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Flower Bud Induction Overview and Advisory

NOTICE FOR CITRUS EXTENSION AGENTS & SPECIALISTS AND GROWER NEWSLETTERS
The following information has been developed as part of the Decision Information System for Citrus

L.  Gene Albrigo, Horticulturist Emeritus
Citrus Research & Education Center, Lake Alfred, FL

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12/3/1999

FLOWER BUD INDUCTION ADVISORY #1 for 1999-2000-12/3/99

The 1999-2000 Fall-Winter weather in Florida is expected to be influenced by a continuing La Nina event that affected Florida weather last Winter. Meteorologists predict that, overall, warmer and dryer than normal weather will occur in Florida, but that also means the chances of a freeze are increased. Warm average conditions suggest that inadequate cool temperatures for induction of citrus flower buds is likely. A prolonged period of cool temperatures (below 75 o F) with no tree growth is necessary for good flower bud induction to occur. Warm periods can initiate bud differentiation (growth) and discontinue the induction process. Last winter season at least four cool periods were followed by week-long warm periods. This led to the observed 4 blooms that occurred from January through late April to early May. Since a single prolonged cool period is less likely this year, methods to increase citrus flower bud induction may be useful.

The other natural process for flower bud induction in citrus is by drought stress sufficient to stop tree growth and cause induction to proceed. Drought stress is the primary process of flower bud induction of citrus in low land tropical climates. In some years, drought stress is operative under Florida winter conditions and also can be responsible for ‘June blooms'. If little rain occurs during a warm winter (more likely in this La Nina year), significant drought stress may develop without irrigation. Of course, if significant rain occurs drought stress will be diminished or not occur. Withholding irrigation for at least 30 to 40 days should be long enough to increase flower induction. Drought stress also can stop initiation of differentiation of potential flower buds if intermittent warm periods occur during the winter. Some growers avoided multiple blooms last year by keeping their trees drought stressed until they were ready to initiate flower bud differentiation in mid-January.

In spite of the general prediction for relatively warmer temperatures this year, the Fall has been cool and excellent induction temperatures are occurring in the 1st week of December. Through November 30th nearly 200 more hours below 700 F have accumulated than last year. This helps to stop growth and get the flower bud induction process underway. As IFAS did last year, the FAWN (Florida Agric. Weather Network) WebSite will provide additional information on accumulated hours of cool temperatures. The hours of temperatures below different thresholds can be viewed for each of 15 available sites by going to FAWN (linked through http://www.crec.ifas.ufl.edu) and to option 4 (select data).. In the four fields, choose the location, summary data and specify the dates to be included, then click on search. This will provide a summary of hours accumulated below different temperatures between the specified dates. You should choose October 15th or November 1st as your starting date. I will attempt to relate the cool temperature accumulation and warming spells to previous good and poor flower induction years as this winter progresses. Weekly advisory reports will continue until mid or late January, after which it is less likely that any attempts to alter flower bud induction will succeed. Since 7 to 10 day temperature and rainfall forecasts are available, we can anticipate when a given cool period will end. We will discuss more on the use of this information later.

Moderate drought stress may be maintained on flatwood bedded groves with the irrigation system shut down. By maintaining the water table just below the bottom of the water furrows, daily stress can occur with some recovery each night from moisture taken up by the tree roots that extend to near the bottom of the water furrow. If the water table is one or more feet below the water furrow, severe drought stress will occur if irrigating is stopped and their is no rain. Also, if severe root pruning occurred due to high water levels during the previous two years, then a water table at the bottom of the furrow may still result in a severe stress level. Visual observation of temporary leaf wilting can be a good indicator of proper stress. Wilting by 10 or 11 am and recovery over-night is ideal. Drought stress is much more difficult to develop on deep sandy soils due to the deeper rooting zone. Two or three weeks may be required on deep sands in order to reach an adequate level of stress to begin flower bud induction. Drought stress can be used on Valencia trees, trees already harvested or Hamlin trees that will be harvested during December. Trees with fruit intended for fresh market should not be subjected to severe drought stress. Valencias hold their fruit very tightly, but Hamlins for later harvest or Pineapple oranges are likely to experience excessive drop of the current crop if subjected to even moderate drought stress.

