Greenhouse Grower Notes: November 2006 3

January 25, 2008
Written by Wayne Brown
Throughout November, a poinsettia crop and Botrytis can make for quite the combination, much to the chagrin of many growers. Almost every year, a portion of many crops has been damaged by our old nemesis, Botrytis.

And why has this happened? Usually it is because we did not develop a strategy or plan to prevent this ever-present fungus from attacking the very susceptible tissue of the bracts.

November, weather-wise, is very unpredictable from one day to the next, let alone from one year to the next. There have been years when the weather has been cool, damp and cloudy for periods of 10 to 12 days, while other years the weather has been mild and sunny interspersed with only a few cloudy days. However, outside temperatures are much colder.

The focus for most growers – and rightly so – has been on the profitability of the crop, or in other words, the bottom line. Natural gas prices remain high, while wholesale prices have only increased modestly. So, the focus on reducing input costs and heat and humidity control are easy targets.

November is also the month when greenhouse temperatures are being lowered from 18-20˚C, down to 15-16˚C to tone the crop and develop the best bract colour possible. By mid-to-late November, depending on cultivars being grown and whether or not the crop was shaded or lit, the bracts have fully expanded and the crop canopy has filled in all the available space.

We all know, but try to ignore, the fact that cool air temperatures and high relative humidity favour Botrytis development.

Wow! When one puts this web of intrigue into perspective, it sounds like we are setting a trap, in which, from a crop management perspective, we can easily get caught. The short and simple answer is … yes, it is easy to get caught!

The life cycle of Botrytis is not going to be discussed because most know the “drill” very well. If you want to review the disease cycle read the section in Chapter 7 of the 2006 edition of the OMAFRA Publication 370 Production Recommendations for Greenhouse Floriculture.

However, it is good to be reminded of some of the scenarios that can result in a Botrytis episode and how to avoid getting caught in the trap. Growers with less sophisticated environmental controls need to be even more vigilant.

Too often, Botrytis appears during or just after a period of several days of wet, damp weather when the outside temperature is in the 5-8˚C range and the greenhouse temperature has been lowered to tone the crop. Tan spots suddenly appear “overnight” on the bracts. It is important to remember to boost the minimum pipe temperature beginning early in the afternoon to raise the temperature 1-1.5˚C to create buoyancy in the air. Exhaust this moist air by fan or vents. The cool air brought in, even though it is contains moisture, will absorb moisture from the greenhouse as the outside air is warmed up.

Botrytis is often more problematic when growing with forced air furnaces. Less accurate thermostats, cold spots and often the variation in temperature from the front to the back because of air circulation patterns can be lead to Botrytis outbreaks. Check your thermostats with min/max thermometers to ascertain their accuracy and raise and/or tighten up the set points to reduce the variation in temperature.

Are all HAF (horizontal air flow) fans the same? Fan blade size and design have an effect on how much air is moved and the pattern with which it is moved. Fans with a variable speed motor have helped. Growers who have older houses with lower gutter heights need to be more vigilant, because the fans are closer to the crop below. If you feel a draft when standing eight to 10 metres in front of a fan, then too much air is being moved, thereby creating turbulence. The temperature of the bracts will then be lower than that of the surrounding air, creating the perfect scenario for microscopic moisture to condense on the bracts, an ideal situation for spore germination. If you think your fans are moving too much air, consider turning them off!

At this time of year, it is important at night to balance the heat between the   overhead and crop heat lines. The radiant energy from the overhead pipes will maintain a warmer bract temperature, thereby preventing the formation of moisture that stimulates spore germination. Yes, it may cost slightly more from a heating perspective, but why risk the potential loss when 90 per cent of the costs have already been put into the crop?

Watering the crop becomes a necessary evil this time of year. Be sure to water early on a sunny day, whenever possible, to allow the crop time to dry. This also allows you to manage the added humidity, especially if on drip irrigation, through additional ventilation.

Unless you like to gamble, lowering the night temperature below 15˚C is foolish and, metaphorically, is like playing with fire.

Be aware of the scenario that can result in Botrytis infection at the shipping stage. If conditions are sunny and warm while sleeving and boxing, close the shade curtain to keep the plant temperature as low as possible. Once plants are boxed, move the cartons quickly to the warehouse to prevent any heat buildup, which can give rise to condensation forming where the bracts are touching the sleeve.  There is nothing worse than the phone call or e-mail with pictures from the customer located six hours away informing you of a major Botrytis problem.

All of the discussion in this article has focused on managing the environment in an intelligent way to prevent Botrytis. For most growers, the application of a fungicide to protect the bracts is also good insurance. I agree, but only when done in conjunction with good production practices. Don’t rely on the fungicide as a crutch! The use of Decree (fenhexamid) has proven to be quite effective when applied by low-volume or high-volume spray and with little or no residue evident.

What’s the bottom line? Quality is in the details, and don’t be caught being penny-wise and pound-foolish!
P.S. – I hope the Christmas shipping season is a good one and free of problems for all poinsettia growers.
 
Wayne Brown is the greenhouse floriculture specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, University of Guelph, in Vineland.

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