Posts Tagged: nutrient

Drought Induced Problems in Our Orchards

Drought Induced Problems in Our Orchards

Abiotic disorders are plant problems that are non-infective.  They are not caused by an organism, but through their damage, they may bring on damage caused by organisms.  Think of a tree hit by lightning or a tractor.  The damage breaches the protective bark which allows fungi to start working on the damaged area, eventually leading to a decayed trunk.  It was the mechanical damage, though that set the process in motion.

Too much or too little water can also predispose a plant to disease.  Think of Phytophthora root rot or even asphyxiation that can come from waterlogging or too frequent irrigations.

Salinity Effects from Lack of Water

Lack of water and especially sufficient rainfall can lead to salinity and specific salts like boron, sodium and chloride accumulating in the root zone.  This happens from a lack of leaching that removes native soil salts from the root zone or the salts from the previous salt-laden irrigation from the root zone.  These salts cause their own kind of damage, but they can also predispose a tree to disorders, disease and invertebrate (insect and mite) damage. 

Lack of water and salt accumulation act in a similar fashion.  Soil salt acts in competition with roots for water.  The more soil salt, the harder a tree needs to pull on water to get what it needs. The first symptom of lack of water or salt accumulation may be an initial dropping of the leaves.  If this condition is more persistent, though we start to see what is called “tip burn” or “salt damage”.  Southern California is tremendously dependent on rainfall to clean up irrigation salts, and when rain is lacking, irrigation must be relied on to do the leaching

As the lack of leaching advances (lack of rainfall and sufficient irrigation leaching) the canopy thins from leaf drop, exposing fruit to sunburn and fruit shriveling.

Leaf drop and fruit shriveling in avocado.

In the case of sensitive citrus varieties like mandarins, water stress can lead to a pithy core with darker colored seeds, almost as if the fruit had matured too long on the tree.

Total salinity plays an important factor in plant disorder, but also the specific salts.  These salts accumulate in the older leaves, and cause characteristic symptoms that are characteristic in most trees.   Boron will appear on older leaves, causing an initial terminal yellowing in the leaf that gradually turns to a tip burn.

Often times it is hard to distinguish between chloride, sodium and total salinity damage.  It is somewhat a moot point, since the method to control all of them is the same – increased leaching.  There is no amendment or fertilizer that can be applied that will correct this problem.  The damage symptoms do not go away until the leaf drops and a new one replaces it.  By that time hopefully rain and/or a more efficient irrigation program has been put in place.

The Impact of Drought on Nutrient Deficiencies

Salinity and drought stress can also lead to mineral deficiencies.  This is either due to the lack of water movement carrying nutrients or to direct completion for nutrients.  A common deficiency for drought stressed plants is nitrogen deficiency from lack of water entraining that nutrient into the plant.

This usually starts out in the older tissue and gradually spreads to the younger tissue in more advanced cases.

The salts in the root zone can also lead to competition for uptake of other nutrients like calcium and potassium.  Apples and tomatoes are famous for blossom end rot when calcium uptake is low, but we have also seen it in citrus.  Low calcium in avocado, and many other fruits, leads to lower shelf life.  Sodium and boron accumulation in the root zone can lead to induced calcium deficiencies and increased sodium can also further lead to potassium deficiencies.  Leaching can help remove these competitive elements.           

Drought Effect on Tree Disease

Drought and salt stress can also lead to disease, but in many cases once the problem has been dealt with the disease symptoms slowly disappear.  They are secondary pathogens and unless it is a young tree (under three years of age) or one blighted with a more aggressive disease, the disease condition is not fatal.  Often times, in the best of years, on hilly ground these diseases might be seen where water pressure is lowest or there are broken or clogged emitters.   The symptoms are many – leaf blights, cankers, dieback, gummosis – but they are all caused by decomposing fungi that are found in the decaying material found in orchards, especially in the naturally occurring avocado mulch or artificially mulched orchards.  Many of these fungi are related Botryosphaerias, but we once lumped then all under the fungus Dothiorella.  These decay fungi will go to all manner of plant species, from citrus to roses to Brazilian pepper.                                

Another secondary pathogen that clears up as soon as the stress is relieved is bacterial canker in avocado.  These ugly cankers form white crusted circles that ooze sap, but when the tree is healthy again, the cankers dry up with a little bark flap where the canker had been.