If one or more winter-time rain fronts negate the attempt to maintain water stress, the associated cool weather behind the front (a typical occurrence) may provide some cool temperature induction. The issue will be whether enough induction has occurred. Even in cool years of good induction conditions, winter sprays of stress-boosting chemicals can provide benefits by increasing flower bud induction and subsequently yields of at least Valencia trees on flatwood soils. Four years of tests on Valencia oranges on bedded grove sites have shown a consistent enhancement of total yield and pounds solids per acre from urea or potassium phosphite (PO3) sprays (Albrigo, Proceedings Florida State Hort. Soc., 1999, in press). Some grower tests using these materials have provided similar increased flowering and yield results on other cultivars as well as Valencia trees.

After some cool temperature induction or 30 days of drought stress, foliar sprays of either 50 to 60 lbs of spray grade urea per acre or 2.6 quarts of 28 % P as potassium phosphite per acre can enhance flower bud induction. Foliar sprays of urea or phosphorous acid products presumably work only if some induction has taken place from natural cold or drought stress. Lovatt, et al.(1988) demonstrated this for foliar sprays of low biuret urea. In our previous tests, the timing of this spray was between Christmas and January 5th. In years of strong El Nino or La Nina events, suitable timing may be earlier or later. In any case, the timing must be before significant high temperatures occur (3 or 4 days of maximums exceeding perhaps 85o F) which will start the flower bud differentiation process. It is currently still difficult to make precisely-timed recommendations, and we can not guarantee benefits. None-the-less, advisories on the accumulation of cool temperatures, generated from the FAWN weather system, and recommended spray timing for Winter sprays will appear on the Citrus Research and Education Center Web Page each Tuesday morning starting the week of December 13th. This website is at http://www.crec.ifas.ufl.edu (look under Grower Aids). This information will be forwarded to County Extension offices and other publishing sources. It should be noted that waiting for these written announcements may cause delays in management practices which may result in missing an important timing window.

If you are interested in applying either urea or phosphorous acid this winter as a test or on substantial acreage, you need to be prepared. The product must be on hand and the spray equipment available on short notice. Low biuret spray grade urea is needed (< 0.5 % biuret on a dry weight basis) or a suitable clean source of buffered phosphorous acid. If you plan to use an aerial applicator, they must use a micronair or similar distribution spray system and they must be booked in advance. This of course complicates timing. Ground sprayers can be used at low volume and fast speeds. Air blast sprayers will work at 50 to 80 gals per acre. Ultra low volume sprayers give good distribution in the outer canopy at 25 to 30 gals/acre. The amount of acreage you can cover will determine your priorities. Healthy trees that had (have) a good current crop should be a higher priority. Weaker trees may benefit, but have less capacity to carry a heavy crop. Navel and other low yielding cultivars (Ambersweet and some Rhode Red strains), that often over-flower, should not be treated to enhance flowering. We have some on-going experiments that suggest that GA3 at the same timing as the urea spray may be useful to reduce excessive flowering on these cultivars and create more mixed inflorescences with good leaf to flower ratios. We will report on these results in a later advisory. Look for our first weather advisory on December 14th.

If you have any questions, please contact me (albrigo@ufl.edu


12/14/1999

FLOWER BUD INDUCTION ADVISORY #2 for 1999-2000-12/14/99

Please refer to the first advisory of this year (12-6-99) for general background information of factors affecting flower bud induction. So far this Fall-Winter, there has been better cool temperature induction of citrus flower buds than last year. For Lake Alfred through Sunday, there were 1100 hours below 75o F compared to less than 900 hours below 75o F last year to 12-12-98. This, however, is still more than 200 hours short of the cool temperature accumulation by 7 January last year when urea foliar sprays enhanced flower bud induction. At the current rate of cool temperature accumulation, it will take another 2 weeks to reach the level on January 7th last year. The National Weather Service (NWS) predicts that cooler weather is likely during the next 2 or 3 days and then highs again in the high 70s are expected for the remainder of the 8 day forecast. These predictions can be seen on their WebSite . Again like last year, we probably will be short of the 2800 hours below 75o F that occurred in the 94-83 season by the time 100 % of flower bud induction (FBI) had occurred. Based on summary data from FAWN weather stations, Lake Alfred has accumulated about 100 to 200 hour more inductive cool temperatures so far this year compared to Ft. Pierce or Immokalee. This is probably typical of the difference between South and Central Florida in most years.

