Now, you’ll be forced to make a choice with your poinsettia crop. Do you stay the course with a biocontrol program? Or spray for pests? And, as always, the question is “spray with WHAT”?
This post has tips on pest management decision making when it comes to common poinsettia problems in October.
Bemisia Whitefly Issues
Some of you may be familiar with the “whitefly tipping point” I’ve referred to in several blog posts and articles (see this issue of Greenhouse Canada Magazine). Essentially, this is when growers have to decide whether or not to switch to chemicals for whitefly control, so they can end up with a clean crop. Switching to pesticides later than the first two weeks of October may mean you don’t have time to get on top of your whitefly population if repeated applications are needed.
The generally guideline is that if MORE than 20% of your crop is infested with whitefly (using presence/absence sampling) in the first week of October, you should switch to chemicals. This is based on sampling at LEAST 100 plants, or 5% of your crop. (See here for more details on how to sample Bemisia in poinsettia).
However, your decision should ultimately be based on several factors particular to your farm, NOT just on the rough metric of percent infestation. Other questions to ask include:
- Is your main red variety heavily affected? In all areas or just one?
- Are you little seeing evidence of your biocontrol program working? (Again, in all areas?)
- Are you seeing whitefly colonization on new growth, or is it on old growth only (original cutting leaves)?
- Do most plants have at least 1 densely colonized leaf? Or are there only 1 or 2 adults or pupae here and there on each plant?
Ideally, to stay the course for biocontrol, you’d like to see less than 20% of your crop infested. BUT, you’d ALSO like to see evidence of host feeding/parasitism, few dense clusters of whitefly colonies, low whitefly density on new growth, and less pressure in your main crop than in the colours.
If 3 out of 5 of these things are NOT true for your crop, then it may be time to seriously consider pesticide application. But which ones? This is always the $64,000 question, as Bemisia whitefly pesticide resistance can vary year-to-year. One thing you can do to be MORE sure is a quick pesticide trial (see this post on pesticide efficacy trials in poinsettia from 2021).
Generally, though, we find combinations of different active ingredients to be most effective for Bemisia whitefly, as this helps overcome their resistance mechanisms. It will also reduce the number of overall sprays needed. (Always check the label to see if products have a general clause in favour of tank mixing!)
If you decide to spray, it can be a good idea to start with softer chemicals, so you can continue to receive benefits from your bio program. Products could include Ference (cyantraniliprole; IRAC 28) and Beleaf (flonicamid; IRAC 29), as they are soft on parasitic wasps. If this doesn’t work, you’ll want to consider moving up to “harder” chemistries like Kontos ( spirotetramat; IRAC 23) and Intercept (imidacloprid; IRAC 4). Additionally, a new product, Rycar (Pyrifluquinazon; IRAC 9B) could also make a good option in a chemical rotation. It’s effects on whitefly natural enemies are currently unknown.
Although Lewis mite: come in on cuttings, their populations build up slowly. This means plants can show damage as late as October. The upper foliage will turn brown and the mites will form unsightly webbing.
What to do if you start finding them now? First, immediately throw out infested plants in a “circle of love” (i.e. the centre infested plant and all those immediately touching it). Then, consider a spot spray of miticides for nearby plants (a spot spray being less likely to interfere with bios for Bemisia control).
For many, Floramite (bifenzate) seems to work for Lewis mite. However, it is a contact insecticide, and if the canopy is dense, systemic miticides might be more effective (e.g. Avid (abamectin), Kontos (spirotetremat)). Note, however, that these products run the risk of disrupting biocontrol programs for whitefly, and you should avoid spraying them farm-wide, unless you’ve already decided to move to chemical control for Bemisia.
Pythium and other Root and Crown Rots:
If you did not apply any preventive fungicide applications in propagation, now is the time when root and crown diseases can rear their ugly heads. Look for rapid yellowing and sudden wilting, which could be signs of Pythium aphanidermatum or P. ultimum.
The additional sign of stem cankers right at the crown indicates either Phytophthora or even Fusarium (showing up more commonly in poinsettia crops).
You’ll need to get your plants tested at a lab ASAP, as root and crown rots cannot be distinguished from each other based on just physical symptoms – DNA confirmation is needed. Depending on the disease, chemical treatments may slow the spread of the infestation (e.g. Previcur (propamocarb), Subdue MAXX (metalaxyl), or Truban (etridiazole) for Pythium and Phytophthora) or be completely ineffective at this stage (e.g. any product for Fusarium).
No matter what the disease, rogue out infested plants immediately, especially if you are using flood floors/benches. This will help decrease any secondary spread.
Botrytis Stem Rot: A grower can be right on track for a quality crop, only to be suddenly derailed by Botrytis stem rot (caused by the same fungi as foliar Botrytis) in October. Look for sunken cankers near stem branches and sudden defoliation/branch death on one side of the plant (above the canker).
The main culprit in Botrytis stem rot is improper spacing at the beginning of the crop, and is likely more common in 4-inch crops, where growers are trying to maximize profits. Once it appears, reduce your watering, increase ventilation, and get an application of a product like Medallion (fludioxonil) or Decree (fenhexamid) on before the canopy closes and it’s too late to penetrate the crop properly. Other chemicals can be effective but risk discolouring bracts.