Bacterial leaf spots caused by Xanthomonas spp. challenge growers because they are spread so easily — via splashing, handling or insects. Growers of the florist’s geranium (a hybrid Pelargonium) rely on culture-indexed cuttings or seed to avoid Xanthomonas hortorum pv. pelargonii, which can be found on hardy Geranium spp. and elsewhere in nature. Leaf spots caused by X. axonopodis pv. begoniae are seen often on Rieger, Rex, Nonstop and trailing begonias, but are fairly rare on Begonia semperflorens. Whereas latent infections are the most likely source of disease outbreaks in begonia and geranium, the source of disease for flowering kale (or cabbage) is sometimes the seed itself. With kale, look for patches of yellowing on the foliage in plug trays to detect bacterial (X. campestris pv. campestris) infections called ‘black rot.’ The diseases on begonia, geranium and kale have a lot in common, but they are caused by different host-specific bacteria.

Photo: Margery Daughtrey

Signs to look for

  • Tiny round spots, 1/16-1/8 inches in diameter on geranium or begonia leaves
  • Yellowing around the leaf spots, often
  • Wedges of yellow or brown (V-shaped) at the edge of the leaf in geranium and begonia; roughly rectangular yellow patches on kale leaves
  • Speckling at the leading edge of V-shaped wedges on a begonia leaf
  • Black veins within the collapsed and yellowed leaf areas on kale and cabbage
  • Wilting of leaves or collapse of stems in advanced infections
Photo: Margery Daughtrey

How to manage

  • Detect symptoms as soon as possible so that young plants can be rogued out, rather than finished crops.
  • Eradicate infected plants promptly.
  • Space plants to get air movement.
  • Irrigate so that plant leaf surfaces stay wet only briefly.
  • Use permeable (e.g. wire mesh) benching so that air can move up between plants.
  • Keep greenhouse humidity low (<85% RH) to prevent condensation.
  • Use fans to circulate air.
  • Avoid hanging ivy geraniums over crops of other kinds of geraniums.
  • Keep the portions of your crop that originated from different suppliers separate.
  • After an outbreak, protect nearby plants with fungicides. Use copper applications, in alternation with biofungicide treatments that have Bacillus subtilis as the active ingredient.
  • Clean up organic debris and use disinfestants on pots, trays, benches, etc. after an outbreak of bacterial disease.

Margery is a plant pathologist specializing in ornamentals at Cornell University's Long Island Horticultural Research & Extension Center.