Drought Effect on Pests

Water/salt stress also makes trees more susceptible to insect and mite attack.  Mites are often predated by predacious mites, and when there are dusty situations, they can't do their jobs efficiently and mites can get out of hand.  Mite damage on leaves is often noted in well irrigated orchards along dusty picking rows

Many borers are attracted to water stressed trees and it is possible that the Polyphagous and Kuroshio Shot Hole Borers are more attracted to those trees.

And then we have conditions like Valencia rind stain that also appears in other citrus varieties. We know it will show up in water stressed trees, but we aren't sure what the mechanism that causes this rind breakdown just at color break.  Could it be from thrips attracted to the stressed tree or a nutrient imbalance, it's not clear?

Water and salt stress can have all manner of effects on tree growth.  It should lead to smaller trees, smaller crops and smaller fruit.  The only way to manage this condition is through irrigation management.  Using all the tools available, such as CIMIS, soil probes, soil sensors, your eyes, etc. and good quality available water are the way to improve management of the orchard to avoid these problems.

Scroll down for Images

Tip Burn, notice sun burn bottom right hand fruit

Endoxerosis with dried out core

Boron toxicity

Nitrogen deficiency

Blossom end rot

Potassium deficiency

Bot gumming in lemon

Black Streak in Avocado

Bacterial Canker

Citrus red mite

Polyphagous Shot Hole Borer damage on avocado

Valencia Rind Stain                        

 

avocado drought canopy

nitrogen deficiency

endoxerosis 4

boron toxicity citrus 1

blossom end rot lemon

potassium deficiency avocado

gumming dothiorella

avocado black streak 1

bacterial canker avocado

citrus red mite

PSHB damage

Blossom End Rot in Citrus - Another Result of Drought

Another impact of the drought? There have been reports of a sunken, leathery patch around the blossom end (opposite of the stem end) of citrus fruit. This has been reported on lemons, limes and mandarins, but I am sure growers are seeing it on oranges, as well as other citrus relatives. This is an abiotic problem caused by a lack of calcium to the fruit, a problem with the plant's growing conditions, not a disease. This is a serious disorder found in various fruits and vegetables, such as tomatoes, melons, peppers and eggplants

Blossom-end rot begins as small tan, water soaked lesions on the blossom end of the fruit. The lesion enlarges and becomes sunken, dark, and leathery. On peppers, the lesion is more commonly found on the side of the fruit towards the blossom end. Also, on peppers it can be sometimes confused with sun scald. Fruit infected by blossom-end rot ripen often become infected with secondary organisms such as Alternaria spp (most likely the surrounding tissue in the photo below).

This is a physiological disorder of low calcium in the fruit. Calcium is required for normal cell growth and in relatively high concentration for new tissue growth. Rapidly growing fruit will begin to breakdown at the blossom end because that is the last place of the fruit tissue to receive calcium and also the area with the lowest concentration of calcium.

In rapidly growing plants, calcium cannot move to those rapidly growing areas quickly enough. Because calcium moves with water, fluctuations in water supply can cause blossom-end rot. Large fluctuations in soil moisture inhibit uptake and movement of calcium within the plant. Excessive nitrogen promotes rapid plant growth, which can cause low concentrations of calcium to occur in plant tissue. Leaf tissue can often not disclose a low calcium in fruit because of the lag in movement of calcium to the rapidly growing fruit tissue.

Other causes such as low calcium levels in the soil or high amounts of cations in the soil which compete with calcium uptake can also cause blossom-end rot. This is especially true in areas of soils derived from serpentine rock that are high in magnesium. The magnesium competes with calcium uptake.

Proper fertilization and water management help to minimize this problem. Avoid over fertilizing the crop. Also avoid allowing the soil to become too dry and then overly wet. Wide fluctuations in soil moisture inhibit calcium uptake and movement. If calcium is deficient or high salts occur in the soil, gypsum applications can help, but delayed uptake may not help fruit tissue content. Often, foliar applications of calcium may be beneficial.

blossom end rot lemon

Silicon as a Nutrient for California Fruit Trees

Silicon is currently under consideration for elevation to the status of a "plant beneficial substance by the Association of American Plant Food Control Officials (AAPFCO). Silicon has been shown in university and field studies to improve cell wall strength and structural integrity, improve drought and frost resistance, decrease lodging potential and boost the plant's natural pest and disease fighting systems. Silicon has also been shown to improve plant vigor and physiology by improving root mass and density, and increasing above ground plant biomass and crop yields.