An advantage in the accumulation of flower bud induction temperatures this year over last year is that, so far, no intervening week of temperatures above 85oF has occurred. These high temperature events occurred four times last year and resulted in 4 separate blooms. We do not know, however, if the temperatures of near 80oF maximum each day with a low of 60oF last week will trigger some flower bud differentiation. Some water stress during this period would have kept the trees in sufficient rest to prevent any flower bud growth. On flatwood soils, 3 or 4 days of withholding water without rain would create such a stress. On the induction side of the temperature responses, we also do not know yet if temperatures from 75o F to 70o F are as good as those below 70o F for induction, we suspect that they are not as good. Studies of specific value of different temperatures for induction are still underway.

If currently predicted or cooler temperatures persist for two more weeks (until Christmas), chilling accumulation would be about equal to last year at the end of the first week of January. That timing would have more probability for a response from enhancement sprays for flower bud induction. The current predicted spray timing is Christmas to New Year. If the current cool temperatures are not sufficient to stop flower bud differentiation from starting, bud swell (separation of the bud scales) will appear, can be observed and sprays would be too late. If the current temperatures are coupled with some water stress, the trees should remain dormant and allow some additional natural stress induction to occur before the preferred time for foliar sprays of urea or buffered phosphorous acid to enhance flower bud induction. However, any period with warmer than 85oF highs will assuredly start flower bud differentiation if there is no water stress to counteract the hot period.

Flower bud induction (FBI) sprays need to be applied no later than the beginning of a hot spell (defined as 85oF or higher daytime maxima). The current moderate cool period is predicted to last more than this week according to the NWS. If so, spraying should be scheduled just before the next predicted period of daytime highs in the mid to high 80s that is likely to last more than 2 or 3 days. These are best guesses only. Unfortunately, we do not have sufficient data to predict more accurately either our exact FBI status nor just how much warm temperature will start the differentiation process. Last year, every 5 to 7 day period of daytime highs in the mid to high 80s started the differentiation process of all buds with sufficient induction and more buds were promoted through induction with each additional cool period. Once the differentiation process is sufficiently started, a foliar spray cannot alter the determination of a buds status as vegetative or flowering. Based on last year, it appears that the foliar sprays were still effective as much as 2 to 3 days into a hot period.

Based on some higher than recommended spray rates used last year, it is even more likely than previously believed that the action of either urea or phosphorous acid at winter time is by enhancing stress induction of flower buds. The urea action may be by the phytotoxicity of the urea itself and the rapid release of ammonia in the plant tissues. Various forms of PO3 can be phytotoxic at high enough rates. The recommended rates of both are just below the threshold to cause tissue necrosis. The urea should be low in biuret because leaf tissues do not recover from biuret toxicity and remain yellow. Suppliers should provide a written analysis of biuret content (less than 0.5 % on a dry spray grade basis). Forms of PO3 should be free of heavy metals or other contaminates. Good sources of PO3 are essentially clear liquids, but an analysis of trace elements should be provided.

For urea, remember that the recommended rate is 25 to 28 lbs of N as urea/ac or 53 to 60 lbs of urea/acre. Six bags of urea (300 lbs) per 500 gal tank sprayed at 100 gal/ac is a simple guide. If you calibrate a speed sprayer to deliver 60 or 80 gal/ac, an acceptable volume to get good outer canopy coverage and maximize sprayer use, then 10 bags or 7.5 bags of urea per tank, respectively, will give the 60 lbs of urea/ac rate. In our previous studies with phosphorous acid, we used 2.6 quarts of an 0-28-26 liquid product. The current label of some products may be for 2 quarts per acre and by law is the limit you can apply.

At this time, it appears that any foliar sprays should be scheduled for 2 weeks from now or slightly later if cooler temperatures were to develop and persist. If trees are drought stressed before the next hot weather returns with no irrigation or intervening rainfall, an additional week or two of induction may be possible before sprays are applied. Severe water stress should not be allowed if the trees have a current crop, and the crop is intended for fresh market. Next weeks advisory will update temperature distributions, expected weather for the following 8 days, and a firmer recommendation for the timing window for winter sprays to enhance flower bud induction.

