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 foliarapplied 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.

Avocado leaves. Photo by David Rosen.

Water prices in San Diego County continue to increase and there is no end in sight, especially with periodic drought years and California losing some share of its Colorado River water.  It is easy to see the response from growers; water is being turned off in many of our districts leaving acres and acres of dying trees.  The water districts get nervous because there is not enough money coming in to cover their fixed costs, so they raise the price of water.  And, they raise it again.

The math is simple.  Some of our water districts are selling water to growers for $1200 - $1300 per acre foot.  At a water requirement of about four acre feet per acre for avocados in the inland areas of San Diego County, water will cost $4800 - $5200/acre per year.  If you are producing 5000 lbs per acre (the average yield in California for the last five years) and receive $1/lb for your fruit, you get less than your water costs.  And that doesn’t consider labor costs, fertilizer, taxes, insurance, vehicle costs etc.  Profits?  Are you kidding?

Are we done?  Either prices for our fruit have to increase, or we have to increase the yield per acre.  As for prices, we don’t have control over market prices; they rise and fall with demand by consumers, on and off years in our groves and interference with Mexican, Chilean and now Peruvian imports. 

Can we increase yield per acre?  The private consultants and the farm advisors have spent our careers trying to help growers with proper irrigation scheduling and balancing out the pressures and flows, proper fertilization, controlling  thrips and persea mites at the right time, and dealing with avocado root rot (which continues to be a huge problem).  Despite good farming practices yields per acre have not increased dramatically for most growers.  To be fair, some growers are doing quite well with good farming practices, good weather and good soil. but It will probably take a dramatic increase for most of the growers to stay in business in San Diego County.

I saw a dramatic increase in yield per acre recently in two groves, one in Temecula (owned by John Cornell) and one in the southern area of Escondido (owned by Steve Howerzyl).  The Temecula grove produced over 30,000 lbs/ac in the sixth year from planting and the Escondido grove produced 24,195 lbs/ac in the fifth year.  Both are high-density groves planted on a 10’ x 10’ spacing (435 trees/acre).  This kind of production is exciting and might bring hope to avocado production for the future in San Diego County.   Yield data is supplied in Table 1 and 2.

Table 1.    Hass avocado yield data supplied by the grower for a high density planting in Temecula.  384 trees /acre = 0.88 acre.  Yield data adjusted to pounds/acre.

Table 2.  Hass avocado yield data supplied by the grower for a high density, non-pruned planting in Escondido.  Yield data is supplied in pounds/acre.

 However, both groves have problems!

The Temecula grove had a low fruit set in the spring of 2011.  (This was the reason I was called out to look at it).   Of course a large part of the problem is the inherent on/off cycle in avocados.  However, in order to keep these trees in a high density situation without crowding, the grower had to prune the trees.  In the late winter/early spring of 2011 every tree was pruned on all sides and topped at 8-9 feet.  This effectively removed a lot of the fruiting wood and the trees had a reduced flowering and fruit set as a result.  Remember, the Hass avocado flowers and sets fruit primarily on the outside of the tree canopy.

The Escondido grove was not pruned and all of the trees had grown into each other, creating an incredibly crowded grove.  It was so crowded that the irrigator was complaining that he couldn’t get through the grove to check the sprinklers.  The grower commented that his plan was to remove all of the trees in the eighth year and start over again according to the Hofshi idea that was suggested several years ago. (More on this later in this article).

I like the idea of close spacing for increasing yield per acre, but both groves need a good idea for maintaining the spacing and yet produce fruit every year.  I proposed pruning in a three year rotation; the southwest side would be pruned the first year, the northeast side pruned in the second year, and the tree would be topped at eight feet in the third year.  Then the whole process would start over again.  By using this method there would always be fruiting wood on the tree.  And it is an easy method to teach grove workers. 

The only problem with this pruning idea is that we have never tried this in a trial.  This is why I proposed a trial to the California Avocado Commission to set up a high density trial with Hass and Lamb Hass, comparing two pruning methods: complete pruning each year vs. the three year rotation idea.  The Commission liked the idea and they funded the trial, along with grower education classes.   The trial and the classes will commence in the summer of 2012.

High Density Avocado Plantings.  High density plantings were proposed in California in an article in Subtropical Fruit News by Fallbrook grower Reuben Hofshi in 1999.  Hofshi stated that the underlying premises for planting on close spacing were:

1. To compete in the international market with low avocado prices will require more efficient farming and a significant increase in productivity.
2. Young trees are vigorous, produce large fruit early, have better canopy to root ratio and reach peak productivity approximately by 7 to 8 years.
3. Smaller trees are easier and less expensive to harvest, particularly when sixe picking is done, and are very amenable to snap harvest.
4. Spraying for different pests may become a way of life; smaller trees are probably the only ones that could be efficiently sprayed by ground rigs in hilly terrain.”

