Watch for Botrytis on Poinsettia
It’s time to get your poinsettia crops past the finish line and out the door. There has a myriad of disease issue on poinsettias this growing season including bacterial soft rot, Pythium and Phytophthora root rots, Rhizoctonia stem rot, Alternaria leaf spot, powdery mildew and Botrytis to name a few. As bracts are coloring and temperatures are cooling, it is important to watch for anything that may damage the bracts. Calcium deficiency and Botrytis are two major issues that can cause bract burn and unmarketable plants.
Botrytis blight can be explosive within greenhouses on damaged or senescing tissues where humidity levels are high (greater than 85%). The pathogen produces an abundant amount of spores on infected tissues that can be dispersed by water-splash and air movement to nearby leaves and adjacent plants. If you are not already using a fungicide to reduce Botrytis, you should consider it depending upon the stage of growth, sale date, and weather conditions in your area. Overcast, humid, and wet conditions will favor Botrytis infection.
Scouting for and managing Botrytis in greenhouse crops has been written about before in eGRO Alerts including "Scouting and Treatment of Gray Mold in Greenhouse Crops” (https://e-gro.org/pdf/2021-10-21.pdf) and "Watch out for Botrytis” (https://www.e-gro.org/pdf/2019_810.pdf). The biggest concern about applying fungicides at this time of the poinsettia production cycle is concern for product residue and phytotoxicity on bracts in color. At this time, when the plants are about to head out of the greenhouse, Decree (fenhexamid) and Astun (isofetamid) are good choices. Other fungicides that are also good on reducing Botrytis are Affirm WDG (polyoxin D zinc salt), Broadform (fluopyram + trifloxystrobin), Medallion WDG or Spirato GHN (fludioxonil), and Palladium (cyprodinil + fludioxonil). Use of a surfactant can help reduce potential product residue problems. Always use products according to label rates and directions.
It is this time of the poinsettia production season that I used to get an annual call from a grower two weeks before Thanksgiving who always asked if he should spray for Botrytis. We would discuss the health of his crop, cultivars he was growing, his sell dates, the current and forecasted weather pattern, his greenhouse conditions, product choices, and eventually conclude that he should spray. We always ended the conversation with him saying, "Well, I’m going to spray, and then I’m going to drink some whiskey and pray my plants will be OK.” I was right there with him: drinking and praying all would turn out well and his crop was beautiful. He is no longer with us, but I always think of him at this time of the year. Good luck getting your plants out the door. Cheers!
Jean Williams-WoodwardAssociate Professor, University of Georgia
Jean is an Associate Professor and Extension Plant Pathologist at the University of Georgia. Jean provides statewide plant disease diagnosis and management recommendations for ornamental plants in greenhouses, nurseries, and landscapes. She also teaches courses in plant disease diagnosis and ornamental pest management. She brings over 27 years of experience in working with ornamental plant producers and extension outreach providing education programs and on-site plant problem diagnostics.