If you have any questions, please contact me (albrigo@ufl.edu).


12/20/1999

FLOWER BUD INDUCTION ADVISORY #3 for 1999-2000-12/20/99

Please refer to the first advisory of the year (12-6-99) for general background information of factors affecting flower bud induction. I have made 2 corrections in the earlier information. This is a La Nina Winter, but in one sentence it was written as El Nina. The other correction was on the formula of the phosphorous acid product used as a foliar spray. In later test, a 0-28-26 formula was used at 2.6 qts/ acre not a 30% P product. The earlier tests were with an 0-40-0 product at 2 qts/ac that was buffered with KOH on-site.

This weeks report might be titled ‘The Good, The Bad and The Ugly'. The good is that temperatures have continued to stay in the upper part of the effective induction temperature range. This is much better than the frequent jumps into the high 80s that occurred last year. Further good is that the 8 day forecast has some cooler days expected starting Wednesday. The bad is that the general rains (1 to 2.5 to inches, except none reported on FAWN for the Immokalee Center) eliminated any water stress that growers may have initiated during the last two weeks. This is not too bad since as expected, the rains are being followed by some cooler temperatures. The ugly part is that the extra hours of induction temperatures accumulated so far this winter are in the upper range of effective temperatures for flower bud induction. We don't know yet how effective this range of temperatures is. The total numbers, however, are becoming favorable.

For Lake Alfred through Sunday, there were 1258 hours below 75o F compared to 1058 hours below 75o F last year to 12-19-98. The below 70o F hours are only about 100 hours short of the cool temperature accumulation by 7 January last year when urea foliar sprays enhanced flower bud induction. At the current rate of cool temperature accumulation, it will take another week to reach the level on January 7th last year. The National Weather Service (NWS) predicts that cooler weather is likely after the next 2 days and should continue for the remainder of the 8 day forecast. These predictions can be seen on their WebSite (put link to Website here, thank you very much). Based on summary data from FAWN weather stations, all citrus locations are still about 200 accumulated cool hours ahead of last year. The expected window for a foliar spray of urea or phosphorous acid to enhance flower bud induction is anytime after next weekend until a warm front of several days in the 80s occurs.

Urea just prior to such a warm front can stimulate the start of differentiation and lead to an earlier bloom. The possibility of a January freeze should be considered in planning. If no warm spell occurs through the first week of January, it is advisable to wait until the second week of January to spray in order to minimize the start of differentiation and the associated loss of cold hardiness. On-the-other hand, the marginally cool temperatures so far this winter indicate that winter spray to enhance flower bud induction is likely to be beneficial in most cultivars except heavy blooming ones like navels and Ambersweet. The decision as to whether to spray or not will be difficult if a warming spell appears before the 1st week of January ends. After that, the stimulation of growth by a late winter spray (1st or 2nd week of January) will be after the most likely period for a hard freeze.

For those planning to use gibberellic acid on navels or Ambersweet, it is not yet legal to apply GA after December 31st. Therefore, sprays should be applied before January. We have posted a new information sheet prepared by Dr. Ed Stover and myself on this site (see under Grower Aids).. This is an update of our work with GA on these cultivars and the preliminary recommendations we can make at this time.

If currently predicted cooler temperatures persist for this week (until Christmas), chilling accumulation would be about equal to last year at the end of the first week of January. That timing would have good probability for a response from enhancement sprays for flower bud induction. The current predicted spray timing is Christmas to New Year or later if cool weather persists.. Any bud swell (separation of the bud scales) that has been observed will probably stay at that stage during the cool temperatures this week. The additional natural stress induction expected to occur this week and at least part of next week should increase the likelihood for foliar sprays of urea or buffered phosphorous acid to enhance flower bud induction. However, any period with warmer than 85oF highs will assuredly start flower bud differentiation as there is no water stress to counteract a hot period.