Silicon (Si) is the most abundant element (27.2%) present in the earth's crust following oxygen (45.5%). Silicon is known for a number of important chemical and physical properties, i.e. semiconductor property that are used in various scientific and technical applications. In most soils near a neutral pH, the composition is a complex of iron, aluminum, oxygen and silicon. Silicon is one of the most important constituents of dust, which is carried by winds all over the world. Geologists know silicon as the rock quartz and the many silicate materials, such as opal. Formally, silica (SiO₂) is a silicic acid (H₄SiO₄), which is water soluble and stable in highly dilute aqueous solutions.  There are many forms that silicon can take in the natural environment, often complexed with water.  Plants take up a form of silicic acid and in highly leached, low pH environments, much of the silicon may have been removed over time. 

It appears that grains, such as wheat and especially rice have an absolute need of supplemental silicon to improve plant growth.  Few non-grass plants have shown this need other than cucurbits apparently.  Much of the improvement typically is for improved disease control and improved stature (prevention of lodging).

Many of the studies showing benefits of silicon amendment have occurred in low soil pH environments or in solution culture where it has been possible to create low silicon growing media.  Several years ago, potassium silicate was being promoted as a fungistat for controlling Phytophthora root rot in avocado.  A number of field and greenhouse trials were tried in California during the early 2000s to assay its effect.  For whatever reason, the material showed no effect on the disease.  Potassium and calcium silicates are liming materials, raising soil pH.  The effect that was noticed in its use in other countries may simply have been a soil pH effect on either the avocado tree, the Phytophthora or both.

silicon

Hunger Signs in Plants

Since Greek and Roman times, the appearance of a plant has been used to help identify plant health. The plant speaks through distress signals. The message may be that there is simply too little or too much water. Or the sign may tell us of a disease caused by a microorganism, such as a bacteria, virus or fungus. The plant may show symptoms of attack by nematodes, insects or rodents or from injuries from frost or lightning. According to the plant species these signals may differ slightly, but frequently they can be generalized.

 

It is also possible to generalize about the signals linked to the nutritional status of a plant. Learning these symptoms can alert us to appropriate steps to correct the toxicity, deficiency or imbalance of nutrients.

 

There are 17 elements essential for plant growth. Hydrogen, oxygen, and carbon come either from the air or water. The others come from the soil. Depending on the quantity needed by the plant, these are called either primary or trace (micronutrients) nutrients. The micronutrient nickel is required in such small amounts (50 -100 parts per billion) by plants that it was identified only last year as being an essential nutrient. Other micronutrients are iron, manganese, boron, chlorine, zinc, copper and molybdenum. Some other nutrients have been identified as being essential for only certain plants, such as silicon for sugar cane.

 

The primary nutrients are measured on a percent (parts per 100) dry weight tissue basis. These are nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. The trace elements are measured on a part per million dry weight basis. For example, a typical analysis of a dried leaf from a healthy cherimoya might show 2% nitrogen, 1% potassium, 100 ppm (parts per million) iron and 50 ppm boron.

 

Although plants require more primary than trace nutrients, all the essential elements need to be present for a healthy plant. An excess, deficiency or even an imbalance of these elements will lead to individual symptoms which are characteristic to most plants. Because of our climate and soils, Southern California has different nutritional problems from those of much of the rest of the country. What is a problem in Massachusetts may rarely be a problem here. The following list is a description of the more common nutritional problems in most plants in Southern California.

Excess or toxicity (usually related to irrigation practices)

*Boron - chlorosis (yellowing), leading to tissue death (necrosis) along the margins of older leaves.

*Sodium , Chloride - necrosis of the leaf tips and margins on older leaves.

 

Deficiency

*Phosphorus - frequently the only symptom is smaller plants, but occasionally the leaves are darker than normal or may have a reddish cast, a common symptom in sweet corn. Phosphorus deficiency in California trees is rare.

*Potassium - scorching or firing along leaf margins that usually first appears in older leaves. Plants grow slowly and have a poorly developed root system. Stalks are often weak and fall over.

*Nitrogen - plants are light green or yellow. Older leaves are often affected first, but in trees the chlorosis may appear on any part of the plant.

*Zinc - depending on the plant there may be interveinal (between the leaf veins) chlorosis on younger leaves, but frequently the leaves are small and appear in a rosette.

*Iron - very sharply defined interveinal chlorosis of younger leaves, with little size reduction. Can often be associated with wet soil conditions.

