Welcome back! I was very encouraged by the positive reception part 1 of Philip’s Mac101 content received and am excited to round out the wisdom with part 2.

In this edition we’ll cover:

  • Orchard Floor Management
  • Irrigation
  • Nutrition
  • Pruning
  • Pests & Diseases

ORCHARD FLOOR MANAGEMENT

Young Orchards:

Given the well-known fact that I am a fervent fan of focusing on soil health above all else, I find it necessary to highlight that the following information is Philip’s advice and, given the fact that he is a renown consultant with decades of experience in macadamias, I felt it was worth sharing, even if my own (completely unqualified) opinion differs a little …

Philip does advise that it really is not necessary to keep the tree lines so clean and weed free as to render the orchard floor almost sterile, but to rather doing everything possible to mimic the forest floor conditions that exist in their natural habitat, especially when the trees are young.

He then goes on to recommend the use of plastic mulch to conserve moisture and restrict weed growth around newly planted trees. “While the laying of plastic mulch does add cost to the establishment phase, the cost of this operation is roughly equivalent to one round of herbicide spray using glyphosate,” says Philip, “in addition, the use of copious quantities of compost applied in the drip area of the trees can only be beneficial to root growth.”

On the topic of composting, Philip gave a very valuable recommendation that I had never heard before; “A simple method of gauging when the composting is complete is to place a steel fencing dropper in the compost pile. While composting is in progress, the dropper will be hot to the touch, reaching temperatures around 60°C. As composting slows down the dropper will get cooler and will be at ambient temperature when composting is complete, when the compost material can safely be applied to the trees.”

Philip concedes that while the orchards pictured below are beautiful, it really isn’t necessary to mow this often and that having some taller grass and even a few weeds in the inter row space provides a habitat for pollinators and natural predators.

Picture taken by Philip Lee

 Mature Orchards:

Philip’s advice for productive orchards is:

  • Ensure that the orchard floor is cleaned before mature nut fall (after November drop of immature nuts) in December/January.
  • Remove all nuts left from the previous season, immature and insect damaged nuts. Failure to remove these nuts will mean that some of them will be delivered to the factory when harvesting begins. This will not only reduce your sound kernel recovery rate, but will increase the chances of rancidity build-up in the final product, thereby reducing its shelf life.
  • Remove all leaf litter or coarse mulch from under the tree canopy. This can cause an increase in the germination of mature nuts which fall, especially in wet periods.
  • Leaf litter and immature/old nuts can be raked into a windrow between tree lines and chopped up with a mulcher, to produce a fine mulch under the tree canopy and in the area between tree rows. Doing this will improve soil moisture retention and organic matter preservation in the root zone.

Picking up all nuts at every harvest and thorough stripping of trees at the end of the season, will make next season’s clean up job that much easier.

 Cover Crops:

Philip recommends the use of cover crops citing the following as the leading benefits thereof:

  • protect the soil (erosion)
  • ‘green manures’ (left on the soil surface where possible to decompose rather than ploughing in)
  • improve soil fertility (act as a slow-release fertiliser),
  • improve soil structure,
  • improve soil water management,
  • reduce weed pressure
  • control pests and diseases by acting as trap crops to lure pests away from macadamia trees and promote populations of beneficial insects.

He also mentioned that care should be taken when using cover crops in years with below average rainfall as they can draw available moisture from the soil. Use of cover crops in this situation is a trade-off between the benefits of crop growth and the drawbacks of reduced soil moisture for crop production that season.

Philip expanded by saying that the choice of cover crop/s will depend on your requirements but most importantly will be based on the season. Winter crops generally perform poorly in summer and vice versa. Winter legumes produce less biomass than summer legumes and there are fewer winter choices especially in cooler parts of the country. He then went on to share his current favourite: Arachis Pintoi, aka the wild peanut.

Arachis Pintoi

Philip continues; “Young macadamia orchards provide the perfect situation for growing cover crops in the inter-rows to maintain or improve the soil environment or may be planted as temporary windbreaks around young trees (taller varieties such as sun hemp and / or sorghum are effective as illustrated below).