For urea, remember that the recommended rate is 25 to 28 lbs of N as urea/ac or 53 to 60 lbs of urea/acre. Six bags of urea (300 lbs) per 500 gal tank sprayed at 100 gal/ac is a simple guide. If you calibrate a speed sprayer to deliver 60 or 80 gal/ac, an acceptable volume to get good outer canopy coverage and maximize sprayer use, then 10 bags or 7.5 bags of urea per tank, respectively, will give the 60 lbs of urea/ac rate. In our previous studies with phosphorous acid, we used 2.6 quarts of an 0-28-26 liquid product. The current label of some products may be for 2 quarts per acre and by law is the limit you can apply.

At this time, it appears that any foliar sprays should be scheduled for next week or slightly later if cooler temperatures were to persist. If trees again become drought stressed before the next hot weather returns, with no irrigation or additional intervening rainfall, another two weeks of induction may be possible before sprays are applied. Severe water stress should not be allowed if the trees have a current crop, and the crop is intended for fresh market. Next weeks advisory will update temperature distributions, weather predictions for the following 8 days, and expectations for closing the timing window for winter sprays to enhance flower bud induction.

Enhancing Cropping of Low-Yielding, Heavily Flowering Citrus by Reducing Floral Initiation

Ed Stover, Citrus Extension Specialist, Indian River Research & Education Center

Gene Albrigo, Research Scientist, Citrus Research & Education Center

Many Florida blocks of Navel orange, Ambersweet, and Minneola tend to flower very heavily yet set poor crops. It appears that tree resources are wasted by extravagant flowering, compromising the trees' ability to set fruit, support early fruit growth, and carry fruit to harvest. Research in several citrus areas indicates that productivity of heavily blooming blocks may be increased by reducing flower formation. This approach has produced very encouraging results in Florida Navel and Ambersweet in two years of trials. While we don't have sufficient information to make strong recommendations, results have been so positive, that some citrus growers may want to try these methods on low-yielding but heavily-flowering groves on a limited scale. The purpose of this article is to provide an update for citrus growers on this promising technique.

A single winter GA spray to Ambersweet or Navel has increased boxes/tree by more than 22% in our trials.

How does this work?

GA (also known as GA3 or gibberellic acid) is a naturally occurring plant growth regulator that has a number of effects on tree and fruit development. GA is known to inhibit flowering in citrus, and when it is applied during the period of bloom induction, it typically reduces the number of flowers and increases the proportion of leafy inflorescences.

When should GA be applied to reduce flowering?

The period of flower bud induction in Florida citrus typically occurs from late November through mid-January in Florida. In 1997-98, we applied GA to Navel in Ft. Pierce on Dec 23, 1997 and Ambersweet in Okeechobee County on Jan 6, 1998. Flowering was reduced by about 40% in each trial and total yield was increased by 22% in Navel and 45% in Ambersweet. We have only looked at a range of GA timings for one year, and found that in that year, time of GA application markedly influenced both time of flowering and final crop harvested. Application of GA to Ambersweet on Jan 6, 1999 reduced and delayed flowering but had little effect on yield, however, application on January 23 accelerated flowering and greatly increased cropping, from a very poor 25 fruit / tree in controls to 275 fruit / tree. In Navel, yield increased following application on Dec 17, 1998 (42% increase) and Jan 6, 1999 (25% increase). Nov 23, 1998 and Jan 25, 1999 treatments to Navel reduced flowering and altered time of bloom but had no effect on yield. It appears that optimum time of GA treatment will likely depend on variety and time of cool temperature periods that induce flowering. Weekly advisories on flower bud induction status, and the best time to apply sprays that alter flower bud induction are posted on the web at http://www.crec.ifas.ufl.edu/crechome/groweraids.shtml. Best results may be achieved by following the 1999-2000 flower induction advisory on this website. For 1999-2000GA spray timing, we expect that a Christmas to January 5th window will be acceptable. Trials are beginning in Minneola, and we suspect that this late-blooming variety may need to be treated with GA about one week later than the earlier blooming Navel and Ambersweet.

Because of recent temperatures, our best guess is that Ambersweet should be treated with GA in early January 2000, while Navel should be treated in late December 1999. See website.

Where can GA be legally used to reduce flowering?

