Poinsettia Prep Refresher

JF14Now that poinsettias are safely tucked into their prop trays and the threat of Erwinia (Pectobacterium) is almost over, it’s time to think about other Poinsettia issues.

Root rots, nutritional issues, environmental stress and PGR mistakes can all be costly in this high-value crop.  Read on for common pitfalls and how to avoid them.

Diseases  in Propagation That Can Sideline You in September

Although you may start seeing issues from root rot in plug trays now, many of these diseases (including Pythium, Phytopthora, Rhizoctonia and even Fusarium) won’t kill the rooted cutting outright.  Often disease will proceed slowly, with large numbers of plants suddenly wilting and dying sometime in September or October.  So make sure you’re regularly inspecting your cuttings to get the jump on these diseases.

Evidence of root rot including poor rooting, stunted growth, discoloured (black or brow) roots, loss of lower leaves, or cankers on stems.  If you see these, then you’ll seriously want to consider getting the disease identified by the Guelph Lab Services so you can make a plan of attack.

Phytophthora crown rot. The pathogen is Phytophthora drechsleri, which causes a root rot that may extend upwards into the lower stem. It is not possible to distinguish this injury from that caused by Pythium aphanidermatum, which is also commonly found on poinsettias. Photo  courtesy of Cornell Cooperative Extension

Generally, with any root rot, here’s how to get on top of it:

  • Rogue out any infected plants immediately to make sure the inocculum doesn’t spread
  • Once you’ve had your disease identified, apply an appropriate preventative chemical fungicide in propagation to help knock it back.
  • Follow this with an application of a broad-spectrum microbial fungicide like Actinovate, Cease, PreStop, Rootshield, Taegro or Trianum on rooted cuttings that will boost root growth and help fight off disease. You may even be able to drench these directly with the chemical fungicide you’re applying, depending on the chemical.  Ask your rep!
  • Don’t over-fertilize or over-water plants, as this can promote disease
  • Keep your greenhouse sanitary: these means no hoses on the ground, no pools of algae or potting mix on the floor that can promote pests like fungus gnats, which can spread plant disease.
  • If necessary, control fungus gnats with applications of nematodes or insect growth regulators like Citation or Dimilin as soil drenches.
  • Repeat up your microbial fungicide applications RIGHT at potting.  This step is key in continuing the fight disease as they get placed in a bigger container.

If these steps are followed, there’s a good chance you won’t start losing plants in September, and need to follow up with even more applications of chemical fungicides.

Proper Nutrition

Typically water soluble feeds are used, but a few growers have been trialing controlled release fertilizers (CRFs) for poinsettia production with good results.  The CRF evens out nutrient availability between liquid feeds, and can allow you to fine tune your fertilizer program throughout production by adding in liquid feed where and when necessary.  Talk to you supplier about your options and ask them for recommendations and trial results if you are considering trying a CRF/liquid feed nutrition program in 2018.

Points blog graph

“Christmas Day” poinsettias showed little to no difference in height & plant mass after finishing with 0, 25, 50 and 100% of the recommended levels of Mo. (Credit: J. West, FCO & W. Brown)

Molybdenum (Mo) has always been considered a limiting nutrient for poinsettias, but research done by OMAFRA and Flowers Canada has shown that it may not be as critical in the final stages of production.  In their study, they looked at reducing the rates of molybdenum once rooted liners of the variety “Christmas Day” were spaced, and there were no significant differences in height or weight of the plants finished at 100%, 50%, 25% or 0% of the recommended Mo level.  The lower levels (0 and 25%) showed a decrease in Mo levels in plant tissues, but no visible deficiencies were seen at the shipping date.  Its important to note that this study didn’t look at propagation or young plant development, where fertilizer recommendations for all nutrients, including Mo are still suggested.

Environment: Temperature and Humidity control

Water and Nutrient for Greenhousehouse and Container Nursery Pro

Well spaced poinsettias as bracts begin to colour.

Spacing can be a big contributor to temperature and uneven plant growth. Pots that are too close together will have weak, elongated stems as the plant competes for light and resources with it’s neighbours. In addition, the tight spacing can lead to uneven canopy expansion and contribute to disease and pest pressure in  humid micro-climates.

Remember that temperature differentials (DIF) can be very valuable for controlling plant growth.  A negative DIF (cool day/warm night) can hold plants at a height, while a positive DIF (warm day/cool night) can help to increase plant height. The end result will depend on the variety and difference between the day and night temperatures.  Stem growth is most active at dawn, so some growers choose to lower the temperature for only a few hours around dawn, with good results.

Proper Application of PGRs

Most newer poinsettia varieties and hybrids have been bred to have a compact, upright shape so pinching and PGR application are perhaps not as critical as they were in the past (although every grower has their preference!). PGR mistakes can be costly, so the more information a grower can gather before making a decision to apply is better. Graphical tracking of plant development against your sales specs for height and diameter can assist in making height management decisions, weather they are PGR applications or DIF adjustments.

growth curve

Example growth curve for a greenhouse crop such as Poinsettia.

To graphically track your crop over the season, measure and record the height of at least 10 plants of each variety that you grow weekly (different varieties may have different growth curves).  Make sure to be consistent in measuring height from either the pot lip or the floor/benchtop.  Indicate dates for any production changes or product applications and ensure the amounts and application methods of PGRs are noted as well. Once you have a few seasons of this data, it’s easy to compare where you are to where you should be overall and make more informed management choices. 

Research out of MSU has shown that early low-dose drenches of Paclobutrazol (e.g. Bonzi) can effectively control height for poinsettia.  However the response by variety plays a big part in both the number of applications and the rate.  In general, more vigorous varieties required more applications or a higher rate (details on what they found for specific cultivars is here). In general, sprays of Daminozide (e.g. B-Nine) or Chlormequat Chloride products (e.g. Cycocel) will provide shorter-term inhibition of stem elongation which may be desirable for small height corrections or holding crops later in production.

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