UCCE Tulare County, Citrus Notes ( February 1999)

Tulare County

Citrus Notes (February 1999)

Disclaimer: This newsletter is geared towards a Tulare County audience and may not be applicable to other geographical areas.

Reprint freely with credit to: Citrus Notes, Neil O'Connell, editor, a publication of the University of California Cooperative Extension, Tulare County.

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For more information contact: Neil O'Connell, Farm Advisor, nvoconnell@ucdavis.edu

Spring Citrus Meeting

9:00 A.M. - 12:00 P.M.
Wednesday, March 24
Exeter Veterans Memorial Building

9:00 a.m.	Nitrogen Management in Citrus With Low Volume Irrigation Dr. Lanny Lund, University of California, Riverside; Dr. Mary Lu Arpaia, Kearney Agricultural Center
10:00	Regulated Deficit Irrigation Studies in Citrus Dr. David Goldhamer Kearney Agricultural Center
10:30	Minumum Temperatures in Citrus Orchard With a Cover Crop Neil O'Connell, University of California Cooperative Extension - Tulare County
10:45	Break
11:00	Impact of Foliar Sprays on Fruit Set Size Dr. Carrol Lovatt, University of California, Riverside
11:30	Ant Bait Studies in Citrus; Emergency Registrations for Red Scale Control Materials Dr.Beth Grafton Cardwell, Kearney Agricultural Center

Continuing Education Credits Requested

Citrus Nutrition

Now is the time to be considering the fertilizer program that will be applied in 1999. Decisions as to what materials to apply and quantities required should be based upon the results of a leaf analysis taken this past fall. Other factors to be considered are the amount of fruit produced, fruit size and quality and fertilizers applied. A related issue is the impact on production and fruit quality of raising the level of a particular nutrient in the tree. Establishing production and quality goals for each block based upon production and quality information helps produce the desired results.

Method of Application

Nutrient materials can be delivered to the tree as soil applications to the orchard floor, as foliar applications to the tree canopy or introduced into the irrigation system. Fertilizer broadcast on the orchard floor during the winter is moved into the soil by rainfall and is taken up by the trees and stored for later use, particularly during periods of high demand such as spring growth flush, flowering and fruit set. Single applications applied prior to spring flush result in equal production and higher quality fruit than split or later applications. Because under some conditions some of the nitrogen may be moved below the root zone and be unavailable to the tree, applying a portion of the nitrogen requirement by foliar application increases the efficiency of the program-foliar nutritional sprays are generally very efficient means of delivering nitrogen to the tree. Foliar applications should be discontinued once warm weather arrives because of the tendency to aggravate regreening of the rind. Some forms of nitrogen applied to the orchard floor are immediately available to the tree while others must be converted into an available form. The nitrate form of nitrogen is the primary form taken up from fertilizer applied to the soil. Fertilizer applied in the nitrate form is immediately available for uptake. Ammonium forms must be converted to nitrate and urea is converted to an ammonium form and then to nitrate. Time of conversion during the winter and early spring may be 2-3 weeks, so applications should be made in time to allow for conversion and assimilation and having the nutrient available for the high demand period of spring growth. Foliar applications of nitrogen as low biuret urea are generally assimilated within 48 hours or less. Delivering fertilizer by means of the irrigation system is becoming more widespread. Fertigation can be a very efficient method of applying nutrients. Care must be exercised to see that the irrigation wets no deeper than the root zone to avoid deep leaching and negative impact on ground water.

Well timed tissue analysis gives a reading of the current level of the various elements in the tree and how the tree has responded to nutrients available from soil reserves as well as those applied from fertilizer applications. Tissue levels should be interpreted in light of the amount of fruit produced as well as the quality and sizes of fruit produced..Reports from the packing house will provide information on field boxes produced as well as size distribution and quality (grade). From all this information goals can be established for the fertilizer program. One other factor is essential in setting these goals. Being familiar with the possible effect on production and fruit quality from increasing the level of various elements in the tree is critical. Increasing the level of a nutrient can have impacts on production as well as quality - rind texture, peel thickness, time to maturity, etc. In general, if all the nutrient levels are in the recommended optimum range, only nitrogen, phosphorous and potassium will have significant effects on quality and size. Past fertilizer applications will also be helpful in deciding on the quantity of material to be applied. Maintaining all nutrients in the optimum range is the first target. Adjustment in nutrient levels based on type and quantity of fertilizer to be applied can then be made if desired. Establishing goals for the nutritional program is helpful in maintaining focus on critical issues such as yield, fruit size, quality and marketing requirements.

Temperature Durations and Freezing

Experiments several years ago in Florida with large cold chambers showed citrus fruits on a tree don't all freeze simultaneously. Some fruits will undercool several degrees for considerable periods of time.

At 26 °F, 58% of the oranges on a tree were frozen after 11 hours at this temperature. However, it took 4 hours to freeze the first one.

At 25 °F about 5% of the fruit was frozen in 30 to 60 minutes. After 2-1/2 to 3 hours nearly 50% was frozen. At 8-1/2 hours at 25 °F, 80% were frozen. At 20 °F all fruit was frozen within an hour. Most fruits undercooled to 24 °F and 25 °F before freezing.

Should Frozen Fruit be Stripped?

Neil O'Connell and John Pehrson,*
University of California Cooperative Extension

The answer to this question depends on the variety.

Lemon fruits that are damaged will drop mainly on their own and there are no trials to show effects of delayed harvest on subsequent crop. The multiple blooms of coastal districts suggests lemon trees can carry multiple crops.

Navel oranges are harvested over an extended season, November to June, and any adverse effect of delayed picking has become a "built-in" cultural practice. Freeze damaged navels shed fairly easily except when treated with 2,4-D which will cause abscission to be slower and less complete. After the 1937 freeze a navel orchard was divided to allow natural drop on half and stripping of the rest. Next year's crop favored the stripping by about 8%, hardly a significant difference.

Similar orchard treatments were done with valencias following the 1937 freeze with the results demonstrating this variety's general tendency to have a smaller crop after a delayed harvest. The frozen fruit effect is about 50% of the response for late picked sound fruit. In other words the picking of sound valencias in September, October and November causes a greater reduction on the subsequent crop than leaving frozen fruit to drop on its own.

Present indications are that good fruit will be found as a more thorough assessment is made in each orchard. Some salvage is a possibility whereas stripping results in an immediate expense. With potential income so devastated by this year's freeze, the expense of dropping fruit may be better applied to culture for next year's crop, particularly for high priority items like water.

*C.E. specialist emeritus