Approaching the Finish Line for Poinsettia Production

Lewis Mite Damage poinsettia production

Lewis mite damage on poinsettia. Look for the characteristic yellow stippling of upper leaves. This can mimic nutrient deficiencies, so make sure to check the underside of leaves to confirm the presence of mites. Photo: OMAFRA

Now that poinsettias have initiated bract formation, the end of the season is in sight. But don’t take your eye off the prize too early — a quality poinsettia crop can still be derailed by issues in October. Here are problems you need to watch out for, and what to do about them at this stage of poinsettia production.

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Height Issues 

Early to-mid fall is the time to start looking at the height of your crop, to see if it’s on track to reach commercial standards. Research out of Michigan State University has shown that early, low-dose drenches (versus sprays) of paclobutrazol (e.g., Bonzi) can effectively control poinsettia height. Drenching with plant growth regulators (PGRs) has shown to have fewer effects on bract size than sprays, which is good news if your plants still need some tweaking into October — though it’s still a good idea to be conservative when it comes to rate.

Lewis Mite

Although Lewis mite (a type of spider mite) comes in with poinsettia cuttings, their damage doesn’t appear until populations build. (Lewis mites only lay 60 to 90 eggs per month, compared to 100 eggs in 10 days for two-spotted spider mite). This can be as early as August, but as late as October.

If you did a preventative miticide application at the cutting stage, you’re hopefully in the clear by now. If not, you should be walking the crop regularly to look for Lewis mite symptoms. These include characteristic stippling of poinsettia leaves, followed by the upper foliage turning brown and the appearance of spider mite webbing if populations are high.

If you find plants with these signs, immediately throw out all symptomatic plants, along with all plants immediately touching them, to help minimize spread.

Also consider a spot spray of miticides for nearby plants (a spot spray being less likely to interfere with natural enemies for Bemisia control). Contact insecticides like Judo (spiromesifen), Floramite (bifenzate), or Akari (fenpyroximate) are options, but if the canopy is dense, systemic miticides might be more effective, e.g., Avid (abamectin) or Kontos (spirotetremat).

Root and Stem Rots

Root rots are sneaky. Infected young plants can look fine — for a time. Now that fungal diseases have built up enough to destroy roots or clog the plant’s vascular system, this can be the time of year we see rapid wilting in poinsettia production en masse.

Fusarium in poinsettia

Wilting of poinsettia plants can happen en masse in early October. Causes include various root rots or Botrytis stem rot. Make sure to have the disease verified using a DNA test at a lab, since root rots are difficult to distinguish from each other on symptoms alone.

If you see rapid yellowing and wilting, as well as brown, unhealthy roots, this could be Pythium aphanidermatum or P. ultimum. The addition of stem cankers right at the crown indicates either Phytophthora or even Fusarium (showing up more commonly in poinsettia crops). It’s best to get your plants formally diagnosed at a lab as soon as possible, since fungicides may slow the spread of infestation, but this depends on the disease. At this stage in production, a product like Subdue MAXX (mefenoxam) could help save plants from Pythium or Phytophthora, but no chemical will cure Fusarium. While waiting for test results, rogue out infested plants immediately, especially if you’re using flood floors/benches.

Botrytis stem rot can also suddenly appear in October. Look for sunken cankers near stem branches and rapid defoliation/branch death on one side of the plant, above the canker.

The main culprit in cases of Botrytis stem rot in poinsettia production is usually improper plant spacing. It’s more common in 4-inch crops, where growers are trying to maximize profits, but sacrifice air flow. Once it appears, reduce your watering, increase your ventilation, and apply a product like Medallion (fludioxonil) or Decree (fenhexamid) before the canopy closes. Other chemicals are effective but risk discoloring bracts.

Whitefly

Consultants in Ontario, Canada have developed a general rule when it comes to Bemisia control on poinsettia. This involves using presence/absence sampling on 5% to 10% of pots per bench. If fewer than 20% of pots are infested with Bemisia whitefly (nymphs, pupae, or adults) in mid-September, then your management strategy (chemical or biocontrol) is working. But, anything higher than 20% is a bad sign, as whitefly populations have time to get out of control by sale.

If you’re using biological control for Bemisia, and you’re around 20% to 30% infestation, you may still be OK if there’s only a handful of whitefly per plant. Pockets of higher infestation can be dealt with using Delphastus. But if you’re above that 20% cut off, and whitefly numbers per plant are high, and your major varieties are affected, it’s time to switch to chemicals. If you’ve been using chemicals since day one of poinsettia production and you’re in this boat, consider tank mixes of pesticides with different modes of action, to get you through the next few weeks. Remember to monitor after each application to determine efficacy.

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