Common Problems of Macadamia Trees in Hawaii

Common Problems of Macadamia in Hawaii1

H.C. Bittenbender2 and Howard Hirae3

Reprint From CMS Yearbook 1990

Macadamia growers in Hawaii frequently see striking or odd things in their orchards. What’s this? Is this a problem? Is my tree sick? Will this lower yield? Even the experienced grower might miss the less obvious signs of problems that will seriously lower an orchard’s productivity.

This bulletin responds to a specific request in the 4th Macadamia Industry Analysis, 1987, action 1.4, "Publish a pictorial chart showing 99% of the common Macadamia problems, despite cause, i.e., pest, disease, nutrient imbalance, improper pesticide and fertilizer application." Growers at the 28th Annual Meeting of the Hawaii Macadamia Nut Association were surveyed regarding what were the common Macadamia problems, both minor and major that they have had and that new growers should know about. The problems identified by growers are the basis for the following discussion of common Macadamia problems in Hawaii.

Unlike most bulletins that deal with problems and their causes we’ve organized the bulletin by symptoms and where on the tree the grower is likely to see them. Each problem is described briefly. These show outstanding features for recognizing the problem in the orchard, references for other publications and solutions (if known) are given.

Problems with the Soil

How to collect a soil sample

Soil Sampling prior to planting an orchard and annually when collecting a leaf sample is recommended. Collecting the sample properly to avoid contaminating the sample with fertilizer on top the soil or on your tools is important. Following the fertilizer recommendations from a soil analysis is the most important way to avoid nutritional problems.


In the orchard soil samples should be taken from a zone 2 feet from the trunk out to the edge of the edge of the canopy or drip zone. Using a clean shovel that you haven’t used for applying fertilizer, scrape away the surface leaves and soil until you reach the small and sometimes stubby Macadamia roots. Collect soil from this depth down another 4 to 6 inches using a clean plastic glove or bag over your hand. Place a cup or more of soil only into a clean plastic bag.

If you have just a few acres, collect from beneath three or more trees and thoroughly mix. The soil sample should weigh at least a pound for it to be analyzed for phosphorus, potassium, calcium, magnesium and pH. If you have an area that you suspect has nutritional problems, collect two separate samples, one from this area and one where tree growth is normal.

Label sample bag and complete information forms required by the analytical laboratory. Check with your Extension agent for companies that do soil analysis.


Problems Seen on Roots and Trunks

Macadamia Root Rot

This fungus, Kretzschmaria clavus attacks the roots and lower trunk of Macadamia trees eventually killing the tree. Orchards in high rainfall areas near forests or on recently cleared forestland are most susceptible to this disease. However, the disease doesn’t spread fast or appear to be a major problem.

Besides poor growth and few leaves, the obvious symptom is a black fungus with smooth or slightly knobby appearance on the lower trunks or exposed roots. Young shoots (roots suckers) growing around the trunk are also symptoms. If the tree is cut down, look for black lines and gray areas in the trunk cross and long sections.


No fungicides are registered or likely to be effective. Dead trees and roots should be removed from the orchard. Don’t plant in the same hole or nearby until the disease roots have decomposed.

Remove forest trees completely when preparing new orchards sites in forested areas. Don’t bury or chip trees in the orchard, as they may have the disease and infect the orchard. Don’t pile large rocks at the base of trees when planting in high rainfall areas. Cut and scraped trunks and roots provide entry for this disease.

Trunk Canker

This is an uncommon disease. The fungus Phytophthora cinnamomi can infect trunks at or near the ground generally through a wound. The infected part of the trunk appears cracked and callused or flattened and produces a dark gummy sap. Tree growth is reduced and the tree may die.


No Fungicides are registered. Preventive measures include planting on well-drained sites, not planting too deep, and avoiding cutting or wounding the trunk near the ground.

Inverted bottleneck—shaped trunks

The inverted bottleneck becomes apparent when the trunk of the scion (top part of a grafted tree) grows wider than the rootstock below the graft union. This is a problem primarily in older orchards where smooth shell cultivars are grafted onto rough shell rootstocks. Newer orchards have smooth shell rootstocks, therefore the inverted bottleneck isn’t a problem. Frequently, affected trees will break at the graft union or the yields will decrease over time.