*Manganese - mild interveinal chlorosis of younger leaves, with no size reduction.

 

These and other problems can be corrected with appropriate fertilizers, amendments and manures and also by soil and water management. In well-managed plants you may never see these signs, but learning the signals can help direct your activities if you do. Sweet corn is a wonderful indicator plant which develops very prominent symptoms according to the deficiency. Planting a row of sweet corn (not field) is a tasty way to determine if your soil has a generic nutritional problem.

iron dieback citrus

Foliar nutrients to avocado

Thick, waxy coating on avocado leaves makes foliar nutrients difficult to abosorb.

LTTLE EVIDENCE TO SUPPORT THE USE OF FOLIAR APPLIED NUTRIENTS IN AVOCADO
Simon Newett, Extension Horticulturist.Department of Primary Industries and Fisheries, Maroochy Research Station, Mayers Road, Nambour 4560, Queensland, Australia. Previously published in: Talking Avocados (published by Avocados Australia Ltd), 11(2), 24-27.

Introduction

Foliar fertiliser application is sometimes promoted as an effective means of supplying nutrients to avocado.  On the market are various products being promoted as foliar nutrients for avocado, some proponents even suggest that their products do away with the need for soil applied nutrients.  This article briefly reviews the literature relating to foliar feeding of avocado and examines the anatomy of the avocado leaf and flower in relation to nutrient uptake.

The avocado leaf
The structure of plant leaves has evolved primarily to capture sunlight and exchange gases, roots have evolved to absorb nutrients and water and anchor the plant. Any absorption of nutrients by leaves is therefore likely to be more fortuitous than by design. In some crops passive nutrient absorption by leaves is occasionally sufficient to supplement the supply of nutrients taken up by the roots.  Most often this involves trace elements, which as their name suggests are required in very small amounts (eg. copper and zinc).  However if non-mobile elements or elements with limited mobility in the plant (eg. calcium, phosphorus, zinc, boron and iron) are absorbed when foliar sprayed they are not likely to make it down to the roots where they are also needed.  Most nutrients will move freely in the water stream but the movement of many is restricted in the phloem, hence leaf applications don't meet the requirements of deficient trees.  Occasionally major elements (such as nitrogen and potassium) are applied to make up for a temporary shortfall or provide a boost at a critical time.  Citrus is an example of a crop where some benefits from foliar applied nutrients have been reported.

The ability of the leaf to absorb nutrients from its surface must depend to some degree on the permeability of its epidermis (outer layer) and the presence and density of stomates (pores for the exchange of gases).  Scanning Electron Microscope studies of mature leaves and floral structures in avocado show the presence of a waxy layer on both the upper and lower surfaces of mature avocado leaves (Whiley et al, 1988).  On the upper surface the wax appears as a continuous layer and there are no stomates.  On the lower surface the wax layer is globular and stomates are present.  Blanke and Lovatt (1993) describe the avocado leaf as having a dense outer wax cover in the form of rodlets on young leaves and dendritic (branching) crystals on old leaves including the guard cells (guard cells surround stomates).  The flower petals and sepals in avocado have stomates on their lower surfaces and no wax layers on either surface, which might explain why floral sprays of boron might work.

Literature review
Nitrogen
Based upon total leaf nitrogen concentration, Embleton and Jones (unpublished) in a replicated trial in California in the early 1950's found no response to leaf sprays of urea on mature 'Fuerte' avocado trees in the field.  Up to three sprays a year were applied.

Nevin et al (1990) reviewed urea foliar fertilisation of avocado and found only one study (Aziz et al., 1975) that reported positive results in terms of fruit yield.  This trial by Aziz et al (1975) involved drenching sprays of significant amounts of urea four times a year (250 to 500 g of nitrogen per tree annually).  It is unclear whether or not considerable amounts of the drenching spray reached the ground, nevertheless, the amounts applied were very high for foliar applications.  No leaf analysis data was reported.

Galindo-Tovar (1983) was able to increase leaf nitrogen concentrations in ‘Hass’ avocado seedlings grown in a glasshouse with low concentrations of urea.  However similar treatments on 3-year-old ‘Hass’ in the field for each month during spring failed to increase leaf nitrogen in mature leaves sampled a week after spraying.  The author cited evidence for crops other than avocado suggesting that urea can penetrate leaf surfaces when grown in a greenhouse, but when grown in the field under full sun, leaf surfaces are different and resist movement of nitrogen into the leaf.

