I love this time of year, as hectic as it is. For most people in south central/central Florida – things are winding down, while in north central Florida, we are in the midst of harvesting.
Most people this year had a great season, with great fruit quality and flavor. However, we did have a wet spring (20 inches from January 1 – May 20th) in Citra and north central Florida, while in other years we get less than 10 inches in that same time span. Cloudy and wet weather affects a number of plant processes such as:
Carbohydrate fixation and accumulation – aka making sugar in enough quantities to have good brix content in the fruit. I also saw overripe fruit because growers were waiting for the fruit size to get bigger, but because perhaps of the cloudy/rainy spring, cell division (stage 1) of fruit development wasn’t as good as it has been in years past. It didn’t help the fruit quality in the store – and this was the result:
Wet feet – in orchards that have poor drainage, you can have problems with leaf drop and/or fruit drop due to damage in the root systems due to suffocation of the roots (anoxia).
Increased disease pressure – We also saw a bit of anthracnose ripe rot and botyrosphaeria rot in the fruit after the rains and some of the warm weather we had in the late part of April/early part of May. It’s important that if we do have successive rain events that you apply fungicides and rotate the chemistries to avoid resistance. Common fungicides like Abound and Topsin-M are great, but are categorized as medium or high risks for resistance and should be used sparingly or in a diverse rotation of chemicals.
Fungicide management is easy to do – but if you don’t know what a FRAC code is (and no – Battlestar Galactica fans, I’m not swearing…) here is a website that will take you to the codes for each chemical. http://www.frac.info/publication/anhang/2014%20FRAC%20Code%20List.pdf. You’ll notice that this chart does nothave the company (trade) names of the chemicals so you will have to READ the LABEL to find out what chemical your fungicide contains – and the FRAC code should be listed there as well.
Hints for Next Season
At our winter field day this year in Fort Pierce (typically held the 2nd week of December) we will be talking about thinning. There was a lot of small fruit put into local supermarkets, and much of this can be alleviated especially in UFSun with more aggressive thinning. Instead of 6″ between each fruit as is suggested with other Florida peach varieties, UFSun should be thinned to 9-12″ between each piece of fruit. In addition, anything that can be done to encourage leaf area expansion will be key to getting good fruit size and flavor development.
The use of hydrogen cyanamide is looking promising for peach growers that are down south especially as it appears to help with leaf emergence – an important stage of growth to help fruit develop its maximum potential for fruit size and good flavors. However, we need more research on timing, and I have some key growers set up around the state this year to look at a couple of different timings and see what the impact is on leaf growth and fruit size/harvest date. If we can get more uniform leaf emergence and bloom, then we should be able to narrow our harvest window and minimize the number of pickings necessary to harvest the entire crop. More research to come in 2014-2015!
As always, if you have any questions – please call or email me!
How is it that our peach trees are already blooming in the south central part of the state? It seems like we are too early by at least three weeks in many cases. It makes for a busy holiday season watching our weather to make sure that we stay on top of frost protection. A current analysis of the chill accumulation across our state doesn’t look good – keeping in mind that last year was also a warm year and winter. See AgroClimate: http://agroclimate.org/_agroClimate_email/november-2013/.
As a grower, what is there to do? Well, first I think it would be good to start out with an explanation of the types of dormancy that perennial trees go through (Lang et al., 1987).
Growth cessation and dormancy in peaches is driven by reductions in the photoperiod. Photoperiod is the amount of light that we get during the day vs. the night period, and as we approach the autumn, the light period decreases and the peach trees respond to this by dropping their leaves (defoliation).
Ecodormancy is where the tree stops the visible growth process and goes into a reduced state of metabolic activity because of an environmental stress. This could be water stress (we shut off the water around the 1st of November to hasten this process) or cold temperatures (happens up to the north quite a bit).
Paradormancy is a type of dormancy where physiological factors outside of the bud affect its growth and development, and this might be the production of auxin (a plant hormone that contributes to apical dominance), or a reduction in photoperiod during the season.
Endodormancy is a period of dormancy that is induced within the flower or vegetative bud, and typically includes chilling responses and photoperiodic responses. We see this type of dormancy affect vegetative buds, when we don’t get enough chilling and the resulting response is delayed vegetative bud break, or leafing.
There are two main types of chill accumulation models that can help growers to track units of chill that are accumulated at night. One is the standard chill accumulation model and the other is the Utah chill accumulation model.
The standard chill accumulation model is one in which 1 hour at a temperature less than 45°F (7.2°C) equals 1 unit of chill accumulation (Weinberger, 1950). In this model, any temperature, even those below 32°F are considered temperatures at which chill units are accumulated. However, this doesn’t work that well for peaches, and a researcher in Utah working with ‘Elberta’ and ‘Redhaven’ peaches delved deeper into this subject and devised a second model (Richardson et al., 1974).
The second model, and the one which we use for peaches, is called the Utah chill model. In this model, an ideal range for chill accumulation was established for peach buds to come out of “endodormancy”. For each hour at the following temperature ranges, a chill unit, or portion of a chill unit is accumulated. These temperatures are:
In the Utah chill model, you can see that there is an ideal range at which 1 chill unit is accumulated, and then portions of chill units are accumulated at higher and lower temperatures. So, although we may be above the ideal range (36.5 – 48.4°F), we still have the opportunity to accumulate half-units of chill during our warm nights in Florida. However, the caveat with this model is: if it is too warm immediately after chilling is accumulated, warm temperatures can negate the chilling from the previous two nights. Chilling accumulated prior to this warm period and the previous 2 nights remain valid within our working model.
We can also observe fruit set problems when these warm nighttime temperatures occur during the bloom period with some of our peach varieties, notably, ‘UFOne’ and ‘UFBeauty’ in warm areas of Florida.