If your trees have this problem now, plant 3 or 4 smooth shell seedlings around the tree. Graft seedling to the smooth shell section of the trunk (above the graft union) with an approach graft when they’re established (at least half inch in diameter). Bond 23 produces vigorous seedlings but any healthy smooth shell seedlings will do. Alternatively if the tree has severely declined, remove the tree and replant.

To prevent, never plant trees grafted on rough shell rootstocks.


Problems Seen on Branches or Entire Trees

Poor Tree Shape

This is a serious problem. Narrow angle between the scaffold (main branch and leader trunk) called the crotch angle and too many scaffold branches at the same place weakened the tree. These trees snap off or suffer severe limb breakage as they mature, particularly in windy areas.


During the first two years in the field, trees should be pruned to a single upright leader. Scaffolds branches should be pruned to three per whorl evenly spaced around the trunk. By pruning, space branches at 18-inch intervals up the trunk. Root or trunk suckers should be removed as they emerge.

Dieback or slow decline

A general term for trees with many dead branches. Trees drop leaves at branch ends. Or older leaves drop too soon, so that only a few leaves are left at the end of branches. The condition develops slowly.

There are many causes for this type of dieback.

1.Disease infection in trunk or root system.

2.Prolong droughts.

3.Anaerobic conditions caused by compaction or poor drainage.

4.Poor root structure caused by planting root bound trees.

5.Toxic chemicals from herbicides or over application or uneven application of fertilizers.

6.Nutritional problems.

Dieback related to nutritional problems is common on highly leached soils in high rainfall areas like the Hamakua coast. Acid soils (less than pH 5) interfere with nutrient uptake, particularly phosphorus. Other soils can bind phosphorus, so roots can’t absorb it. Under these conditions aluminum levels in the leaves may reach toxic levels, greater than 200 ppm.


Try to determine the cause of the problem by process of elimination. Proliferation of root suckers generally indicates root, collar or trunk injury. Phosphorus deficiency usually starts on the main terminals first and works down. General loss of leaves of entire tree or confined to particular branches but not necessarily the top is probably phosphorus. Use soil and tissue analysis, and apply fertilizer and lime as recommended. Before planting a new orchard, have a soil analysis, and fertilize and lime as recommended.

Quick decline associated with ambrosia beetle

Leaves quickly turn brown and may stay on the tree or all drop immediately, death of the tree is rapid. This is a problem only in high rainfall areas.

Unknown stress factors are suspected to affect the tree and increase the susceptibility to beetle attack. Waterlogged soil, low soil pH, nutritional problems and fungal root and stem rots are suspected stress factors.

The role of ambrosia beetle, Xyleborus affinis is thought to hasten tree decline. It bores matchstick—size holes in the trunk leaving a trail of white powdery wood (frass) on the outside.


Trunks can be sprayed with endosufan (Thiodan®) to prevent beetles from infesting the trunk. Follow the label.

Don’t plant "Ika Ika (333), it is more susceptible than other cultivars. Dead and dying trees should be removed from the orchard, so the beetles from infested trees aren’t able to reinvest other stressed trees.

Problems Seen on Leaves and Young Shoots

How to collect leaves for tissue analysis

Semiannual tissue analysis is recommended to determine the best fertilizer practice for your orchard and to prevent nutritional problems. Choosing the correct leaves for analysis and understanding the results on the laboratory analysis report or "Elemental analyses of plant tissue by ADSC, CTAHR" form are important. Symptoms of common nutritional problems are discussed later.


If you’ve never taken a leaf or soil sample, talk with your Extension Agent or fertilizer representative before beginning. If your orchard appears normal, sampling one tree per acre is enough. Collect a leaf sample before trees produce new leaves, generally during February and March or before fertilizing in September or October. Pick 3 to 4 branches when the bud at the tip of branch is just opening and beginning to grow. Buds will have three small pale green leaves one quarter to one half inch long, it looks like a claw. If you wait too long the bud will open completely, it turns green and the new shoot and leaves are easy to see. Pick one healthy leaf from the second node (whorl) of leaves below the bud. Fifteen leaves from as many trees as needed for each sample submitted for analysis. Place the leaves in a plastic bag and label the bag with your name, date of sampling, and sample number.