Klein & Zilkah (1986) reported substantial uptake of foliar urea-N when detached leaves of 'Fuerte' avocado were dipped in urea solutions.  Zilkah et al (1987) reported the translocation of 15N from foliar-applied urea to vegetative and reproductive sinks of both 'Fuerte' and 'Hass' avocado.  Despite the apparent response achieved by Aziz et al in Egypt, Klein & Zilkah, and Zilkah et al in Israel, attempts at the University of California to demonstrate significant uptake of nitrogen from foliar sprays have not been successful (Nevin et al., 1990).

Research at the University of California, Riverside, provided evidence that the leaf nitrogen content of 'Hass' avocado was not increased by foliar application of urea at the same concentration that increased citrus leaf nitrogen content two-fold (Nevin et al., 1990).  Maximum uptake of 14C-urea by 'Hass' avocado leaves was physiologically insignificant after 2 days. Over 96% of the 14C-urea applied was recovered from the leaf surface even after 5 days.  Maximum uptake of 14C-urea by leaves of 'Gwen' and 'Fuerte' was less than 7%.  15N, 14C-urea and 65Zn are radioactive forms of nitrogen, urea and zinc respectively that are used to track their movement through the plant.

Potassium
Sing and McNeil (1992) conducted a study on an orchard with a history of potassium deficiency where high magnesium levels in the soil competed with potassium for uptake.  Foliar applications of 3.6% potassium nitrate were applied at half leaf expansion, full leaf expansion and one month after full leaf expansion.  These foliar applications of potassium nitrate were effective in increasing the potassium level in the leaves of 'Hass' avocado trees, however two to three foliar applications per year were required to achieve the same result as one application of potassium sulphate (banded) to the soil once every 2 to 3 years.  Accounting for labour and material costs the foliar sprays of potassium nitrate were estimated to be more expensive than soil applied potassium sulphate applied every three years.  The foliar sprays also affected the levels of other nutrients in the leaf, some negatively.

Calcium
Calcium is receiving attention as an element in avocado fruit associated with better quality and longer shelf life.  Several different calcium products were tested during the 1980’s as foliar sprays in South Africa in an attempt to raise fruit calcium levels but none were found to be effective.

Veldman (1983) reported that the treatment of avocado trees with one, three and six calcium nitrate sprays did not successfully control pulp spot in avocado fruit and there was no increase in fruit calcium levels on sprayed treatments.

Whiley et al (1997) report that calcium foliar sprays during fruit growth have little effect on internal concentrations in most fruit due to poor absorption by fruit, and lack of translocation within the tree.

Boron
Some benefits have been reported from foliar application of boron if applied at flowering.  Timing is important because it appears that absorption takes place through flower structures and not leaves.

Jayanath and Lovatt (1995) reported on results of four bloom studies (two glasshouse and two field experiments) which demonstrated the efficacy of applying boron or urea sprays to 'Hass' avocado inflorescences during early expansion (cauliflower stage) but prior to full panicle expansion and anthesis.  Anatomical analysis of the flowers provided evidence that the boron prebloom spray increased the number of pollen tubes that reached the ovule and also increased ovule viability, but to a lesser degree than urea.  The urea prebloom spray increased ovule viability compared to boron-treated or untreated flowers.  Urea also increased the number of pollen tubes that reached the ovule, but to a lesser degree than boron.  However, combining boron and urea resulted in a negative effect even when the urea was applied 8 days after the boron.  Lovatt (unpublished) provided an update on this work at the World Avocado Congress in 1999, after 3 years of field trials the only treatment to have a positive effect on pollination was the boron in Year 2, the most likely reason why it didn’t work in other years was thought to be low temperatures.  There were only hardened leaves present at the time of foliar applications suggesting that uptake was through flower parts.

Whiley et al (1996) report that despite an increase in fruit set with foliar sprays of boron during flowering there has been no convincing evidence that showed increased final yield.  Root growth has a requirement for boron and in deficient trees it is unlikely that sufficient nutrient from foliar applications would be translocated to the roots.  Foliar applications have the advantage that specific organs can be targeted to enhance their boron concentrations, but with the disadvantage that insufficient boron can be absorbed through leaves to mediate chronic deficiency in trees.  Soil applications have been shown to dramatically improve the health of boron deficient trees. 

