WHITE CHRISTMAS
TEMPERATURES AND CITRUS FRUIT
TREE AND FOLIAGE CONDITION
DEFOLIATION-WHAT DOES IT MEAN-
NURSERY AND YOUNG TREES
DETECTING FRUIT DAMAGE
THERMOMETER CALIBRATION
FRUIT MOVEMENT
WEATHER AND BLOOM

WHITE CHRISTMAS-

I cannot imagine anyone who would have ever expected a white Christmas in the Lower Rio Grande Valley, but if you were out of town, you missed a record snowfall. I do believe that everyone who awakened to the scene on Christmas Morning was absolutely stunned beyond belief, more or less an inch snow covering everything, with a gorgeous blue, sunny sky.

Citrus growers did not necessarily awaken to the winter wonderland, however, as many of us were awake and out-and-about checking thermometers throughout the night. I have heard reports of temperatures as low as 25, but most reports were higher than that.

TEMPERATURES AND CITRUS FRUIT-

At 3:30 am Christmas morning, I noted a temperature of 31 degrees at my home in Weslaco, recording a low of 27 at about sunrise-which was about 7:30 am. Having recently calibrated this thermometer, I am confident as to its accuracy.

Elias Hernandez, farm manager at the Citrus Center, recorded temperatures throughout the night at both the Center and the South Farm, with 28, 27 and 26 being the minima at three locations at 7:00 am. Those same thermometers registered 30, 31 and 30 at 4:30 am, so there was very little time at or below the critical temperature (27-28).

Dr. Bob Wiedenfeld has recording thermometers at the Weslaco Center, at the Annex Farm on Mile 2 East and at Hiler Farm up at Mile 10 North. His data show critical temperatures for only about 2 to 3 hours at all locations which should not have resulted in damage to citrus fruit. Having worked with Bob's data in the past, I have found his temperatures to be a little lower than those recorded at the so-called "official" weather station here at the Center.

I found no ice in fruit from La Feria to north of Santa Rosa on Christmas Morning, and Elias Hernandez reported that he found no ice at either the Citrus Center or the South Farm. Pio Alejandro, fieldman with Edinburg Citrus Association, indicated to me that he had found no evidence of ice or of crystals (white, hesperidin crystals which form in the juice when it has been frozen) in fruit as of Tuesday after Christmas. According to Jim Hearne, Warehouse Farms, ECA and Heald's Valley all were cutting fruit with no evidence of ice formation.

On January 4 and 5, I made a pasear into some groves in west and northwest Cameron County to examine leaf drop. While there, I cut a lot of Valencia, Marrs, Jaffa, and Rio Red fruit looking for evidence of damage. For that purpose, I deliberately selected fruit that was undersized, near the tops of the trees on the north to northwest quadrant, with complete exposure to the sky (obviously, I was selecting fruit most likely to have damage). Of hundreds of such fruit, I found only two fruit with what I consider to be damage-and those two were on Valencia trees which were planted only two-and-a-half years ago (other fruit on the same two trees were undamaged).

Yes, I too have heard reports of temperatures in the low 20's and even a 19-my reaction is either the thermometer needs calibration or somebody has some "oceanfront property in Arizona" that they are anxious to unload.

TREE AND FOLIAGE CONDITION-

Although it does appear that my initial assessment of no damage, with few exceptions, to the citrus crop was correct, there has been more defoliation than I would have expected. The defoliation is primarily of green leaf blades (not the petioles) which means that it is not because of underlying twig damage. Had there been twig damage, the leaves would not fall; instead they would just turn brown and hang on the tree for weeks. I don't recall having observed this phenomenon in the past.

The defoliation is variable in mature and young-bearing oranges, it is generally concentrated in the tops of the trees, with very little drop lower down. In very young, barely bearing oranges, it is more or less general across the trees. It is of no small concern that the buds in the axils where those blades fell are pushing out already?if temperatures remain mild for another week or so, we could see a major leaf flush before the end of January. I don't know about bloom with these emerging buds, as I would have thought it too soon for flower bud differentiation and initiation?processes which occur after flower bud induction.

In mature grapefruit, the defoliation is more widespread in some groves than in others. As a rule, the blades that have fallen show no visible evidence of having been frozen, i.e., they are still green with no attendant darkening that occurs with freeze damage. What is obvious, however, is that most of the fallen leaves have either greasy spot lesions, feeding injury from citrus mites, in some cases leafminer injury and in at least one grove, spray burn or combinations of these damages.

While one might have thought that the newest growth flush was the one most likely to be affected by adverse temperatures, it turns out that that flush is pretty much still there, the fallen leaves are mostly those on prior flushes. In retrospect, the newest (fall) flush is less likely to have insect or disease damages, which is kind of backhanded evidence that the present defoliation is not due to freeze damage.

DEFOLIATION-WHAT DOES IT MEAN-

Significant defoliation is not good for the existing crop or the coming one. Without a full complement of leaves, the normal increases in the size of unharvested fruit will not be realized. In other words, the medium sized grapefruit remaining after an initial ring picking are not going to make size 27 or 32 without adequate leaves. Too, the uncovering of fruit previously not exposed to direct sunlight could result in sunburn, given mild temperatures and especially intense sunlight.

The need to replace the fallen leaves will occur at the expense of fruit set on the coming bloom, as energy that would have gone into fruit set will necessarily be redirected into new leaves. Too, I really don't know whether flower bud initiation has occurred already; if it has, then an early flush will be accompanied by an early bloom. In itself, that's not such a bad thing; unless sufficiently colder weather (i.e., frosty) comes along during or soon after the flush and bloom.

