Gary 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 weeks.

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 Clorox*, Physan*, GreenShield*, or ZeroTolerance*.
• 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 rapidly, aiding in scouting for plants that are diseased.

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.

* Trade Name