The distinct color and nature of poinsettias get many in the Christmas spirit when winter arrives, but for growers, preparing for poinsettias starts with propagation in the spring.
However, many of the conditions favorable for propagation — high humidity and wetness — are also favorable for various diseases. To ensure healthy poinsettia yields, growers need to be on their guard right from the rooting stage. The main contenders threatening this shrub are pythium, rhizoctonia, botrytis and erwinia.
“Growers need to be thinking about it as early on as they can,” says Gary W. Moorman, professor of plant pathology at The Pennsylvania State University. “This time of year, they should be cleaning up their structures where they’re going to be doing propagating.”
Know what to look for
Fungal threats are the most common diseases among poinsettias, particularly pythium root rot. Spread through potting material, the infection can result in stunted and yellowed cuttings. You’ll first note rotting at the base of the stem, which often continues upward and is sometimes accompanied by white, fluffy growth.
“But sometimes, or very often, you don’t see that white, fluffy growth; you just see stems collapsing,” Moorman says.
Another common fungal disease among poinsettias associated with potting material is rhizoctonia, a root and stem rot. It causes brown lesions on plant stems and roots, particularly at the soil line or where the cutting is inserted into the potting mix. This is often accompanied by a coarse, white webbing.
“The webbing grows under high-humidity conditions from the rooting mix to the cutting or from the cutting down to the rooting mix, so you really need to watch out for that on your media,” Moorman says.
Botrytis, also known as gray mold, also has a telltale visual indicator. Its easy-to-spread spores make it one of the most common greenhouse diseases, and true to its nickname, botrytis is made obvious by fuzzy gray growth. This occurs on the stem of the plant or just above the soil line where the cutting is planted.
“You’ll also see it especially on the leaves of the cutting that are laying right on the mix that’s being used for rooting, and then you’ll see that particular leaf yellowing,” Moorman says.
Bacterial diseases also threaten poinsettias, notably soft rot erwinia. Cuttings become soft and mushy, typically from the base of the stem upward, resulting in rotted, brown tissue.
“Oftentimes, it will kill cuttings before they callous off and before they root,” Moorman says. “I have also seen cases where the cutting will still root then you’ll see some rotting just above that developing, or maybe just below it. The grower thinks the cutting is fine and plants it out, but the plant develops wilting symptoms later.”
Focus on prevention
The best stance for growers to take is a preventative one so they never have to experience these blights in the first place. The most important step is to ensure you’re starting with as healthy a plant as possible.
If you buy cuttings, inspect them to ensure you’re receiving healthy plant material that doesn’t already show symptoms of disease.
“You particularly need to watch out looking at the base of the cuttings,” Moorman says.
If you’re rooting your own cuttings, you want to make sure the rooting media you’re using is not infested with plant pathogens. If you’re using a potting mix, ensure it’s coming out of bags or bales that haven’t been damaged, risking exposure to stray soil. If using a soilless option such as cubes, ensure they’re stored away from stray soil to avoid contamination.
“You want to make sure that whatever you’re rooting plants in is not contaminated with pathogens,” Moorman says. “The other thing is if you’re doing rooting in a mist bed, before you start that you want to have the mist bed cleaned up really well.”
Remove plant debris from previous rootings, and disinfect the bed structure thoroughly. Porous materials such as wood will need to be soaked with disinfectant longer than materials such as plastic or metal to ensure cleanliness.
Move new cuttings from the misting bed to the rooting material as quickly as possible, as the mist bed is ideal for disease development. Possibly add fungicides or biological control agents to the potting media as an added precaution. However, Moorman warns to be careful as these measures can themselves inhibit rooting.
“If possible, you want to stay away from chemical fungicides unless your operation has a history of disease problems during propagation and you’ve used the fungicides before and know they’re not going to give you a big problem,” he says. “Use them as early in the process as you can, because fungicides usually don’t cure a plant once it’s already infected.
“That’s even moreso with a biological control agent,” he adds. “If you’re going to use a biological agent, whether it’s a bacterium like bacillus or a fungus like trichoderma, you really have to have them present as early in the process as possible. They definitely don’t cure a plant once it’s infected, and you want them present so that they take up residence on the new roots as the roots are forming.”
All of the above steps should be done in greenhouses specifically reserved for propagation only, if possible, Moorman says.
“You should be doing propagation in houses where are there are no established plants, especially ones kept all year round because they can be a source of not only pathogens, but various insects as well,” he explains.
Botrytis in particular is easily spread from other plants (such as geraniums), so keep your cuttings away from other susceptible crops.
How to treat
Even the most diligent growers may still experience disease outbreaks. With pythium, erwinia and rhizoctonia, Moorman doesn’t recommend attempting treatment.
“That’s really difficult to manage, you really just need to look at the cuttings and just throw out the ones that are affected — you’re not going to save them,” he says. “With pythium and rhizoctonia, there are fungicides that can be applied, but usually you end up missing some and then the plants will develop root rots and stem rots at a different date. So it’s better to go through and really rigorously pull out any that have symptoms and throw them away.”
Fungicides are more reliable for botrytis. You should also go through and hand clean cuttings to remove as much of the infected leaves as you can.
Educating yourself on the pests and diseases prevalent among poinsettias during propagation will limit loss and ensure your Christmas sales include numerous leafy red shrubs to deck consumers’ halls.
Jessica Hanna is a regular contributor to Greenhouse Management magazine.