See:
Geranium (Pelargonium spp.) - Bacterial Blight
Cause Xanthomonas axonopodis pv. poinsettiaecola, a bacterium. Other Xanthomonas spp. can also cause similar symptoms including X. codiaei and one similar to X. hortorum pv. pelargonii. This disease has been reported to spread rapidly especially with overhead irrigation. Considered a rare disease and typically associated with outdoor poinsettia cultivation. This disease has occurred sporadically in greenhouse production. Sources of bacteria may include geranium, croton (Codiaeum variegatum), crown of thorns (E. milli), and zebra plant (Aphelandra squarrosa).
Symptoms Dull, grayish-brown, slightly water-soaked spots first on undersides of leaves. As spots enlarge, they can be seen on the top side of the leaf. Spots may be angular and develop a yellow halo. Spots are limited to the areas of tissue adjacent to and between minor leaf veins, which give them the angular shape. Spots generally are of different sizes and uneven in distribution across the leaf. Spots may coalesce, forming large areas of dead tissue that become dry, papery and easily torn. Heavily infected leaves drop prematurely. Does not cause cankers on stems.
Cultural control The focus is on dry leaves.
- Scout regularly and remove and destroy infected plants.
- Avoid overhead irrigation or any practice that keeps plants wet for extended periods of time. Individual drip watering of pots can also help.
- Keep humidity as low as practical.
- Wash hands and keep cutting tools disinfected.
Chemical control Focus on cultural controls. Chemical (or biological) applications will help reduce spread of the disease to healthy plants. Good coverage is essential.
- Camelot O at 0.5 to 2 gal/100 gal water. Group M1 fungicide. 4-hr reentry.
- FireWall 50 WP at 1.36 oz/25 gal water. Group 25 fungicide. 12-hr reentry.
- KleenGrow at 6 to 38 fl oz/100 gal water. 48-hr reentry.
- Phyton 27 at 1.5 to 3.5 oz/10 gal water. Group M1 fungicide. 48-hr reentry.
Biological control
- Cease or Rhapsody (Bacillus subtilis strain QST 713) at 2 to 8 quarts/100 gal water. Active ingredient is a small protein. Efficacy in the Pacific Northwest is unknown. 4-hr reentry. O
References Benson, D.M., Hall, J.L., Moorman, G.W., Daughtrey, M.L., Chase, A.R., and Lamour, K.H. 2002. The history and diseases of poinsettia, the Christmas flower. Online. Plant Health Progress doi:10.1094/PHP-2002-0212-01-RV.
Rockey, W., Potnis, N., Timilsina, S., Hong, J.C., Vallad, G.E., Jones, J.B., and Norman, D.J. 2015. Multilocus sequence analysis reveals genetic diversity in xanthomonads associated with poinsettia production. Plant Disease 99:874-882.