The current ProGIBB label permits application to Navel during December to reduce physiological disorders and permit a more orderly harvesting pattern. Therefore, application to reduce the physiological disorder of excessive flowering is permitted by the existing label. HOWEVER, THE EXISTING LABEL DOES NOT PERMIT ANY APPLICATION OF GA TO CITRUS IN JANUARY. The makers of ProGIBB are working with us to get special local needs labeling for this use and we will alert you when this is received. A full label for controlling flowering in citrus should be in place for 2001.

The existing label does not permit GA application to citrus in January. A special local needs exemption may be in place for January 2000. Please call your citrus agent for an update before any January GA sprays.

Where should you consider using GA to reduce flowering?

THIS IS STILL AN EXPERIMENTAL PROCEDURE! However, it is worth trying on limited acreage where trees routinely flower very heavily but produce very light crops. We have only tested this procedure on Navel and Ambersweet, but it seems likely to be effective on other bouquet bloomers as well. Trials on more varieties will be conducted in 1999-2000. Because treatments sometimes result in earlier flushing, there is a chance that winter GA may reduce cold-hardiness and we are also testing this possibility. If there are still fruit on the tree at application, you may experience undesirable regreening.

What rate should you use?

All of our trials have been conducted with 20 ounces of ProGIBB (4%) per acre applied at 125-175 gallons per acre with 0.05% Silwet L-77. Lower rates, gallons per acre and surfactants may be effective but we have not tested them. Lower rates of GA per acre will be tested in 1999-2000.

At 150 gallons/acre, this would be : 13.3 ounces of GA (4%) and 6.3 ounces of Silwet L-77 per 100 gallons

What will happen following winter GA application?

Some leaves may be knocked off of trees, especially where greasy spot has damaged foliage or temperatures are high at the time of application, however we have seen increased cropping even after some leaf loss. Fruit still present on trees may quickly and dramatically regreen. Bloom is likely to be reduced, will be associated with more leafy flush, and may be earlier or later and more concentrated than in untreated trees. In our experiments, final crop has been markedly increased by most treatments.

Please tell your citrus agent what you see following GA treatments to reduce flowering and enhance cropping. Your information will help us develop stronger recommendations more quickly.

If you have any suggestions or questions, please contact me (albrigo@ufl.edu)


12/27/1999

FLOWER BUD INDUCTION ADVISORY #4 for 1999-2000-12/27/99

A best guess is that urea or phosphorous acid sprays can be applied starting this week for enhancement of flower bud induction. Thursday or later this week may have the advantage of higher temperatures to increase foliar uptake of the spray material. From a cool temperature accumulations status, this timing is essentially equivalent to last year through the first week of January when sprays were effective. Relative to last year's temperature profile, this year is better because no intervening hot (+ 85 degree F) periods have occurred, but this year, the additional cool temperatures have been in the 65 to 75 degree range, the upper range of temperatures thought to be inductive. So far this Fall-Winter, cool temperature accumulation for induction of citrus flower buds continues to be relatively low. Through Sunday, hours below 70 degrees F were between 900 and 1100 hours in the Central to South citrus districts, while hours below 75 degrees were between 1280 and 1410 hours. To the North, an additional 50 to 60 hours had accumulated below these temperature values. Since the cool temperature induction has been only moderate, flower bud induction is likely to benefit from stress enhancing sprays.

The current week has been of ideal temperatures for flower bud induction but not many hours below 60 degrees F occurred prior to that. The weather prediction for Thursday through Monday (December 30th - January 3rd) is for 78 -80 degree F highs. It is most likely that citrus trees will start flower bud differentiation of induced flower buds as soon as a period of 4 or 5 days with daytime temperatures in the 80s occurs. We do not know if the upcoming weekend with daytime highs of 80 degrees will trigger flower bud differentiation. January in this La Nina year is predicted to be above normal. Flower bud differentiation will start early in the month if the 80 degree weather continues. Sprays should be applied before an extended warm period begins.

In our tests last year, urea was effective in enhancing induction of flower buds at a equivalent accumulation of inductive temperatures. For this foliar spray treatment to be effective, it will need be applied before significant flower bud differentiation begins. The appropriate treatment window would be this week or later as long as a warm period does not occur. The 8 day temperature forecast looks favorable through Tuesday of next week. If daytime highs remain near or below 80 degrees F next week, sprays may still be effective based on last year's test results.