For now, it's a wait-and-see situation.

NURSERY AND YOUNG TREES-

Both young trees and nursery trees received some damage. The very young trees I have seen show some evidence of wood damage, as some of the leaves are brown and still attached. However, they also have green leaves, so it would appear that only the smaller, most tender twigs were affected. If that proves to be the case, then they will recover in short order. Similar damage has been reported in field nurseries.

I have been told that some nursery trees were killed to the bud union. Unless these were very recently budded trees (i.e., a vigorous, tender budling), I don't understand it, as even the most basic cold protection measures should have prevented killing the bud completely. Of course, there remains the possibility that the nursery was in a "cold pocket".

DETECTING FRUIT DAMAGE-

Although there was very, very little fruit damage, I've had enough queries about it to describe what to look for. During a freeze event, the first way to ascertain if damage occurred is to go out and cut fruit, looking for the presence of ice in the juice. With a very sharp knife, slice off about half an inch or more of the stem end of the fruit, and then drag the edge of the knife across the cut surface. If ice is present, the juice that accumulates on the knife edge will look like slush (like the Icee, Slurpee or whatever they call those semi-frozen soft drinks available at convenience stores). If temps are below freezing when you do this, it might be better to do the slicing and checking in the comfort of a warm pickup or Suburban, as supercooled fruit can quickly form ice in the presence of very cold air temperatures.

A second way to check after a couple of days or so is to take thin slices at the stem end of the fruit and look for the presence of whitish looking crystals in the juice. These crystals are hesperidin, which crystallizes out of juice which has been frozen.

Another way to check is to do similar slicing a week or two after a freeze event, depending on ambient temps in the interim. Freeze-damaged pulp will show evidence of apparent separation of the juice vesicles, especially near the peel. They aren't really separating; the juice vesicle membranes were ruptured by ice crystals, so the juice leaked out into the rest of the fruit.

If you wait a little longer, these apparently-separated juice vesicles will become granulated; another way of saying "dried out". The best comparison I can give you is that it looks very similar to the pulp in the stem end of a large, oblong navel orange that is drying out when it is past prime maturity.

THERMOMETER CALIBRATION-

Since I broached the subject earlier, and this winter isn't over yet, I'll give you the quickie version to calibrate a standard mercury thermometer. All you need is a container of finely crushed ice and water about equal quantities of each, as you want to create a slurry of ice and water, the temperature of which will be 32 degrees. Immerse the thermometer bulb in the slurry, wait about a minute and then mark or otherwise note the height of the mercury. I don't care what the scale shows the height of the mercury indicates where the 32-degree mark is supposed to be.

If the marked height of the mercury does not coincide with the 32-degree mark on the thermometer, you will have to evermore add or subtract the difference between the two to have an accurate reading. For example, if your thermometer calibrates at the 29-degree mark on the scale, you have to add 3 degrees to any future reading to be accurate. Conversely, if it calibrates at the 34-degree mark on the scale, you must subtract 2 degrees from any future reading. And that's assuming that the scale itself is correct but just misplaced up or down!

If I had a thermometer that calibrated more than a degree off, I would chunk it and get a new one from a company which has a specialty in making and supplying accurate mercury thermometers for agricultural or scientific use.

If you have one of the newer, remote, recording thermometers, I haven't a clue. About all I can suggest is to place a calibrated mercury thermometer close to the sensor itself (the bulb should not be exposed to sunlight or wind), wait a few minutes for it to stabilize, then call home on your cell phone and have someone check the temperature shown for the remote thermometer at the same time that you are reading the mercury thermometer.

FRUIT MOVEMENT-

As of December 25, the total fresh movement of Texas citrus was pegged at about 85 percent of the volume shipped at the same time a year ago. The shortfall is entirely in grapefruit, as it is running at only 65 percent of last season (to domestic markets); 74 percent when you add in the volume exported. Early oranges were at 117 percent of last season, while navel oranges were at 120 percent.

Given the volume shipped in relation to the USDA crop estimate, about 76 percent of the total grapefruit crop was still on the trees, while 57 percent of the early oranges and navels were still out there. Normally, nearly a third of the grapefruit crop and about half of the orange crop is harvested by the end of the calendar year, which still had a week to go as of the December 25 report from the Texas Valley Citrus Committee.

The f.o.b. prices of fruit are still holding up, with Texas Rio Star Fancy grapefruit ranging from around $8 to $24 per carton (size 56 to 27, respectively). Early oranges are bringing $7 to $10, while navels are about the same, primarily being a little higher for the smaller sizes than earlies.

WEATHER AND BLOOM-

While I haven't the resources to track precise temperatures during the November to January flower induction period, I have made the usual observations of the general weather conditions. It is generally accepted that citrus flower induction occurs in temperate climates with exposure to temperatures below 68 degrees. As a rule, the temperatures in late November and most of December have been very favorable for bloom induction, as there have been a lot of days with temps in the 60's and on down to the 30's and 40's.

While some days have seen temps in the 70's and even the 80's, there has not been a period of several days in which temps were unseasonably warm. That indicates that induction has continued more or less without interruption, and that previously induced flowers have not switched from induction to initiation, which is to say that different blooms are not likely. In other words, the spring bloom should be a single, normal, good bloom in Valley citrus; excepting for the possibilities of an early flush due to defoliation as discussed above.

Obviously, because I cannot track the exact temperatures and accumulations, the preceding should be viewed with just a pinch of salt.

JULIAN W. SAULS, Ph.D.
Professor & Extension Horticulturist
2401 East Highway 83
Weslaco TX 78596