In the last few years in California we are seeing a severe reduction in our labor force for harvesting.  Pickers are getting picky; they have cell phones and they can call around to find the groves that can be harvested from the ground.  Lugging a ladder around on a steep slope is just not desirable, and they can make a lot more money if they don’t have to use ladders.

Researchers in other countries have been interested in high density plantings and many of the new plantings in Chile are planted in high density patterns.  Ernst and Ernst, growers and nursery owners in South Africa maintain that  high density can only be successful if the trees are pruned to a central leader immediately after planting, and maintained in that manner through the life of the tree.  They are working with a Hass-like variety known as ‘Maluma’ which has more of a natural central leader than does ‘Hass’.

Pruning.  Growers in California have traditionally avoided pruning.  Other than stumping periodically if the trees get too tall, not much pruning is done.  One of the reasons is the labor to prune.  It is difficult to determine from work done in foreign countries how much labor is involved in high density because many of the areas use the growth retardant paclobutrazol, a chemical we are not allowed to use in the U.S.  We will keep track of our labor costs in our trial and will report this to growers interested in high density plantings.

Can avocado growers survive in a county with high priced water?  Possibly.  High density plantings may be one solution to a really serious problem.  But growers must continue with good farming practices such as a complete leaf analysis each year along with proper irrigation scheduling.

Literature Cited:

Hofshi, R. 1999. High density avocado planting – an argument for replanting trees.  Subtropical Fruit News. Vol. 8 (1).

Ernst, Z.R. and A.A. Ernst. 2011. High density planting: a case study of central leader pruning with Maluma. Proc. VII World Avocado Congress 2011, Cairns, Australia 5-7 Sept. 2011.

Research to fight Huanglongbing (HLB), the deadly citrus disease carried by the Asian citrus psyllid, is taking place throughout our nation and the world.  Industry-wide urgency is funding a variety of research in search of a solution.

In 2010 a National Academy of Sciences panel concluded that genetic engineering “holds the greatest hope”.  A promising genetic engineering study, developed by a scientist at Texas A&M's Texas AgriLife Research and Extension Center is moving into the field testing stage.  

Using spinach defensin proteins to help protect citrus trees and then exposing the trees to the bacterium, lab and greenhouse studies show infection rates are very low even when the trees are exposed to higher concentrations of infected insects then would be found in a commercial grove.  The type of defensing protein used in the study can be found in plants, insects and mammals.

To learn more please see the AP article, Spinach could be weapon against citrus scourge.

While the Asian citrus psyllid/HLB pest-disease complex has received a lot of press lately, another deadly pest-disease combination has been found in Los Angeles County.

Tea Shot Hole Borer (Euwallacea fornicatus) is a vector for the Fusarium fungus. A native from Asia, this beetle is very small. Females are between 1.8 to 2.5mm (0.07-0.1 inch) long. Males are even smaller at 1.5mm (0.05 inch).

In addition to avocado trees, this insect is a serious pest of tea in Sri Lanka and India. In California, The Tea Shot Hole Borer was first reported on black locust (Robinia pseudoacacia), Lychee (Litchi chinensis), Box elder (Acer negundo), but there were no reports of fungal damage.

What to do:

• Look for a single exit hole with surrounding white powdery exudate.
• Scrape off the bark layer around the infected area to see the canker.
• Follow the gallery to look for the beetle (may or may not be present).
• Avoid movement of infested avocado wood out of infested area.
• Look for other hosts (Castor beans plant, Black locust, Lychee, and Acer) showing symptoms of the beetle/disease.
• Because the beetle tends to colonize both live and new dead wood, chip the dead wood within the grove and cover with a tarp for at least a week to prevent further beetle colonization.
• Sterilize tools to prevent spread of the disease with either 25% household bleach, Lysol (add symbol) cleaning solution, or 70% ethyl alcohol.

For more information please see the Fusarium dieback on California Avocado trees vectored by the Tea Shot Hole Borer Pest Alert developed by researchers at UC Riverside. The flyer is available in English and in Spanish.

Tea Shot Hole Borer (Euwallacea fornicatus). The pest is about the length of Lincoln’s nose on a penny. Photo by G. Arakelian.