Mans (1996) experimented with ‘Hass’ trees that had leaf levels of nitrogen and boron below the accepted norms (N was 1.71% and B was 23ppm).  The aim of this trial was to see if supplying nutrients directly on the flowers could increase the yield of ‘Hass’ trees growing in a cool environment.  Mans (1996) found that if a multi-nutrient spray that included nitrogen and boron was applied as the first flowers started to open then he could increase yield and distribution of fruit size.  The stage of flowering when spraying takes place was very important.  Sprays that were applied pre-bloom, at fruitset or when fruitlets were present were not effective.

Iron
Kadman and Lahav (1971-1972) reported that the only means to control iron chlorosis in already established avocado orchards is soil application of iron chelates since applications of various iron compounds by foliar sprays have not been successful on a commercial scale.  Gregoriou et al (1983) found that the quickest and most successful treatment of trees suffering from iron chlorosis on calcareous soils  was obtained by incorporating Sequestrene 138 Fe-EDDHA in the soil.

Zinc
Kadman and Cohen (1977) found that avocado trees have difficulties in absorbing mineral elements through their foliage.  In spite of this, spraying of apparently zinc-deficient orchards was rather common in California and some other countries.  In Israel, some growers spray their orchards, but as experiments have shown, no apparent improvement occurs in leaves or fruits following such treatment.  The results presented in this paper indicate that the penetration of zinc through the leaves is so slight that there is practically no benefit through supplying it by foliar sprays.

Zinc deficiency is common in avocado and is particularly difficult to address on high pH (alkaline) soils.  Crowley et al (1996) evaluated methods for zinc fertilisation of ‘Hass’ avocado trees in a 2-year field experiment on a commercial orchard located on a calcareous soil (pH 7.8) in California. The fertilisation methods were:
• soil or irrigation-applied zinc sulphate
• irrigation-applied zinc chelate (Zn-EDTA)
• trunk injection of zinc nitrate
• foliar applications of zinc sulphate, zinc oxide, or zinc metalosate.
• 
Among the three soil and irrigation treatments, zinc sulphate applied at 3.2 kg per tree either as a quarterly irrigation or annually as a soil application was the most effective and increased leaf tissue zinc concentrations to 75 and 90 mg/kg respectively. Experiments with 65Zn applied to leaves of greenhouse seedlings, showed that less than 1% of zinc applied as zinc sulphate or zinc metalosate was actually taken up by the leaf tissue. There was also little translocation of zinc into leaf tissue adjacent to the application spots or into the leaves above or below the treated leaves.  Given these problems with foliar zinc, Crowley et al (1996) suggest that fertilisation using soil or irrigation applied zinc sulphate may provide the most reliable method for correction of zinc deficiency in avocado on calcareous soils.

Whiley and Pegg (1990) report that foliar applications of zinc have been found to be highly ineffective in Queensland orchards.

Price (1990) reports that zinc can be absorbed through the leaves (from foliar sprays, e.g. zinc sulfate, zinc chelate) but that insufficient zinc can be absorbed in this manner to meet the plants requirements, especially in avocados.  Since zinc is required at the growing points of new roots and shoots, it is essential that most zinc be taken up by the roots.

Foliar fungicide sprays
If leaf applied nutrient sprays in avocado give inconsistent or nil effects why do foliar sprays of phosphorous acid work for the control of root rot?  The amount of phosphorous acid uptake required for root rot control is small but even so, several applications per year are required to be effective and the canopy must be dense and healthy. The phosphonate concentration required in the roots for effective root rot control is in the order of 30 mg/kg.  To achieve this level either three to four sprays of 0.5% phosphorous acid per year are required at strategic times (Leonardi et al., 2000) or alternatively six or more sprays of 0.16% phosphorous acid per year must be applied.  Another factor contributing to the effectiveness of leaf applied phosphorous acid is that, unlike many nutrients, it is extremely mobile in the plant.

Borys (1986) reports the dry matter distribution of roots to shoots in avocado seedlings average 26% and 74% respectively.  Using these figures and some critical nutrient and fungicide levels in avocado we can get some perspective on the relative quantities required.  In a tree consisting of say 100 kg of dry matter, about 26 kg would be in the roots and 74 kg in the shoots.  This tree with a phosphonate root level of 30 mg/kg would contain a total of about 0.8 g phosphonate in the roots.  With the optimal leaf levels of 50 mg/kg of boron and 2.5% of nitrogen, the tree would contain about 4 g and 1850 g of boron and nitrogen respectively in the canopy alone.  It can be seen from these relative amounts that the fungicide required is substantially less than the nutrients.
Conclusion
Apart from well-timed boron applications at flowering in situations where leaf boron levels are deficient, there is no clear evidence to support the use of foliar nutrient sprays in avocado to correct nutrient deficiencies or to supply nutrients for growth.  Occasionally a foliar nutrient spray may succeed in alleviating leaf deficiency symptoms, however this type of application will not provide the tree’s longer-term requirements for this nutrient which should be addressed through soil applications.

