Friday, May 22, 2015

Bacterial Blight of Geranium

For the first time in several years, the Plant Disease and Insect Clinic received a sample of geranium with bacterial blight caused by Xanthomonas hortorum pv. pelargonii (formerly Xanthomonas campestris pv. pelargonii) from a North Carolina greenhouse. All geranium (Pelargonium) producers should be vigilant.

Small, circular necrotic spots and also larger wedge-shaped marginal lesions.
Typical leaf symptoms of bacterial blight of geranium.
In this case the submitted leaves showed small circular leaf spots and v-shaped necrotic lesions at the leaf margin. There were also edema-like bumps on the underside of the leaves. Petioles appeared healthy in this case, but in the advanced stages of this disease, stems can become infected, resulting in wilting. Note that with bacterial wilt caused by Ralstonia solanacearum there is no leaf spotting.

Small scabby spots and also typical triangular lesion at the margin.
Close-up of upper leaf surface of Xanthomonas-infected geranium.
Water splash, tools, and handling are possible ways this pathogen can spread, and of course through infected propagative material (stock plants or buy-ins). Fortunately this strain of Xanthomonas does not affect any plants other than geranium, though it could survive in infected debris. Avoid overhead watering as much as possible. Keeping plants grouped by source can be helpful in containing and tracing any outbreak that occurs.

When Xanthomonas blight is confirmed in a geranium crop, the affected plants must be discarded, including the potting mix. Workers should avoid working among healthy geraniums after handling diseased plants. Asymptomatic (apparently healthy) plants next to diseased plants should also be destroyed, since they likely have populations of the bacterium on or in their tissues. The same goes for geraniums grown under hanging baskets containing diseased geraniums. All surfaces that had been in contact with these plants should be cleaned and then sanitized. This includes benches, tools, and pots. There’s a table with detailed information about sanitizers in the "Nursery Crops" section of the Plant Pathology Department Ornamentals page.

Preventive applications of a copper-based fungicide/bactericide, rotated with the biological control Bacillus subtilis, may reduce the spread of bacterial blight, but is not an effective strategy on its own. Use of clean stock and rigorous sanitation are the essential steps. For details on chemical applications, see p.448 of Table 10-11 in the 2015 North Carolina Agricultural Chemicals Manual.

For more information, see the following publications:
Bacterial Blight of Geranium by Gary Moorman of the Pennsylvania State University
Bacterial Blight of Geranium by M.B. Dicklow of the University of Massachusetts

A special thank-you to Dr. Mike Benson for reviewing this post.