Forage sorghum / sun hemp mix planted in a young macadamia orchard. (Source: Southern African Cover Crop Solutions)

IRRIGATION

Philip has done research that shows that macadamia orchards produce 60% more when irrigated and therefore believes that the capital investment is justified.

His trials were done in the Levubu area where annual rainfall averages 943mm. In a subtropical climate like this, he says that it is generally accepted that the yield of unirrigated mature trees is about 2.5tons DIS/ha, while mature trees under irrigation will produce yields of 4 tons DIS/ha and more. In this section he shares more details on water requirements of macadamias and the most critical times to irrigate, especially when water is in short supply.

How much water to apply

Philip says that macadamia trees will produce optimal yields with total precipitation of 1200mm per annum. So, if you take your average rainfall away from that, you’re left with the amount you should irrigate. E.g.: in an area where the average annual rainfall is 900mm, the deficit is 300mm over the year.

A further consideration is that irrigation does not cover the whole orchard but is concentrated around the trees i.e.: microjets wet about 60% of the orchard and drip, about 20%. So you can reduce your supplementation further: 300mm x 60% = 180mm for microjets or 300mm x 20% = 60mm for drip.

You then need to consider the loss factor, particularly with micro-jet irrigation as the water evaporates in the delivery process. Generally this is about 15%.

This then changes the amount to 207mm for micro-jets.

Calculating the actual amount of water is done like this:

To put down 1mm on 1 hectare will require 10m³ water.

Therefore, 207mm on 1 hectare = 2070mᶾ (2,07 megalitres or 2,07 million litres) over the year, per hectare.

Or, if you’re using drip irrigation, 60mm on 1 hectare = 600mᶾ (600 000 litres) over the year, per hectare.

The next step is to decide WHEN this water should be put down. This is a decision based on 2 factors: 1. Rainfall – as all good farmers do, we keep track of rainfall so that the shortfall, if any, can be supplemented. 2. Tree needs – and when we consider ‘tree needs’, it’s actually what we need from the tree ie: yield. Knowing when to supplement the tree’s water requirements so that our own yield requirements are met is what we’re really looking for …

What does this mean for your daily planning?

Water shortages cause stress and stress affects yield. The purpose of applying irrigation is to avoid the trees going in to a stressed state. It is interesting to note how stress, during the various phenological stages, impacts yield and thereby know which are the most sensitive stages.

Dr Stephenson’s study (reference at the end of this article) showed this:

What this research revealed is that stress during November dump time impacts yield the most. According to this, if we maintain a stress-free environment through this time, the yield will undoubtedly improve. The phase where water stress least impacts yield is flower initiation.  So, if you’re going to withold irrigation it should be during the cooler winter months, starting in about March, through to October.

Tree age / Canopy coverage

It goes without saying that smaller trees need less water, concentrated in their root zones, so whilst your irrigation system will be designed for a mature orchard, there will be adaptions along the way and the most significant one is water volume and placement.

Soils as water reservoirs

Another piece in the irrigation puzzle is soil type. Different soils retain and release water differently. Ultimately it is important to know your soil so that you understand how much water is available to your trees. Plant Available Water (PAW) is the difference between the Field capacity (how much water your soil can hold on to) and the Wilting point (how much water your soil will release to the trees). Obviously these are not easy things to measure … unless you have capacitance probes linked to interpretive software. Combined, these tools will indicate when you need to irrigate, from a soil moisture perspective.

So now you have Philip’s advice on irrigation, from a plant-needs perspective, from a tree phenology and age perspective and from a soil perspective. The farm situation is the final piece in deciding HOW to deliver the water – through micro-jet sprinklers or drip irrigation. He advises that water availability, management capacity and access to capital are just some of the factors that come into this decision.

