FOLIAR FEEDING and

SAR for CITRUS TREES

Mongi Zekri and Gary England

University of Florida, IFAS

Management practices to improve fertilizer efficiency

♦ Evaluation of leaf analysis data

♦ Adjustment of rates to the level based on expected fruit yield and IFAS recommendations

♦ Selection of fertilizer formulations to match existing conditions

♦ Proper calibration and maintenance of equipment

♦ Careful placement of fertilizer within the root zone or within the canopy

♦ Timing to avoid the rainy season (June-early Sept.)

♦ Split application and good irrigation management to minimize leaching

Management practices to improve fertilizer efficiency

Satisfactory nutritional status for citrus leaves

Nutrient (%)

Nitrogen 2.5-2.8

Phosphorus 0.12-0.17

Potassium 1.2-1.7

Calcium 3.0-5.0

Magnesium 0.3-0.5

Sulfur 0.2-0.4

Nutrient (ppm)

Iron 60-120

Boron 36-100

Manganese 25-100

Zinc 25-100

Copper 6-16

Molybdenum 0.1-1.0

http://edis.ifas.ufl.edu/pdffiles/SS/SS47800.pdf

SL 253

Benefits of foliar fertilization

Foliar fertilization has become important as growers are seeking to improve nutrient use efficiency, reduce losses to leaching and runoff, and fight pests and diseases.

In sandy and calcareous soils, foliar applied fertilizers are 4 to 20 times more effective when compared with soil applied fertilizers.

(Lingle & Holmberg, 1956; Wittwer et al. 1959, annual crops, beans, and tomatoes, http://www.ecochem.com/t_foliar.html).

Benefits of foliar fertilization

Factors affecting foliar nutrition

Response to foliar nutrition is dependent on: Fertilizer form

Nutrient concentration

Frequency of application

Timing (temperature, humidity, wind)

Spray volume

Spray coverage

Leaf age or growth stage

Advantages of foliar feeding

When properly conducted, foliar fertilization was found to: increase yield,

enhance fruit quality,

provide drought tolerance, and

improve tolerance to diseases and pests.

http://edis.ifas.ufl.edu/pdffiles/HS/HS118100.pdf

Effects of Mineral Nutrition on Health and

Performance of Citrus Trees

Arnold Schumann, Tim Spann, Tim Mann, Tom Obreza, and Mongi Zekri

Citrus Industry magazine, July 2010, Vol. 91 (7): 22-24.

How does nutrition

improve tolerance to

pests and diseases?

1. Formation of mechanical barriers, primarily through the development of thicker cell walls.

2. Synthesis of natural defense compounds which provide protection against pathogens.

Foliar feeding was found to increase yield from 12 to 25% compared with conventional fertilization. (http://www.ecochem.com/t_foliar.html.)

Advantages of foliar feeding

Foliar urea or phosphorous acid

Foliar applications of low-biuret urea (25-28 lbs N/acre) or phosphorous acid (2.6 quarts/acre of 26-28% P2O5) in late Dec-early Jan have been known to increase flowering, fruit set, and fruit yield.

(Albrigo 1999, FSHS 112:1-4).

Foliar potassium

Postbloom foliar applications of potassium nitrate or mono-potassium phosphate (8 lbs/acre K2O) in late April have been found to increase fruit size and yield.

(Boman and Hebb 1998, FSHS

111:128-135; Boman 2001, FSHS 114:83-88).

Foliar feeding with micronutrients

Considering the wide variations in soil types and pH, foliar sprays of the micronutrients (Mn, Zn, B, and Cu) are a more effective, more economical, and a quicker way to supply these nutrients than soil application (when included with postbloom or summer foliar sprays at about full expansion of the new flush).

Sulfate forms are less expensive and nitrate forms appear to facilitate the uptake of micronutrients.

Soil vs. Foliar fertilization

There is still a general consensus that foliar fertilization should not be considered a substitute for a sound soil-fertility program.

Foliar nutrition can economically supplement a good ground fertilization program.

Foliar fertilization can also provide some macronutrient needs and all micronutrients except Fe.

Iron (Fe)

Foliar application of Fe is not recommended for citrus trees due to lack of effectiveness and risk of leaf and fruit burn.

Soil application of Fe chelates is still the most reliable means of supplying Fe to citrus trees.

Usefulness of foliar feeding

Foliar feeding is useful under any condition that decreases the tree’s ability to take up nutrients when there is a demand.

When does foliar nutrition make sense?

Soil conditions prevent nutrient uptake

Small amounts of nutrients are needed which makes ground application inefficient

Nutrients are not mobile in the tree

Transient deficiency

Need nutrients fast

Application is profitable

When do soil conditions prevent nutrient uptake?

