Bacterial Blight of Geranium: Symptoms, Spread, and Control Measures

V-shaped yellowing that ends on the vein.

Pathogen and Importance

Bacterial blight, caused by Xanthomonas campestris pv. pelargonii (specific to geraniums), is the single most important geranium disease. If suspected, contact a plant pathologist immediately for diagnosis.

Symptoms

Leaves

Small water-soaked spots (≤1/8", up to 1/4" on some cultivars) become sunken, well-defined, and necrotic. Characteristic V-shaped yellow areas (wide part on leaf margin, point on vein) are common. Bacteria enter leaf, move into vascular tissue, and spread systemically.

Stems

Leaves of a branch wilt from margin toward petiole (umbrella-like), yellow, and die. All leaves on that branch die. Cross-section reveals darkened vascular tissue; hours later, slimy bacterial ooze may exude. Disease progresses: dead leaves fall, stems blacken, entire plant dies.

Spread

Cuttings from infected stock plants are the primary spread mechanism—stock may be asymptomatic but infected. Bacteria spread on cutting knives, via water splash, and through infested propagation media. The bacterium survives in plant debris in soil for up to a year; outdoor beds with prior blight can reinfect new crops.

Disease Characteristics

All cultivated geranium varieties are susceptible; ivy geraniums especially so. Pelargonium × domesticum types may carry the pathogen asymptomatically. The pathogen resides in water-conducting vessels; root entry slows disease development. Below 60°F, development is slow; at 70–81°F, symptoms appear in 7–10 days (up to 3 weeks). Verticillium wilt (fungal) causes similar symptoms; lab culture on selective media is needed for differentiation.

Control

• Purchase disease-free plants annually.
• Discard all geraniums at season end; wash benches with disinfectant (Clorox, Greenshield, ZeroTol).
• Do not save outdoor-grown plants for stock.
• Discard all customer returns; do not allow them into growing areas.
• Break out branches from stock for propagation rather than using knives.
• Water to prevent splash.
• Immediately discard infected plants; wash benches, disinfest all tools, flats, pots. Phyton 27 sprayed on infected plants causes rapid yellowing, aiding scouting.

* denotes trade name.

Sources: Geraniums (1971, J.W. Mastalerz ed.); Geranium Disease Control Guide (1977, A.H. McCain); Resistance of Pelargonium to Xanthomonas pelargonii (1967, J.F. Knauss & J. Tammen).

Ooze from cut water conducting tissue; Leaf spotting; Typical wilting; Leaf spotting on cranesbill (Geranium). Prepared by Gary W. Moorman, Professor of Plant Pathology.