As poinsettia season gets started with cuttings being rooted, greenhouse growers should watch out for bacterial soft rot. The following is more information.
We have had couple of reports of bacterial soft rot, Erwinia carotovora, showing up on poinsettias in greenhouses. E. carotovora is found everywhere and attacks stressed and weakened plants. Often the problem starts because of how the cuttings are handled. Most cuttings are produced offshore and when they arrive in the United States they are cooled by third party companies before shipment. The trucks they are shipped in are cooled to keep the poinsettia cuttings out of stress. If the cuttings are not kept cooled enough at any stage in this process then they become stressed. When the cuttings arrive at your greenhouse, reach into the box to check the temperature of the poinsettia cuttings. If cuttings feel warm, take them out and cool them down using ice packs, by misting them or by moving them into your cooler and lowering the temperature to 50°F. If you have to stick the cuttings right away, soaking them first will help reduce the temperature stress. Watering the oasis cubes or substrate before sticking also helps reduce stress on the cuttings. Do not heat the greenhouse at night for the first 2 - 3 nights. Greenhouse temperatures of 80-85°F/ 26-29°C during the day and 72°F/22°C at night are optimum for poinsettia propagation. Shading the greenhouse to 1500 - 2000 foot candles/16,140 - 21,520 lux will help reduce temperatures and stress on the cuttings. Remove all infected cuttings from the growing area. If you can get the plants past the first 4 to 5 days, you are usually “out of the woods” with this disease.
Reprinted from the Greenhouse TPM/IPM Bi-Weekly Report from the University of Maryland Cooperative Extension Central Maryland Research and Education Center http://www.ipmnet.umd.edu/09Jul24G.pdf