches from stock plants for propagation rather than using cutting knives.
- Water plants in such a way as to prevent splashing.
- Immediately discard all plants found to be infected, wash down benches, and disinfest all tools, flats, pots, etc. used to handle the plants. Infected plants sprayed with Phyton 27* tend to yellow quickly, aiding is scouting for diseased plants.
Some of the information presented was excerpted from the following sources:
Geraniums. 1971. Edited by J. W. Mastalerz. Pennsylvania Flower Growers.
Geranium Disease Control Guide. 1977. A. H. McCain. Univ. of California, Berkeley. Leaflet 2603.
Resistance of Pelargonium to Xanthomonas pelargonii. 1967. J. F. Knauss and J. Tammen. Phytopathology 57:1178-1181.
DISCLAIMER
Notice: The user of this information assumes all risks for personal injury or property damage.
Warning! Pesticides are poisonous. Read and follow all directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams or ponds.
Issued in furtherance of Cooperative Extension work, Acts of Congress, May 8 and June 30, 1914, in cooperation with the U. S. Department of Penn State College of Agricultural Sciences research and extension programs are funded in part by Pennsylvania counties, the Commonwealth of Pennsylvania, and the U.S. Department of Agriculture.
Visit Penn State Extension on the web at extension.psu.edu.
Where trade names appear, no discrimination is intended, and no endorsement by Penn State Cooperative Extension is implied.
This publication is available in alternative media on request.
The Pennsylvania State University is committed to the policy that all persons shall have equal access to programs, facilities, admission, and employment without regard to personal characteristics not related to ability, performance, or qualifications as determined by University policy or by state or federal authorities. It is the policy of the University to maintain an academic and work environment free of discrimination, including harassment. The Pennsylvania State University prohibits discrimination and harassment against any person because of age, ancestry, color, disability or handicap, national origin, race, religious creed, sex, sexual orientation, gender identity, or veteran status. Discrimination or harassment against faculty, staff, or students will not be tolerated at The Pennsylvania State University. Direct all inquiries regarding the nondiscrimination policy to the Affirmative Action Director, The Pennsylvania State University, 328 Boucke Building, University Park, PA 16802-5901; Tel 814-865-4700/V, 814-863-1150/TTY.
