Greenhouse Canada

Features Crop Protection Inputs

January 28, 2008  By Wayne Brown

Watch out for those late-season glitches.  In the previous couple of issues, I have discussed many of the
important cultural aspects of which we as growers need to be mindful
when establishing and growing-on quality poinsettias.

32In the previous couple of issues, I have discussed many of the important cultural aspects of which we as growers need to be mindful when establishing and growing-on quality poinsettias.

The last six weeks are often just as stressful as the first six weeks because of all the potential “potholes” we may encounter on the final run to the finish line! It’s like being in a cycling race … you see the finish line but don’t notice the pothole in the road. You hit the pothole, get thrown off onto the pavement, and can’t finish the race because the front wheel is bent out of shape.


Common late-season disease issues often catch growers off-guard! Often, the production/management decisions made for “short-term gain lead to long-term pain.” Many of the disease problems that arise can be traced back to decisions to save on the heating bill by lowering temperatures and eliminating dehumidification programs.

Powdery mildew is one of those sporadic late-season diseases that can be very devastating, but colourful on red bracts when it strikes in early November, as the bracts are sizing and colouring. Powdery mildew shows up after the canopy has filled in and the weather outside is cool and wet. The disease, unfortunately, just doesn’t show up! It is present low in the canopy prior to being visible on the bracts, but conditions are either not favourable or its visibility on lower foliage is less detectable. Like all powdery mildews, it requires three to four hours of microscopic moisture on the bract tissue to germinate and become established. This condition occurs when the plant temperature is lower than the air temperature or when the air temperature is allowed to drop sufficiently to allow moisture to condense on the bract tissue.

For most growers, the disease never appears when an active dehumidification program is in place. It is important to have a minimum pipe temperature if using hot water in the overhead pipes so that the radiant energy will maintain warm dry plant surfaces. If using steam, short bursts of steam should be programmed to be introduced on a regular basis to create the radiant effect without necessarily raising the temperature.

A few growers, as a precaution, apply Nova 40W by low-volume application in late October or early November, especially if the crop is being grown on the ground. It leaves little or no residue when applied this way and provides that added insurance that allows some growers to “sleep better at night.”
However, our old friend Botrytis is the most common disease for which we need to be most vigilant. To the chagrin of growers, it shows up every year in a few poinsettia crops, usually because of penny-pinching to save on fuel costs. Fuel is expensive and represents a large percentage of the production cost. But from the perspective of the “big picture,” it actually is cheap if a portion of the crop ends up in the compost because of bract spotting.

November is often a tricky month because of the cool, cloudy, often damp weather conditions outside, while inside we are slowly lowering our temperatures as bracts mature to tone the crop. This leads to a low temperature differential between the inside and outside temperatures, which results in little demand for heat. The relative humidity rises to nearly 100 per cent during the night, giving rise to microscopic moisture condensing on the bract tissue; perfect conditions for germination of Botrytis spores.

The opposite is also true. Some November days can be very sunny and the temperatures are allowed in run up (free heat), but then the temperature drops with a steep gradient beginning mid-afternoon leading to high humidity and an environment favourable to Botrytis as the crop enters the night period.

If control of the environment is computerized, then make sure the dehumidification program is active.

Check your minimum hot water pipe temperature settings. For growers with less sophisticated control, make sure that the fans are set to run for a few minutes every hour to exhaust the humid air and bring in fresh air.

The objective of any dehumidification program should be to exhaust a percentage of the humid air, while bringing in cooler drier air which, when heated, will absorb moisture from the greenhouse environment. Adding heat to raise the temperature slightly prior to exhausting some of the air is the most efficient approach because it increases the buoyancy of the air.

Evaluate your situation honestly and objectively. Avoid dropping your holding/toning temperature below 16ºC if you have a history of Botrytis or have less than ideal environmental controls.

When the appropriate management strategy has been implemented, it is usually not necessary to apply a fungicide. Smaller growers with low poly hoop houses and forced air furnaces need to be more vigilant because control of the environment is less sophisticated. An application of a fungicide can be beneficial. Decree 50WDG is very effective with very minimal residue even when applied with a high-volume sprayer. Just remember that relying on a fungicide is not necessarily addressing the real problem.

Pythium root rot is always a concern and there are a few things that should be a standard practice by every grower. Check the salts weekly. If the EC is higher than desirable but the roots are healthy, ease back on the fertilizer. (This assumes it is not the 20th of November.)

If uncertain about whether to water the crop, almost always err on the “don’t water” side. The roots of poinsettias are always stronger and healthier late in the season when maintained on the drier side.

Provide shorter irrigation cycles. Finishing poinsettias on concrete floors is a challenge because it is next to impossible to provide the crop with just a “little water” … it’s all or nothing.

With sub-irrigation, a fungicide application in late October should seriously be considered even if the roots look fantastic. Experience over the years has shown that Subdue Maxx is very effective when applied at this stage of crop development. Water-logging of the media is a recipe for disaster. Root regeneration is slow once a plant is in its later stages of floral development.

EC readings from the various zones will determine fertilization decisions. At this point, the crop should have been fertilized with 15-0-15 since mid-October to avoid stretching. A fertilizer EC of 1.5 to 1.8 mS/cm is adequate in most situations. In general, during the last two weeks prior to shipping fertilization should be done with clear water.

In November, as temperatures slowly are being lowered and with the lower light levels, this means often watering only every six to seven days.

Every year, a portion of the early planted crop gets too big for the spacing given, or the 15 cm crop being grown under 20 cm pots hanging above them begin to stretch in early November.

What to do?
The first obvious step is to get some of the baskets down from above the crop. They are getting all of the limited light and as well influence the red:far red ratio. If we are serious about quality, then we can’t fill the air above the benches with product during low-light conditions … it’s a false economy! Most people are growing fewer poinsettias and have open production space.

If certain benches of the crop are stretching, then remove some and re-space them. It is amazing how poinsettias stop stretching and flatten out as soon as the plants have some light penetrating all around. The plants maintain rounded habits, stem breakage is less, and leaf yellowing is not an issue.

Yes, it costs money in labour. But if we are serious about selling a quality product that the consumer will want to come return to purchase more of, then we have to do what it takes.

Growing poinsettias always seems to be a challenge. Watch out for those proverbial “potholes.” Focus on the production details and then roll with the punches that the crop is sure to throw your way.
The only constant with poinsettias? Christmas Day is always Dec. 25.

Wayne Brown is the greenhouse floriculture specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, University of Guelph, in Vineland. • 905-562-4141, ext. 179;

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