Poinsettias may only in the greenhouse for a relatively small time, but they can have BIG problems compared to other crops. Lewis mites, Pythium, and the dreaded Q-Biotype of whitefly all have the potential to make a significant number of plants unsaleable. So what’s a grower to do?
By implementing some preventative measures RIGHT NOW (i.e. while cuttings are still in trays, or just potted up), you can save yourself a lot of headaches later.
Warding Off Whitefly:
I would LOVE to write a post on measures that will prevent whitefly completely. But since pigs don’t fly, here are some tips to at least reduce your starting whitefly population.
- Dipping your cuttings: In previous posts I’ve mentioned how dipping your unrooted cuttings in insecticidal soap and biopesticides can reduce whitefly populations by 70%. Although most cuttings are rooted by now, I’ve heard many anecdotal reports of growers successfully dipping after rooting (while the cuttings are still in trays). If you’re going to go this route, trays should be turned upside-down in the dip tank – contact of the dip solution with the root area should be avoided. Here’s information on dipping products, rates and procedures.
- Apply Biocontrol Agents early: luckily, there are many whitefly biocontrol agents on the market (check out GreenhouseIPM.org for details). By using a mix of parasitoids and predators early in the crop, you can keep whitefly populations low enough to delay the need for pesticides (see next point!).
- Avoid pesticides for as long as possible: At this point, most growers are aware that the Q-Biotype of Bemisia whitefly thrives when pesticides are applied. Spray too early and too often, and all you’ll get is a greenhouse full of these unkillable monsters. To prevent this, delay your first pesticide spray until the END OF AUGUST at the EARLIEST (see Fig. 1). I’d even prefer you wait a little longer than that. This way, the Bemisia population will have enough time to switch predominately to the pesticide-susceptible B-Biotype (favoured under no-spray conditions). This applies whether you are using predominately biocontrol, or an all-pesticide program for your whitefly (see Fig. 1). Against “B”, you’re more likely to see results with chemicals like Distance (pyriproxifen) and Kontos (spirotetremat), as well as Dyn0-Mite (pyridaben; used as a crop clean up for adult whiteflies).
Levelling Lewis Mite:
You may not notice Lewis mite (a species of spider mite) until mid-October, when populations build up enough to start causing noticeable damage. But, that doesn’t mean it’s not already there. Early detection of this pest is difficult, since the symptoms are rather subtle at first, and the mite is almost impossible to see without a microscope.
Webbing caused by a severe infestation of Lewis mite. Photo courtesy of Ohio State University.
Given that they are difficult to detect but can cause a lot of damage at a critical point in the crop, the easiest solution is a single, preventative application of a miticide at the cutting stage. These include Avid (abamectin), Floramite (bifenazate), Vendex (fenbutatin oxide), Shuttle (spinosad) or Forbid (spiromesifen).
Control of Lewis mite with pesticides is more difficult later in the crop when the canopy is filling in, since Floramite, Vendex and Shuttle are contact miticides. Systemic miticides applied later can interfere withwhitefly biocontrol programs (being hard on parasitoids and predatory mites).
Now that the plants are off the misting bench, our focus turns from Erwinia to Pythium root rot, which can cause severe wilting an yellowing.
Although Canadian growers don’t seem to be having the same issues with fungicide-resistant Pythium strains as the U.S. is having, we may not be able to dodge that bullet forever. Even without the complications of resistance, timing of fungicide applications is still an issue – if Pythium gets a significant head start, then fungicides are unlikely to rescue it.
Because of this, many growers in California currently apply drenches of microbial fungicides (e.g Rootshield, Trianum, Cease) at the cutting stage to prevent disease and avoid fungicide resistance. By drenching poinsettias while still in the trays, then again at potting, the beneficial organisms not only occupy the space where pathogenic fungi would normally grow, but also actively fight them off. As long as Ontario seems to avoid resistance issues, preventative drenches of Subdue (metalaxyl), Truban (etridiazole) and Previcur (propamocarb hydrochloride) may also be effective.
However, microbial fungicides – being far more “general” than most chemical fungicides – also have the added benefit of fighting multiple plant diseases that affect poinsettias under the right conditions (e.g. Fusarium, Botrytis, Rhizoctonia). This avoids potential calamities from misdiagnosis and incorrect fungicide selection.
Unfortunately, even with applying all of these preventative measures, some sort of problem will likely still arise within your poinsettia crop (I like to call this “Graeme Murphy’s Law”). But by implementing these tactics upfront, you’re more likely to minimize the impact of most issues, and avoid some entirely. Perhaps most importantly, you’ll also avoid a case of the dreaded “Shoulda, Coulda, Woulda’s” when sale time comes ’round.