NUTRITION & FERTILISATION

Most farmers I visit push their chair back from the table when I ask about this topic; they raise their arms defensively and say that they leave this completely up to the experts. And I completely understand that. After all, it’s an incredibly complex science and not everyone is wired to master it – least of all me. But … there are a few among us who want to know more and are not ready to completely surrender to the ‘specialists’, all of whom are selling something at the end of the day. So, if you’re keen to know a little more about feeding your trees, put your concentrate-now cap on and listen to Philip … (who gains nothing by sharing this info, I promise 🤣) … and who also suggests that finding a trust-worthy specialist to guide you through the science, and how it plays out on your farm, is essential.

Philip explains how we have windows into the orchards (trees and the soil) that should be used regularly in order to understand what’s happening and how we can support it. These windows are leaf and soil sample analysis results. His recommendations in this regard, I am sharing verbatim:

“It is very important that accurate soil and leaf analyses data be obtained.  Identify and mark specific trees within each sampling block. These trees serve as your “window” into the orchard and are the only trees from which samples should be taken and on which visual assessments can be based. For both soil and leaf sampling the size of the “sampling block” should preferably be no more than 10 hectares and 20 trees (soil sampling sites) need to be marked within each sampling block, following a Z or X pattern through the block. Samples taken from these marked trees every year (or in the case of soil sampling every 2 years is sufficient to monitor soil fertility over time) will greatly reduce the variability of the results, ensuring that you are better able to compare like with like in building a history of soil fertility and nutrient status in your orchards.

When taking soil samples, growers must ensure that the soil is taken from more or less the same depth every year and at the same time each year. July is a good time to standardise on as it allows sufficient time for the last fertiliser application of the season (April) to be absorbed in the soil and utilised by the trees. For topsoil samples a depth of 0 to 15cm is fairly standard and for subsoil samples 15-30cm and 30 to 45cm. The soil sample hole should be dug to the required depth (15cm for topsoil samples) within about one metre radius of the tree stem (closer to the stem for small trees, further away as trees grow – to ensure the sample is taken from the active root zone of the tree). Soil from the hole can be placed beside the hole and then one or two handfuls from each hole placed in a clean bucket. When the 20 sites per sampling block have been sampled, soil in the bucket can be tipped onto a clean sheet of plastic and quartered by drawing an X in the soil. Taking one handful at a time from each quarter, the sample bag (clean plastic bag or clean strong paper bag) can then be filled until it contains around 500 grams of soil, sealed and clearly labelled immediately. Cedara and SASRI labs have standard sampling boxes.

Leaf samples must be taken during October/November each year by sampling the fourth leaf behind the growing point on an actively growing shoot or a shoot that has recently completed a growth flush on which the terminal bud is dormant, as illustrated below. Only leaves from healthy plants must be sampled. They must be free from sunburn, insect damage or any deficiency symptoms or signs of disease. As indicated, four leaves taken from alternate sides of each of the 20 marked trees per sampling block will make up a sample of 80 leaves. After the leaf is removed from the tree it is placed in a suitable clean paper bag and once the block has been completed the bag can be sealed and clearly labelled immediately. If necessary to store the leaf samples for a few days before delivery to the laboratory, this should be done in a fridge at about 5oC. Do not freeze leaf samples before delivery.”

Macadamia leaf sampling method showing position of the leaf to be sampled. (Source: The Cultivation of Macadamia – ARC)

Once you have the test results back, what’s next? Philip continues …

“Analysis of the data is more meaningful when recommendations are based on historical trends rather than on only one year’s results. Any nutrition management programme must take cognisance of the following:

– a visual appraisal of the trees (including yield data)

– historical soil analysis data

– historical leaf analysis data and

– any previous fertilizer programmes.”

For all these assessments, there are ‘norms’ – indictors as to what level the soil nutrients should be and what level the tree should be holding (in the leaves). Visual assessments should corroborate the shortages or toxicity issues shown in the analysis data. All this information will be taken in to account when planning the nutrition going forward.

e.g.: If leaves are dull and pale green (visual appraisal) then nitrogen might be low, if the analysis data substantiates this and the historical fertiliser programme shows that we’ve been tight with nitrogen, it’s safe to plan more nitrogen into the upcoming programme (and adjust future programmes accordingly) – problem sorted.

