Low Chill Hours In Blueberries And Using Dormex

We’ve received a very low amount of chill hours compared to previous years. What does this mean for blueberry growers?

Below shows the chill hours from October to January 5 for the past seven years.

This information is from Jeffrey G. Williamson, Professor and Extension Specialist at the University of Florida:

As you know, many of our growers routinely use hydrogen cyanamide (Dormex or Budpro). Below are some observations that I have made over the years:

1.     Plants respond much better to HC treatment when they have received some pre-chilling conditioning prior to spray applications.
2.     We don’t know the exact pre-chilling needed, but even 70 – 80 hours is much better than 20 or less hours (which can happen in south-central Florida). 150 to 200 hours or more seems to be quite helpful for us with our low-chill cultivars. More chilling always seems to be better as long as it comes before the time to apply HC. I think many of our cultivars are chronically under-chilled in Florida since they respond well to HC treatment, even during what we consider “high-chill” years.
3.     As you know, the stage of flower bud development is critical. I tell growers in Florida to wait as long as they can to get as much pre-chilling as possible but spray before a significant portion of the bloom has progressed past stage 2. Flower buds past stage 2 are often damaged by HC sprays (even on tolerant cultivars).
4.     There is significant variation in tolerance to HC sprays among cultivars. Primadonna, Jewel, and Windsor have been shown to have greater than average susceptibility to injury in Florida. Emerald and Star have been pretty tolerant. I don’t know how one would predict a cultivar’s tolerance level to HC without testing it. I’m not that familiar with the cultivars grown in Georgia so I can’t comment on this except to say that I would not recommend widespread use of HC on a unknown cultivar without first testing it for potential injury. I believe that many years ago, we sprayed Climax successfully in FL with HC but I don’t know the winter conditions (pre-chilling, etc.) under which this was done. You guys hopefully have some experience with your most popular cultivars to draw from.
5.     In Florida, when faced with extremely low chilling conditions (like this year), many growers who make the decision to go ahead and use HC will apply lower than usual rates in hopes of reducing the potential for injury. This seems logical, and anecdotal reports suggest that this may be a good strategy. However, when you get down to 1.25% and below, are you really getting a beneficial response, or are you wasting your time? I’ve always seen the best (and safest) responses from HC with sprays ranging from 1.5% to 2.0 % (formulated product, v/v). But this was under much better conditions than we have in Florida this year. I think most growers in Florida that used HC this year applied lower rates. This year, most of our HC sprays were applied from early to late December. We are now way past the window for safe applications in Florida.

Note that the Dormex and BudPro labels state the product should only be applied to dormant plants and there is cautionary language about incomplete dormancy on the labels.

My best guess about HC use in Georgia this year – in situations where you have accumulated 200 or more chill hours, plants appear fully dormant, flowers are not past stage 2, and the cultivar is known to be tolerant to HC, it may be beneficial. My guess is you are already past stage 2 with many of your southern highbush cultivars. There may be some situations with rabbiteyes where the above conditions are met. For rates, I would defer to your experiences locally, but probably no more than 1.5 % (v/v) of formulated product would be my guess.

Another thing to consider – If HC is applied in early to mid-January and is effective, that is you get early, strong, leafing, this may complicate freeze protection for the rest of the spring as ice loads would be heavier on plants that have more developed canopies.

I can’t think of any reason to apply ethephon on blueberries at this time.

From UGA Specialist, Erik Smith:

Homerville and Alma hover in the 100 hr of chill to date. Many of the farm visits I have made before Christmas show many of the apical buds of Farthing, V1, Emerald, Star, and Rebel at stage three bud swell. In addition the forecast predicts highs over the next two weeks in the 70’s. Each hour above 45 F subtracts an hour (lows are predicted in the fifties). I have been suggesting forgoing dormex applications this year.

If a growers intent is to use lime sulfur or calcium polysulfide give a buffer of 14 days between a dormex application. Also, there should be careful consideration of hort-oil use, which can consume 14 days’ time between applications. ‘Do not apply within 14 days of an oil spray or within 30 days of a copper fungicide spray’ is a direct quote from the IPM guide for growth regulators.

Source: uga.edu