High pH (primarily calcareous soils) makes Mn, Zn, Cu, B and P insoluble and unavailable

Low pH reduces availability of Mo and P

Excess soil moisture (flooding)

Drought

Low soil temperature

Restricted and damaged root system

Requirements for foliar feeding

When no rain is expected for 24 to 48 hours

Unlike most pesticides, nutrients need to enter into the leaves

pH of spray solution should be 5.5 to 7.5

Spray when wind is minimal

Maximize coverage of the underside of leaves (where stomates are located)

Requirements for foliar feeding

Spray when weather conditions are humid Spray at 60° to 85°F temperatures Spray fully expanded young leaves of major

growth flushes (spring, summer, fall) Most effective foliar feeding is done late in the

afternoon or early in the morning (temperature and humidity are right, wind is minimal, and the stomates are open)

Addition of a surfactant (and nitrogen source such as urea) may increase absorption

Nutrient uptake after treatment

Absorption by underside leaf surface is 20-25 times greater than by the upper surface.

Absorption is enhanced by light, even at low levels

Heavy rain within 4 hours would reduce potential uptake by half.

Rain at 12 hours can reduce uptake by a third.

24% of urea was absorbed in 1 hour. However, only 3% of potassium nitrate (KNO3) was absorbed in 1 hour.

(Lea-Cox & Syvertsen, 1995, ASHS, 120: 505-509.)

Tank-mixing foliar nutrients

Be careful about possible chemical interactions among foliar fertilizers and other chemicals

No nutritional benefit, but phytotoxicity problems can occur using unfamiliar tank mixes

Some materials may precipitate, tie up nutrients, and clog nozzles

Tank-mixing foliar nutrients

Be sure to read all product labels and do the jar test if uncertain

Be consistent in tank mixes, timing, and formulations once you find something that works

Foliar feeding and Systemic Acquired Resistance (SAR)

Proponents of foliar feeding consider foliar fertilization an effective means of stimulating the natural defense mechanisms of plants.

Systemic Acquired Resistance (SAR) is analogous to the innate immune system found in animals.

When a pathogen attacks a plant, the plant reacts to survive. Gene switches are turned on to produce proteins that in some way help the plant to defend itself, resist diseases, and recover.

Systemic Acquired Resistance (SAR)

SAR is induced when a plant turns on its defense mechanism in every part of the plant, in response to a chemical or pathogen.

There are several commercial chemical sprays that turn on genes in plants, producing Systemic Acquired Resistance (SAR) to diseases.

Salicylic acid was found to induce/activate SAR and provide a certain level of disease control.

Some used products

Some growers have been experimenting on their own with compounds such as Salicylic Acid, SAver (10% salicylic acid), Actigard, Safeguard, Serenade, Oxidate, Phosphites, Hydrogen Peroxide, KeyPlex, and various fertilizers to reduce the damaging effects of diseases including HLB, to slow down tree decline, and to enhance tree growth and fruit production.

Some products evaluated by Researchers

Soil-applied Admire (Imidacloprid), soil-applied Platinum (Thiamethoxam), and soil-applied Actigard (4 applications at 2-month interval) were found to induce SAR and suppress citrus canker, but did not suppress citrus greening (HLB).

(Graham, 2010, http://research.fcprac.com)

SAR Facts

SAR is not curative or completely disease preventative.

Typically, disease prevention by SAR is broad spectrum but incomplete, ranging from 20 to 80% efficacy.

Where SAR has shown to work, it only slows down disease progression and reduces disease severity.

SAR Facts & Recommendations

SAR alone cannot solve a problem completely. It has to be a component of an integrated comprehensive management program.

Data from other crops consistently show that SAR works best at low disease inoculum levels.

Thus, SAR management programs should be implemented early in the disease cycle.

Caution for SAR-products

If using SAR-induced products, be sure that they are labeled for use on citrus, and that they will not lead to residue issues that may make the fruit unacceptable and unmarketable.

Is it a good, efficient way to foliar spray nutrients at night?

Questions?

NO.

The stomates are closed at night and nutrients need to enter into the leaves to be useful.

Is it a good, efficient way to foliar feed by air?

NO if the coverage of the lower side of leaves is poor.

The stomates in citrus are located on the lower side of the leaves and nutrients need to enter into the leaves to be useful.

Is it a good, efficient way to foliar feed with solutions that have a pH lower than 5 or higher than 7.5?

NO.

Leaf absorption of nutrients is poor.

The solution may cause leaf and fruit burn.

At high or low pH, some fertilizer materials may precipitate, tie up nutrients, and clog nozzles.

YOUR Questions?