The challenge comes in when things don’t line up e.g.: tree is showing deficiency symptoms, leaf analysis verifies this but the soil is full of it because you’ve been applying it generously. Now you need to find out why the trees aren’t absorbing it. This can be as simple as an oversupply of an inhibitor element. When you cut back on this, the ‘deficient’ nutrient may be unlocked. But, because everything you tweak has a knock-on effect to everything else, this can get messy. And is also a very big reason to allow nature to manage the provision of nutrition – but this is another topic entirely.

Nevertheless, here are some guidelines picked from Philip’s highly technical chapter on fertilising …

  • Macadamia trees need between 25 and 50 grams of nitrogen per tree per year per age of tree.
  • Then the balance of N:P:K appropriate to young trees from planting up to age 3 when the first small crop is produced is in the ratio 4:1:2, changing to a ratio of 5:1:5 once trees are in production. While it is acknowledged that this general guideline is an over-simplification of the complex process of making accurate fertiliser recommendations for specific orchards growing in a wide range of soil types and climatic conditions, there are a number of orchards in Southern Africa producing sustainable yields upwards of 3.5 tons DIS/ha using these general guidelines.
  • Philip gave a table of ‘normal’ nutrient levels which I thought might be a useful resource:
Soil and leaf analyses norms for macadamia (Kuperus & Abercrombie, 2003)

 
Analysis result NORM
SOIL
pH [H2O] 5.5 – 6.5
pH [KCI] 4.5 – 5.5
P 20 – 80 ppm* (14-35 for AMBIC analysis)
K 80 – 150 ppm 0.2 – 0.4 meq %**
(1 meq % = 390 dpm)
Ca 400 – 800 ppm 2.0 – 4.0 meq %
(1 meq % = 200 dpm)
Mg 100 – 200 ppm 0.8 – 1.6 meq %
(1 meq % = 120 dpm)
Na < 40 ppm <0.2 meq % (1 meq % = 230 dpm)
Total CEC 3.2 – 6.2 meq %
Ca (% of total CEC) 72.5
Mg (% of total CEC) 15
K (% of total CEC) 7.5
Na (% of total CEC) < 5
LEAF
N 1.2 – 1.5 %  
P 0.07 – 0.09 %  
K 0.4 – 0.6 %  
Ca 0.6 – 0.9 %  
Mg 0.09 – 0.11 %  
S 0.2 – 0.4 %  
B 50 – 90 ppm  
Zn 15 – 30 ppm  
Cu 5 – 10 ppm  
Mn 150 – 250 ppm  

*ppm = parts per million = mg/kg = miligrams per kilogram

**meq %             =             milli equivalents % or per 100g soil

1 meq % Ca         =             200 ppm Ca

1 meq % Mg       =             120 ppm Mg

1 meq % K           =             390 ppm K

1 meq % Na        =             230 ppm Na

TOTAL CEC          =             Total Cation Exchange Capacity

=             Sum of Ca + Mg + K + Na as meq %

Here are Philip’s recommendations of soil optimisation through the various ages:

