A Botrytis lesion on a poinsettia bract
Photo: Margery Daughtrey

Come holiday season, poinsettias are a must-have for many consumers. But production of these crops in the summer and fall can be a difficult time for growers. Poinsettias have no profit margin, says Dr. A.R. Chase, president of Chase Agricultural Consulting, so when diseases infect these plants, production of them becomes less practical.

Furthermore, the plant diseases that attack poinsettias are tenacious. They can infect poinsettias in the cutting stage, then not display obvious symptoms for a while, until events such as high temperature or overfertilization cause symptoms to flare up into root rots, explains Margery Daughtrey, plant pathologist specializing in ornamentals at Cornell University.

It is for reasons such as these that growers need to proactively prevent and treat diseases on poinsettias.

Several diseases appear again and again on poinsettias. “Pythium root rot, bacterial soft rot, Rhizoctonia stem rot [and] Botrytis blight are the most common,” Daughtrey says. “Other diseases happen, but they are ‘surprises’ that need to be scouted for.”

Poinsettia diseases often appear because of poor growing conditions, Chase says. “The Botrytis could be controlled if [growers] spaced the plants and improved air flow. Pythium can be better controlled if they refrain from over-fertilizing and over-watering. The early diseases like Erwinia on the cuttings [are] mainly due to bad shipping conditions and poor water management when they are under mist. The Rhizoctonia is something that they usually get on the cuttings — so accepting poor quality cuttings impacts this disease.”

Out of the common diseases on poinsettia, each has their own unique sets of symptoms, as well as prevention and treatment methods, Daughtrey says.

Bacterial soft rot caused by a Pectobacterium sp.
Photo: Rose Buitenhuis

Pythium root rot symptoms include “soft, grayish brown discoloration of roots” and white strands at the ends of the roots when the plant is pulled out of a mix, Daughtrey says. Above the soil line, she adds, Pythium root rot-infected plants are characterized by stunting and chlorosis on the lower leaves, followed by wilting.

Pythium can be curbed by tried-and-true greenhouse sanitation practices, using a well-drained mix, being careful not overfertilize or cause root injury from a dry-down, and following a preventative fungicide drench, Daughtrey says.

Bacterial soft rot can begin with “soft brown decay at the base of cuttings during propagation,” and can result in complete collapse of the cuttings, Daughtrey says. “Later on, cuttings that might have been infected during propagation may have grown into well-rooted potted plants, but a heat spell may trigger the development of systemic infections that cause the plant to wilt down from internal infections,” she says. Examples of these infections can involve Pectobacterium and Dickeya bacteria, which were formerly known as Erwinia species.

Bacterial soft rot can be avoided by not stressing cuttings, Daughtrey says. This means immediately sticking them and controlling fungus gnats, which can create wounds in the cuttings that can let the pathogen in.

Rhizoctonia stem rot is often first detected after potting, through wilted cuttings, Daughtrey says. “The fungus Rhizoctonia solani attacks plants at the soil line, and when and if the stem becomes girdled, the plant wilts,” she says. “A brown canker may be seen at the soil line. If Rhizoctonia contaminates the propagation area, it is not uncommon to see leaves blighted by the fungus where they come into contact with the rooting media.”

Rhizoctonia can be prevented by using a preventative fungicide drench and through greenhouse sanitation best practices, including not using field soil, Daughtrey says.

Botrytis blight will usually appear during propagation, and unless there is not enough sunlight, it usually does not occur again until finishing, Daughtrey says.

Growers can do their part to prevent Botrytis by keeping relative humidity under 85 percent and not allowing condensation to form on plant surfaces, Daughtrey says. “There is nothing absolutely safe to spray during the finishing of poinsettias, so preventing Botrytis blight in the fall should be done with very, very careful environmental management,” she says. “Don’t let condensation occur on the plants — and make sure they have plenty of calcium in them so they aren’t easier prey for the Botrytis.”

Propagators can be the source of poinsettia diseases, such as poinsettia scab and Xanthomonas, Daughtrey says. Boom irrigation and irrigation splash can spread Xanthomonas bacteria or scab fungus. Propagators may use fungicides to prevent scab from spreading, and copper and biocontrols against bacteria.

Regional variations

Climate and region can influence which diseases appear on poinsettia crops, Daughtrey says.

For a powdery mildew epidemic to flourish, daytime temperatures need to be less than 86° F, so the disease occurs in the northern United States earlier in the year than in other regions. In areas where poinsettias grow outdoors throughout the year, poinsettia powdery mildew spores could, as Daughtrey says, “float into the greenhouse.” If powdery mildew appears on poinsettia cuttings in the north, it can become more noticeable as temperatures drop in the fall.

In the case of Thielaviopsis root rot, the disease prefers high pH, and will appear in areas where pH is high.

In Chase’s experience, she says region and climate do not influence poinsettia diseases as much as growing conditions. “The fact that the growers start growing them when it is very hot and finish when it might be pretty cold makes them an extreme challenge,” she says.

Insect pests and diseases

On poinsettia crops, the presence of diseases and insect pests can be linked, Daughtrey says. “Poinsettias are one major crop where growers don’t have to worry about thrips spreading Impatiens necrotic spot virus or Tomato spotted wilt virus. They do, however, have to worry about the thrips for their own sake!” she says. “Fungus gnats are often there to enjoy the feast when Thielaviopsis, Phytophthora, Pythium or Rhizoctonia is attacking the stem base, and they can be the means of moving some of these around the greenhouse. Even shore flies, which don’t feed directly upon the poinsettias, could move bacteria around the greenhouse.”

Whether influenced by temperature, pH, pests or propagation, diseases infect poinsettias in a range of ways. Early prevention and detection are key for growers to head successfully into fall and winter.