If you have re-established water stress after last week's rainy cold front, your trees can be held another 2 weeks until after the second week of January before spraying to enhance flower buds. Irrigation should then be resumed. This delay should help to avoid the stimulation of growth before the most likely freeze danger period has passed.

Several cultivars may benefit from reducing flowers, increasing leafy bloom and thereby getting higher fruit set. For navels, Ambersweet and other heavy flowering but low setting cultivars, application of GA sprays should be considered this week. See the accompanying extension advisory for further information on the use and benefits of GA sprays, but note that the legal window for application is only until January 1st.

In summary of the current status of flowering potential in Florida citrus, flower induction is probably low to moderate, but better than last year based on fairly continuous cool temperature accumulation. Foliar sprays of urea or phosphorous acid to enhance flower bud induction are likely to work if applied this week or early next week. If you have established water stress after last weeks cold front, the stress should be continued for one or two more weeks. Irrigation should then follow any enhancement spray. For heavy blooming cultivars, a GA application is appropriate late this week. These various procedures should provide a reasonable level of bloom within the normal bloom period. Currently, it appears that we should have a single bloom with only a slight chance that it will be prolonged due to a previous period when daytime highs were in the low 80s. People that had their trees water stressed at that time should not experience a prolonged bloom.

Further delay before using a flower induction enhancing spray may be beneficial due to additional accumulation of cool temperatures. But that delay increases the risk of shortening the period for effective spraying if warm weather continues next week or a warmer period occurs in the next 2 weeks. Delaying also increases the chance of a prolonged bloom and decreases the chance of stimulating an earlier bloom for fresh market cultivars.

For those that hold out another week, we will briefly update flower bud induction conditions next week. Further advisories will occur if there are unexpected climatic events in January. One advisory on bloom-time options for fruit set will be posted later.

If you have any questions, please contact me (albrigo@ufl.edu)


1/3/2000

FLOWER BUD INDUCTION ADVISORY #5 for 1999-2000-1/3/00

The progress of Florida citrus trees toward induction of citrus flower buds from cool temperatures was favorable early last week, but warmer days last weekend and continuing today and tomorrow may start flower bud differentiation. This is particularly true for South Florida and East Coast locations were low 80 degree F highs are expected throughout the week. If you have not applied a flower bud enhancing spray of urea or phosphorous acid and you planned to, the earlier this week you do so the better.

The cool temperature accumulation below 70 degrees F at Lake Alfred through yesterday is now 200 hours more than through Saturday 8 January of last year which was 875 hours below 70 degrees F and 1125 hours below 75 degrees F. This exceeds the hours related to the 75 % induction stage for the 94-83 season. Fort Pierce and Immokalee are 160 to 175 cool inductive hours, respectively, ahead of last year also. As usual, Tavares has been cooler than Lake Alfred and has about 50 more hours below 70 degrees F through yesterday.

It would appear that it is still likely to be a moderate to low flower induction year, unless the temperature range from 65 to 75 degrees F is more effective than anticipated. Enhancement sprays should be effective and should have been applied by last weekend. There still may be some benefit if sprays are applied in the next 2 or 3 days. Delays beyond that are not advised as earlier studies in Gainesville showed that flower bud differentiation started, even that far North, by January 15 to 20. We of course still have the possibility of another cold spell in mid-January to early February, which could lead to a second set of induced buds. If flower bud induction sprays were applied last week or early this week, it is less likely that a significant second bloom will occur even if another cool spell occurs.

In summary, induction by cool temperatures is on the moderate side and enhancement sprays may still be beneficial for flower bud induction if applied early this week. We should expect a fairly concentrated or slightly protracted bloom this year. Foliar nutrition or PFD control sprays during the bloom period should be easier to time because of a less protracted bloom period than last year.

This will be the final advisory for this Winter, unless another significant cold spell occurs. I hope they have proved useful and my best guesses are not too far wrong. I would appreciate email responses this Spring as to your observations of bloom intensity and spread, your enhancement sprays experiences, and if applied, did they appeared to help you bloom situation. My email is albrigo@crec.ifas.ufl.edu I will post an advisory closer to bloom time as to final assessment of bloom time and intensity and foliar nutritional options you may wish to consider.