  • Prior to planting:
    • the correcting of soil acidity and phosphate levels through the addition of lime, gypsum, potassium and phosphate.
    • The aeration that takes place when these elements are ripped and disced into the soil also stimulates healthy root growth once the trees are planted.
    • Increasing organic content of the soil is strongly advised especially if the soil is sandy or has been cultivated before. Here, Philip had two pearls of wisdom to add: 1. “Organic material used must be thoroughly composted before application, otherwise the composting process will continue on the soil after application, actually withdrawing nitrogen from the soil surface until the composting action is complete. 2. Beware of using large volumes of chicken manure in the compost, as the high phosphate proportion of chicken manure can lead to phosphate-induced iron deficiency where chicken manure-rich compost is used continuously over a number of years.”
  • Planting to 3 years (non-productive years)
    • For the first 2 months, while the newly planted trees settle in and their roots start to grow beyond the potting medium, no fertiliser should be applied.
    • After that ‘little and often’ is how it should be administered.
    • As the main aim, at this stage, is to stimulate vegetative growth and develop vigorous, healthy root systems, the appropriate fertilisers to use are Limestone Ammonium Nitrate (LAN 28%N) or Greensulf (26% N) to supply the nitrogen required for vegetative growth and Mono Ammonium Phosphate (MAP = 11% N; 21% P; 3% S; 1% Ca) in which the phosphate component will stimulate root growth. Remember, macadamias only need small quantities of phosphates so try correct the soil P levels before planting your trees.
    • As macadamias are extremely sensitive to salt burn, it is most important to avoid the use of any fertilisers containing chlorides, so as to limit salt build-up, especially in more alkaline soils and during drought years.
    • In the early years it is important to get the trees to a size and shape so as to optimise the amount of bearing wood in their canopies as quickly as possible, the regular use of foliar nutrient sprays is also beneficial. Lowest cost options are products such as Low Biuret Urea (LB Urea) or Ammonium sulphate, with micronutrients added.
    • Philip warns that, while many of the commercial foliar mixes will have the micronutrients macs need most often (zinc, boron and copper), they are generally at such low concentrations as to hardly have any impact on correcting observed deficiencies. Growers need to be aware of this and supplement accordingly.
  • Bearing Trees (4 years and older)
  • As trees begin to bear nuts, generally from tree age 4 onwards, the transition from a focus on vegetative growth to reproductive growth requires a shift from a high N:K ratio (N:K of 2:1) to a lower N:K ratio (N:K of 1:1 to 1:1,5).
  • Timing of fertiliser applications:
When Nitrogen
June 100% of Calcium & Magnesium application (could also be done in Feb/March)
July/Aug 25% of Nitrogen application

20% of Potassium application

50% of Phosphorous application
Sept/Oct 35% of Nitrogen application to optimise summer vegetative growth

25% of Potassium application

50% of Phosphorous application
Nov/Dec 25% of Nitrogen application to further stimulate flush

35% of Potassium application
Jan 10% of Nitrogen application to minimise late summer vegetative growth

10% of Potassium application
Feb/March 100% of Calcium & Magnesium application (could also be done in June)
April 5% / 15%* of nitrogen application

*for dryland orchards – to take advantage of the last rains

10% of potassium application to ensure that sufficient potassium is available to stimulate fruit growth and development during the period September to December

Philip emphasises that although sulphur is included in a number of fertiliser products, it is important that farmers make sure it is present in all fertiliser programmes as it important for oil accumulation.

Boron is critical in promoting the growth of the pollen tube to ensure successful fertilization to set as many nuts as possible

TREE TRAINING AND PRUNING

As the mac industry in South Africa matures, we are building up some experience. Unfortunately, in the department of pruning, it’s more about what NOT to do … so we can thank the early farmers, who didn’t prune, for the evidence that this is a not a profitable strategy. As a result, we know that sustainably profitable mac orchards need to be pruned but there are some differing opinions on what the best tree shape is and how canopy management is best handled.

Philip has a very solid strategy in this department, which I’ve summarised below – it requires that you start from young trees …

Commercialising macadamia trees necessitates manipulation (irrigation, fertilising and pruning are all forms of manipulation) but nowhere is it more literal than in training. The ultimate outcome of Philip’s strategy is a tree with only one vertical stem (central leader) with main wide crotch angled framework branches spaced roughly every 0.5m up its length. To achieve this he starts by cutting the young tree off at knee height (50cm high). The continuation of the central leaders as well as the first set of framework branches will grow from this cut. Here’s how: in the axil of every leaf, just where the leaf stalk attaches to the stem, there are 3 vegetative buds, arranged in a vertical line on the stem, known as the primary (top), secondary (middle) and tertiary (bottom) buds.

When these buds grow, the top ones will have a very narrow crotch angle (less than 15 degrees). The middle ones will have an angle of between 15 – 40 degrees and the bottom ones will grow out at about 40 degrees. So, we select the strongest of the top shoots to become the central leader and remove the other top shoots and all the secondary shoots, leaving only the tertiary shoots which will become the new framework branches at this level.

Regrowth after the young tree was cut back to 50cm height.

