Vein-bounded spots caused by Xanthomonas campestris pv. zinniae on zinnia. Note yellow haloes.
Photo: Margery Daughtrey

Bacterial leaf spot diseases are one of the stickiest (pun intended) areas for disease management. Many of these are due to one of the bacteria (coated in sticky polysaccharides) in the genus Xanthomonas that attack particular species, genera or families of plants. Shaped like teensy-tiny sausages, these bacteria can work collaboratively to have dramatic effects on their host plants, even though individual cells are only one to several microns long. Avoiding introduction of these single-celled hell-raisers is far easier than coping with them once they have established in a crop. There are few chemical tools to use against them, and they are notorious for their ability to spread.

Growers of begonia, English ivy, lavender, hydrangea, poinsettia, ranunculus, zinnia, geranium, and ornamental kale, cabbage and pepper are probably already painfully aware that their crops have a particular vulnerability to bacteria leaf spot, but these are just some of the most common examples of plants that may be found with Xanthomonas infections. Keep on the lookout for additional Xanthomonas victims. These diseases are moved within the crop during transplanting or other handling and when bacteria are splashed from one place to another by irrigation or rainfall.

English ivy with haloed bacterial leaf spots caused by X. hortorum pv. hederae.
Photo: Margery Daughtrey

The bacteria enter leaves through the natural openings (stomates or hydathodes) or through wounds (made by hail, insects, wind-driven sand, etc.). Overhead irrigation spreads bacterial leaf spot diseases in the greenhouse; pounding rain leads to spread in an outdoor crop.

Xanthomonas bacteria causing leaf spots are in some cases known to be spread on seed, but they may also be moved from business to business on invisibly contaminated plugs or cuttings.

Xanthomonas bacteria causing leaf spots are in some cases known to be spread on seed, but they may also be moved from business to business on invisibly contaminated plugs or cuttings.

Ornamental hosts of Xanthomonas are damaged by the enzymes and toxins produced by the myriad of bacteria multiplying inside the leaf: chlorophyll production is disrupted (so you see yellowing) and leaf tissue is killed (so you see browning). In some Xanthomonas diseases, as for geraniums and begonias, a vascular wilt disease may develop when the bacteria become systemic.

Geranium leaf infected by X. hortorum pv. pelargonii showing both small, round spots and V-shaped wedges.
Photo: Margery Daughtrey

Monitoring

  • Watch for small spots that appear across the leaf blade and don’t follow on the heels of a spray treatment that might have caused phytotoxicity.
  • Look for spots that aren’t uniformly distributed and vary slightly in size and appearance, which are water-soaked when they first form.
  • Look for yellowing in a halo around the spots (a variable trait).
  • Notice if there is a combination of spots and wedges, or vein blackening (most often seen in kale and cabbage).
  • Consult books, online references and a diagnostic lab to confirm your hunch.
X. campestris pv. campestris on ornamental kale may be seedborne; the yellow wedges at the edge of the leaves sometimes show blackened veins.
Photo: Margery Daughtrey

Prevention

  • Grow less-susceptible species or cultivars when available.
  • Consider hot water treatment of seed for kale and cabbage, but be wary of reducing germination with an imprecise technique.
  • Avoid condensation on plant surfaces, overfrequent irrigation, or overcrowding that will keep foliage from drying off quickly after irrigation.
  • Consider regular preventive spray treatments with competitors of Xanthomonas, such as those found in biofungicides containing Bacillus subtilis or Bacillus amyloliquefaciens.
  • Scout plants often for the appearance of symptoms.
Xanthomonas isn’t the only bacterial threat to your crops: Dianthus is one of many plants susceptible to a bacterial leaf spot caused by a Pseudomonas sp.

Treatment of pest when present

  • If a bacterial disease strikes, immediately rogue out the spotted individuals or remove symptomatic leaves.
  • Treat the remainder of the crop to kill bacteria that have already been distributed by handling or irrigation.
  • Use a copper material in alternation with a biological control agent containing a Bacillus species at a weekly interval for most ornamental crops; agricultural streptomycin is registered for use on peppers. Be aware that sprays made to control insects or fungal diseases will spread bacteria if no copper or biocontrols are present on the foliage.

Margery is a plant pathologist specializing in ornamentals at Cornell’s Long Island Horticultural Research & Extension Center. She aims to help growers outwit diseases.