The AgroClimate site which covers chill accumulation (http://agroclimate.org/tools/Chill-Hours-Calculator/) offers two options when you choose a weather station: <45°F, and 32-45°F. Although not quite within the ranges that we need to calculate the proper chill accumulation with half-units, the 32-45°F model does a pretty good job of an estimation. So, be sure to choose this one when tracking your chill unit accumulation this winter.
For those of you that use Weather Underground, I found a neat tool that calculates the chill accumulation for a particular station. It’s called Get Chill Hours! (http://getchill.net/), and calculates the chill accumulation with three different models. One caution: when the model calculates the Utah Chill Model accumulation, it will most likely be negative because of our warm nights interspersed with our nights that accumulate chill. Thus, the 32-45°F is again a better depiction of our chill accumulation with certain caveats as mentioned in the Utah Chill Model explanation.
Results of Low Chill Accumulation?
So, now understanding some of the background about dormancy, you can see that in South Central Florida with our low-chill peach varieties, trees in most years appear to not enter into endodormancy, which can hinder the ability of the tree to push vegetative buds when they should. Observation of trees in commercial orchards indicate that trees enter into ecodormancy in late October and early November when irrigation and fertilizer are reduced in the orchard.
Warm temperatures during the past three years in December have resulted in trees flushing and blooming approximately December 15th without significant accumulation of chill units. Fruit set in varieties like ‘UFSun’, ‘Flordaglo’, ‘UFBest’, and ‘TropicBeauty’ has been excellent under these conditions.
The result of not leafing shortly after bloom or having the delay of the vegetative buds push, is that the tree must rely on stored carbohydrate and nitrogen reserves to produce fruit, and both cell division (which affects fruit size) and the cell enlargement (when sugars and peach flavors get transported and develop in the fruit) can be reduced, ultimately producing a “flat” tasting fruit. The leaves, through photosynthesis, produce the sugars and flavor compounds needed to give the fruit that “tree-ripe” flavor that we sell to the consumers.
A second result of low chill unit accumulation is an extended bloom period. This results in an extended fruit set period, and thus thinning must be done several times during the late winter/early spring, rather than once during the thinning period. Multiple trips through the orchard results in more labor dollars spent, reducing the orchard’s bottom line.
What can be done?
All of this is wonderful information, but how will it affect how we grow this year’s crop? In years with low chill unit accumulation like this one, experience tells us:
The trees will bloom early. Be sure that your frost protection systems work, and you are ready for any cold fronts that sweep down the state.
Because the trees are blooming early, if you have >50% bloom, start watering and fertilizing. Stress during the fruit set and cell division period will result in fewer and smaller fruit.
Be sure to know what your fruit developmental period (FDP) for your peach variety. These ranges still hold true for overall fruit development, but cool, rainy weather during the spring will delay any gains in the bloom period that we have achieved. In addition, if you bloom around January 1st you will most likely harvest ‘UFSun’ around April 1st. But, if you bloom around January 20th, then you would probably harvest ‘UFSun’ around mid-April – because it is warmer from Jan 20 – April 15th then from January 1 – April 1st. Warmer temperatures will compress the fruit developmental period.
If you have fruit on the trees, start your disease and insect protection programs along with your irrigation and fertilization programs.
In the future, we hope to gain some knowledge about the use of rest-breaking chemicals like hydrogen cyanimide (Dormex® or BudPro®); however, with the products there is a great potential to do significant bud or branch damage with high concentrations and/or application at the wrong time. In cooperation with a few growers around the state, I’m tracking the results of various applications and rates and will have more to report by the end of this year.
I saw this article as I was perusing the Puget Sound Wine Growers Group List that I am still a part of – http://www.wired.com/design/2013/07/inforpr0n-winemapz/ – and I was surprised to see that the premium wine growing regions in Washington State and California will at some point be too warm to grow great fruit.
What does this mean for the rest of the country and even the globe?
Increasing carbon dioxide (CO2), rising temperatures, and changes in precipitation pattern will affect agricultural productivity.
Livestock production systems will be greatly affected by temperature stresses.
Climate change effects on crop and livestock production for the next 25 years will be mixed.
Climate change will make current stresses (e.g., drought, flooding, salinity) even worse on plants and animals.
Agriculture is dependent on processes that support crop productivity including maintenance of soil and water quality AND quantity.
Higher incidence of extreme weather events will have increasing influence on agricultural productivity.
The vulnerability of agriculture to climate change is strongly dependent on the responses taken by humans to moderate the effects of climate change.
We can adapt.
What was really interesting when I was with WSU in Prosser, WA – the viticulturist there examined possible increases in maximum temperature as a driver of climate change in the region. However, what we found was that while minimum temperatures were increasing, maximum temperatures were fairly steady over 85 years.
What impact might minimum temperature increases have? We could start observing more varieties with fruit set problems, like that of ‘UFOne’. With ‘UFOne’, researchers at UF found that when the night temperatures were over 55 °F, fruit set was poor. It could be that the pollen is being shed before the ovary is ready, or that pollen tube growth (on its way to fertilize the ovary) is arrested and the pollen never makes it to the ovary.
The warm night temperatures might also affect acid levels in the fruit – which are important in developing quality flavor profiles. Fruit with little acid often end up tasting sweet, but flabby and don’t linger very long on your palate, making for a bland eating experience.
I’ll end this with a picture of surface temperature projections through to 2099 in two different scenarios, one in which CO2 emissions are controlled (low) and one in which CO2 emissions continue to remain high. I am hoping that technology will help us moderate emissions so that we can achieve the lower temperature scenario for the future.