Only 5 shoots have been left – the new central leader (far, left hand side) and the four bottom shoots that will become the lateral branches. This is a Beaumont, hence 4 shoots – from the four leaves. Integs would have 3, as they have 3 leaves per whirl.

We continue to do this exercise of cutting off the central leaders to stimulate lateral branches at 1m, 1,5m and 2m so that there is a new framework of lateral branches at each cut. Sometimes the tree will naturally produce the side shoots – we just need to make sure the crotch angle is correct by removing the top and middle shoots.

A similar exercise is done on the lateral branches … any of these that grow longer than 0.5m in length without, in turn, making side (secondary) branches, must be cut back by about one third of their length, in order to stimulate additional lateral branching.

Another way to train young trees is by using “toothpicks”. These can be especially useful in the upright growing cultivars (816, 741, 344). Thin (0.5 to 1mm diameter) steel wire, cut into lengths of about 8cm using a chisel to make a sharp point at each end of the wire, make very effective toothpicks. So, the first action required is to identify the main framework lateral branches on the central leader and to begin bending these branches outwards by inserting the wire spacers between the central leader and the lateral branch. This is best done by gently pulling the lateral branch downwards and, while holding it at a widened crotch angle, insert the one side of the wire spacer into the central leader, then insert the other end of the wire into the lateral branch as you release the lateral branch, so that the wire is stuck into the bark at both ends making a relatively tight fit so that it will not be blown out by the wind. This action then needs to be repeated at 6 to 8 week intervals, moving the spacer down into the branch angle each time, until the desired crotch angle of about 45 degrees to the main stem is achieved. The wire spacers can then be left in position in the tree as the bark will grow over them in time and they strengthen the attachment of the laterals to the central leader.

Steel wire spacer in place to widen crotch angle of main framework branch in cultivar 788

When the trees have been trained to the correct and productive shape, with a single central leader and main lateral framework branches at roughly 0.5m, 1m, 1.5m and 2m up the length of the central leader by about tree age 4 to 5, the tree can be left to grow without much more intervention. The central leader above the 2m height will continue to grow vertically and produce more lateral branches up its length. Although most of these higher lateral branches will have narrow crotch angles, they will grow more laterally over time and begin to shade out the lower laterals.

 Another golden rule of thumb: when the height of the tree exceeds 80% of the row width, tree canopies become less productive and yield will level off or start to decline.

 At the commonly used tree spacing of 8m X 4m, this less productive state will begin when tree height gets to about 6.5m (80% of 8m = 6.4m). The focus then is to ensure that the bottom half of the tree remains productive, by removing at least one and a maximum of two of the lateral branches in the top half of the tree, which are shading out the laterals in the bottom half of the tree, each year. This branch thinning generally only needs to start from tree age 6 to 8 and should be done each year immediately after completion of harvesting.

Keeping the 4 leading goals of pruning a mac orchard in mind while you’re doing it is helpful:

  1. Maximise sunlight
  2. Maximise spray penetration (for pest management)
  3. Maximise bearing canopy
  4. Minimise wind damage

PESTS & DISEASES

Ag, this is such a loathsome topic, I am always tempted to practice the ostrich strategy BUT that won’t get anyone anywhere so here’s the highlights of the extensive info Philip provided on the subject:

Effective pest control is combination of:

  • Quality scouting
  • Accurate record keeping
  • Cautious and precise spraying
  • Environmental sensitivity
  • Knowledge & Awareness

QUALITY SCOUTING

“The importance of implementing a thorough scouting system as a basis for pest and disease control in macadamia orchards cannot be stressed enough,” is Philip’s opening comment on this topic. He cites the increasing resistance being built up in macadamia pest, as a result of programme or incorrect spraying, as the motivation behind his statement. Quite simply, we HAVE to employ an Integrated Pest Management approach that includes a range of control tactics and minimises pyrethroids and other broad-spectrum killers.