If you have any questions, please contact me (albrigo@ufl.edu)


1/24/2000

FLOWER BUD INDUCTION ADVISORY #6 for 1999-2000-1/24/00

Significant advancement of flower bud induction is occurring in mid- to late January following an earlier warm spell. Over 1400 hours below 70 degrees F have occurred in South Florida (Immokalee and Ft. Pierce) and over 1650 in the Central and Northern districts. An additional 100 or more hours of inductive temperatures can be anticipated before a warm period starts on next Saturday. At the very least, we can now expect a prolonged bloom period or 2 separate blooms if the previous warm periods of low 80 degree temperatures initiated significant differentiation. Overall bloom should now be heavy, particularly in all blocks that had a light crop this current year. 
It is very important at bloom-time to observe blocks treated in late December or early January with urea or PO3 to enhance bloom or GA3 to reduce bloom to see if they have a more concentrated, earlier bloom than comparable untreated trees. We have initiated urea and GA3 timing experiments to see what effect timings throughout January have on flowering distribution and intensity. We will report on those findings before the next bloom induction cycle. Any observations from growers about their experiences this year will be greatly appreciated. 
Although a spray of urea this week will likely increase flower bud induction from the later January period of cool temperatures, the result may be an increased second bloom. For early market cultivars, this may not be an advantage. For processing oranges, flowering from this induction period should still result in a bloom during the normal span of bloom (late February to early April) and the resulting fruit should have satisfactory maturation, but a prolonged or heavy second bloom will increase the diverse maturation status of fruit on the tree and result in a poorer average fruit quality from a single harvest. 
If the projected warm period starting next Saturday is extensive, bloom should come quickly. We will post an advisory regarding expected bloom date after February 5th.

If you have any questions, please contact me (albrigo@ufl.edu)

 


2/11/2000

FLOWER BUD INDUCTION ADVISORY #6 for 1999-2000-2/11/00

The evidence from Florida citrus trees is that we will likely have two blooms on most trees. I know you didn't want to hear that, but it is my best guess at this point. After the more than 1075 hours below 70 degrees F accumulated at Lake Alfred through January 2nd, we experienced 10 days in which temperature maxima exceeded 80 degree F. This amounted to 168 hours above 75 degrees F over a 13 day period. At the time we speculated about whether these temperatures were high enough to start and push flower bud differentiation along far enough to give us a separate bloom. As most trees are pushing buds to varying stages now, even though we have had cool weather, it is obvious that a significant bud break was initiated. 

Since then more than 450 to 500 hours below 70 degrees F have been accumulated in the various citrus districts. This should cause more buds to be induced into flower buds. This cool period has lasted 26 days. A normal bloom lasts about 4 weeks. It would appear that little overlap of a second bloom with the first is likely as the first bloom had 2 weeks of warm temperature differentiation plus 3 ½ weeks of cool temperature development. 
Growers who continued water stress through the 2 weeks of warm temperatures in early January may have prevented the differentiation that started the 2nd or 3rd of January. Other growers that used a urea or phosphorous acid (PO3) spray at that time may have enhanced the induced flower bud level so that a significant 2nd bloom will not be seen. Everyone involved in these programs should observe their trees carefully. Please report your observations as to bloom intensity and occurrence of any second bloom. 

Using a rough model for differentiation and bloom development relative to temperatures, about 70 % of the process may have occurred in the most advance cultivars. Another two weeks of daytime highs averaging over 80 degrees F should put these cultivars at 50 % full bloom from the first wave of flower bud differentiation. This would be about February 26th to March 1st. The second wave should be about 25 to 30 days later assuming average daytime highs are 80 degrees. Watch for a second, April 1st, bloom, no fooling.That is enough guess work. When bloom does come, those with hybrids that require cross-pollination may find a new article useful. This article deals with bee preference within citrus cultivars and some advise on how to maximize efficiency of pollination for your target hybrid cultivars. A short popular version will be posted on this WebSite also under Grower Aids by early next week. The work was supported by the Florida Citrus Production Research Marketing Order. 
Please report full bloom dates by email to help us with our predictive model. Also please report your observations on effects of drought stress and/or flower enhancement sprays.

This will be the last advisory for this year. If you have any questions, please contact me (albrigo@ufl.edu)