Acknowledgments
I would like to thank Drs Chris Searle and Tony Whiley and Mr Garry Fullelove of the Queensland Horticulture Institute for their assistance in compiling this article.  The literature search was conducted using the AVOINFO avocado reference database.

Bibliography
Aziz, A.B.A., Desouki, I., El-Tanahy, M.M., Abou-Aziz, A.B. and Tanahy, M.M., El 1975. Effect of nitrogen fertilization on yield and fruit oil content of avocado trees. Scientia Horticulturae, 3 (1): 89-94.
Blanke, M.M. and Lovatt, C.J. 1993. Anatomy and transpiration of the avocado inflorescence.  Annals of Botany, 71 (6): 543-547.
Borys, M.W. 1986. Root/shoot relation and some root characteristics in seedlings of avocado and Chinini. California Avocado Society Yearbook 70: 175-198.
Crowley, D.E., Smith, W., Faber, B. and Manthey, J.A. 1996. Zinc fertilization of avocado trees. HortScience 31 (2): 224-229.
Galindo-Tovar, G.E. 1983.  Effects of urea spray concentration and surfactants on avocados.  M.S. Thesis, University of California, Riverside, USA. September.
Gregoriou, C., Papademetriou, M. and Christofides, L. 1983. Use of chelates for correcting iron chlorosis in avocados growing in calcareous soil in Cyprus. California Avocado Society Yearbook 67: 115-122.
Jayanath, I. and Lovatt, C.J. 1995.  Efficacy studies on prebloom canopy applications of boron and/or urea to 'Hass' avocados in California. World Avocado Congress III, Proceedings: 181-184.
Kadman, A., and Lahav, E. 1971-1972. Experiments with various treatments to cure chlorotic avocado trees. California Avocado Society Yearbook. 55:176-178.
Kadman, A. and Cohen, A. 1977. Experiments with zinc applications to avocado trees. California Avocado Society Yearbook, 61: 81-85.
Klein, I. & Zilkah, S. 1986.  Urea retention and uptake by avocado and apple trees. Plant Nutr. 9:1415-1525.
Leonardi, J., Whiley, A.W., Langdon, P.W., Pegg, K.G. and Cheyne, J. 2000. Progress on the use of foliar applications of phosphonate for the control of phytophthora root rot in avocados. 
Mans, C.C. 1996. Effect of foliar feeding of ‘Hass’ at various stages of flowering.  South African Avocado Growers' Association Yearbook, 19: 31-32.
Nevin, J.M., Lovatt, C.J. and Embleton, T.W. 1990. Problems with urea-N foliar fertilization of avocado.  Acta Horticulturae 275: 535-541.  International Symposium on the Culture of Subtropical and Tropical Fruits and Crops. Vol. II. (J.C. Robinson, ed.), International Society for Horticultural Science. Wageningen, Netherlands.
Price, G. 1990. Thinking about zincing your trees? Talking Avocados, Third Edition, Aug/Sept, p.5.
Sing, J.L. and McNeil, R.J., 1992. The effectiveness of foliar potassium nitrate sprays on the 'Hass' avocado (Persea americana Mill.), World Avocado Congress II, Proceedings: "The Shape of Things to Come" (Lovatt, C.J. ed.) 1: 337-342.
Veldman, G. 1983. Calcium nitrate sprays on avocados at Westfalia Estate with the objective to reduce pulpspot. South African Avocado Growers' Association Yearbook, 6: 64-65.
Whiley, A.W., Chapman, K.R. and Saranah, J.B. 1988. Water loss by floral structures of avocado (Persea americana cv. Fuerte) during flowering.  Australian Journal of Agricultural Research, 39 (3): 457-467.
Whiley, A.W., and Pegg, K.G.1990. Correction of micro-nutrient deficiencies and control of Phytophthora root rot in avocado. Talking Avocados, Second Edition,  May/June, p. 11.
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