As with all things, there are always shortcuts. The astute mac farmer, nor the industry as a whole, can afford the consequences of sub-standard scouting so farmers need to ensure that scouting teams are well-trained, well-equipped and following a well-thought about plan. Here’s Philip’s top tips:

  • Scouting should continue throughout the year, with each orchard scouted every week during the growing season and every two weeks in the off-season.
  • Train the scouters to look for disease symptoms and nutrient deficiencies while they’re scouting. Equip them with a Smart phone so that they can take pictures and mark locations.
  • Stink bug scouting:
    • Scouting must be done at or before sunrise while bugs are still static and before they start moving and flying around as the temperature increases.
    • Select 10 random trees in an orchard.
    • The same 10 trees must not be sprayed again until at least six weeks have passed ie: each time you scout, it should be 10 different trees.
    • Make sure the base of the trees is clean or lay sheet under the trees.
    • Spray a medium to heavy cover spray of Dichlorvos @ 150ml/100L over the selected trees.
    • Wait for 30 minutes then collect fallen bugs. Do not leave too long as birds can eat the bugs.
    • Record numbers present. Plotting the number of stink bugs per ten trees sprayed per scouting block weekly on a graph with a straight line drawn at the 4 bugs per block level, provides the grower with an instant picture of the bug populations in the orchards as well as a predictive model on which to base spray timing decisions
    • The threshold level which triggers the decision to spray is 4 stinkbugs on the 10 sprayed trees, or 0.4 stinkbugs per tree. When weekly bug numbers are on the rise and reach a level of more than 3 per 10 trees, the grower is advised to accompany the scout on a walk through the orchards very early in the morning, especially to stink bug “hotspots”, to verify stinkbug activity for himself before deciding whether to spray or not. Growers religiously practising IPM will sail close to the wind in deciding when to take corrective action, waiting until two successive weeks of counts at or slightly above the threshold level are recorded before confirming the spray decision. They may even opt to use a higher threshold level, aiming to reduce the number of sprays applied per season to an absolute minimum, in order to preserve the balance of nature and maintain the natural predator levels as high as possible.
  • Scouting for Macadamia Nut Borer (MNB) and False Codling Moth (FCM):
    • Set up specially designed ‘Yellow Delta Traps’ in late July/early August before numbers are high
    • Use one trap for every 3 to 5ha of trees
    • Traps must always be in the top 3rd of the tree
    • If monitoring for both MNB and FCM, place traps at least two trees apart
    • Traps must be checked weekly from the start of fruit set to the beginning of harvest
    • Remove moths after counting (otherwise it is difficult to count future trapped moths)
    • Record numbers from the trap on the back of the trap, in a book or spreadsheet
    • The threshold level triggering the treatment decision is 7 moths/week for MNB and 10 moths/week for FCM
    • Pherolures must be kept refrigerated until ready to be used. Try not to handle excessively when placing in trap. Replace MNB pherolure every four weeks (place in centre of sticky liner). Replace FCM pherolure every seven months (attach to wire on inside of trap and hang from roof of trap).
    • Yellow Delta Traps need only be replaced when perished.

There are a number of ways to avoid resistance build up in pests; one of the most important is to restrict the use of broad-spectrum pesticides (the synthetic pyrethroids) applied as foliar sprays to an absolute minimum and actively seek environmentally-friendly alternatives.

Control of the nut borers can now be achieved by one or two applications of the “attract and kill” pheromone products per season for both MNB and FCM.

Alternative to preventing stink bug damage:

The use of the neonicotinoid thiamethoxam applied as a soil drench at 9ml of product in one to five litres of water per tree, provides systemic control of stink bugs for a three month period. In theory it is now therefore possible to achieve acceptable levels of stinkbug control using two (second application is 6ml/tree) such applications of this soil drench per season (180 days of control), once again precluding the use of too many high-volume, broad-spectrum insecticide sprays to achieve the same level of control.

Note carefully however, that the use of this product is less effective in soils with more than 25% clay as well as in trees older than 12 years. The soil drench is most effective when the orchard has been irrigated the previous day then followed immediately by irrigation via a microjet system, and can be applied through the water in drip systems (or by hand directly under the drip emitters). Only use this product once every second season to prevent resistance build-up.

Thrips:

Philip agrees that the bronzing of husks has no impact on kernel quality or size and is probably caused by mites rather than thrips.

(Left) Thrip and/or mite damage (‘bronzing’) on macadamia husks

(Right) Typical leaf symptoms of thrip damage (Source: Colleen Hepburn)

The more serious impact of thrip damage is the reduction of vegetative growth, especially on the new spring flush starting from about mid-August and aggravated by hot dry conditions at that time. Given that the average length of the life cycle is 19 days and that each female lays about 70 eggs, it is understandable that the thrip population builds up so fast that it is often too late to remedy the situation by the time the damage to vegetative growth is observed.

Damage to the new spring vegetative flush is certainly limiting the size of this flush, which all of our management inputs are aimed at optimising, but on the other hand, it is promoting branching where the growing points are damaged, thereby creating additional bearing wood for future nut production.

For biological control of thrips there is a relatively new product being used in the South African macadamia industry with good results; it is an insect-killing fungus from the soil called Real Metarhizium anisopliae 69.

 Root diseases:

Although macadamias have not been prone to any serious diseases in South Africa, Philip warns that attention needs to be given here to the increasing prevalence of the root rot fungi complex in recent times, especially in poorly drained soils in KZN.

Management of the root rot fungi complex, which includes Phytophthora, Pythium and related pathogenic fungi, is essentially restricted to cultural practices at this point in time, as there are presently no fungicides registered for use on macadamias in South Africa. Cultural practices at our disposal include controlled irrigation, ridging of less well drained soils, the application of compost to improve the carbon level and organic component of the topsoil to stimulate the beneficial micro-organisms in an attempt to repress the pathogenic fungi, and use of mulch to moderate soil temperature so as to stimulate the growth of healthy roots in the topsoil.

Fungicides registered for use in avocadoes in South Africa have been found extremely effective in combatting the symptoms of root rot in macadamias, when applied as either soil drenches around the base of the macadamia tree trunks, stem sprays or foliar spray treatments, up to four times per season. It is likely that one or more of these fungicides will soon be registered for use in macadamias in South Africa. To date the use of these fungicides as a macadamia tree trunk injection, as commonly practiced in the avocado industry, has not been successful.

Blossom blight:

 Risk of infection is higher in the more humid growing areas, like the KwaZulu-Natal coastal belt, and during occasional spells of cooler, misty weather in the flowering period from August through to October. This fungus infects flowers, as illustrated below.

Blossom Blight infected flowers

Dense canopies are more susceptible than well-pruned, open orchards. Preventive foliar sprays can be applied to control the disease with the first application at the time when the raceme has grown to full length but before flower opening and the second application 2 weeks later.

Husk rot:

 Another fungi issue – symptoms can appear as early as November, but are far more prevalent in December and January. Initial infections often appear near the pedicel end of the husk and progress down the seam of the nut, as illustrated below. In severe cases the lesions can cover the entire husk which may well penetrate to the shell and impact kernel quality.

Husk rot damage showing progression of the black lesion along the seam of the nut. (Source: Tracey Campbell)

Affected nuts may drop prematurely although diseased husks also remain attached to the pedicel on the tree and are seen as “stick tights” at the end of the harvest season. The more susceptible cultivars identified to date include 816, 788, Nelmac 2 and 344.

Cultural controls include the removal of all “stick tight” nuts from trees during the final harvesting round of the season. While some of the nuts harvested in this final harvest may be of acceptable quality, it is wise to water sort these nuts after dehusking to discard all floaters. Ensure also that all old nuts remaining on the orchard floor at the end of harvest are removed as they may be a source of infection. If husks are to be used as mulch / compost, ensure that they are well composted before application under the trees.

And that brings us to the end of Philip Lee’s advice. Thank you Philip – your spirit in sharing is perfectly aligned to TropicalBytes’ mission; to equip farmers for excellence and success by sharing knowledge and expertise and, for that, we are eternally grateful.

References:

Stephenson, R.A., Gallagher E.C. and Doogan, V.J. 2003. Macadamia responses to mild water stress at different phonological stages. Australian Journal of Agricultural Research 54 (1): 67 -75.