Element Concentrations Adequate for Growth of Bearing Macadamia Trees.
Element Symbol Concentration Percentage
nitrogen N 1.45 - 2.00
phosphorus P 0.08 - 0.11
potassium K 0.45 - 0.60
calcium Ca 0.65 - 1.0
magnesium Mg 0.08 - 0.10
sulfur S 0.24, - or N:S ratio of 9 to 14
manganese Mn ppm 50 - 1500
iron Fe 50 or FE:P ratio =>600
copper Cu 4
zinc Zn 15 - 20
boron B 40 - 100
aluminum Al less than 20
ppm means parts per million or one ten-thousandth of 1%


Iron chlorosis

Typical symptoms are new leaves pale yellow to white; older leaves are green. This problem is common in a soils with pH greater than 6.5 or are over-fertilized with phosphorous. Trees growing in other soils under similar conditions can develop this problem.

Nursery stock may develop symptoms when excessive amounts of high phosphorus fertilizer such as 10-30-10 used.


Stop phosphorus fertilization until symptoms disappear. Semiannual leaf tissue analysis can be used to monitor leaf phosphorus (P) and iron (Fe) levels. If the Fe/P ratio is less than 600, iron chlorosis is present or will happen. For example, the Fe/P ratio in previous table ''Element concentrations adequate for growth of bearing Macadamia trees'' is 50 divided by 0.08 or 625. (See Hue and Nakamura, 1988. Macadamia chlorosis by phosphorus and iron fertilization, Proc. HMNA 28).

Magnesium (Mg) deficiency

Yellow, older leaves with wide green veins (interveinal chlorosis) are typical symptoms. Leaf tissue analysis show Mg levels less than 0.065%. Dry weather or a soil under high rainfall conditions favors this problem. Over-application of calcium or potassium fertilizers may cause Mg deficiency.

Mg deficiency had no effect on the growth of seedlings 26 months after symptoms appeared (John Bowen 1987. Micro—element nutrition of Macadamia Proc. 27th Annual Meeting of HMNA). We think its effect on yield is minimal.


Use tissue and soil analysis to confirm deficiency exists. Apply dolomitic lime or magnesium sulfate according to soil analysis results. Symptoms on affected leaves won’t go away. When the deficiency is eliminated new green leaves will mask and replace older, affected leaves.

Glyphosate (Roundup) injury

Early symptoms are dead or yellow leaves, the bark of green stems may crack. New growth is quite unusual. Many shoots begin to grow at the end of branches or tops of young trees. The tree or branch has a bushy appearance. New leaves may be stunted, narrow and pale. Drifting spray onto green foliage or heavy application in porous soils causes injury.


Branches or young trees generally survive and return to normal growth. If the excess shoots stay and a new, single stem doesn’t take over within a year, prune off excess shoots. Severely affected young trees may stay stunted for months to years, replanting may be necessary. Be extremely cautious when applying herbicides around young trees. Avoid spraying on windy days or soaking weeds down in porous soils. Follow the label.

Black citrus aphid

Young and adults suck the sap of young leaves and flowers causing a distortion or puckering of the leaves. Under normal conditions, yields aren’t reduced.


Generally enough natural predators are present to prevent serious damage.

Problems Seen on Flowers

Poor flowering

The intensity of flowering isn’t the same every year. In years when the flowering period is quite long, with no distinct period of heavy flowering, growers call this a poor flowering. Growers commonly call a concentrated or heavy flowering period a good flowering.

Research shows that tree yields are the same regardless if flowering was ‘good’, many nuts set and many dropped prematurely or if flowering was poor and few nuts set, and few dropped prematurely.


This isn’t really a problem, see Nagao 1988. Flowering, nut set and premature nut drop of Macadamia, HMNA 28. See also premature nut drop.

Flowers blights

A brown withering of the flowers and raceme is caused by two different fungi, Phytophthora capsici (Phytophthora blight) and Botrytis cinera (Botrytis Blight). Cladosporium is usually secondary or found only affecting the raceme tips. Heavy continuous rain and cool temperature during flowering are ideal conditions for these diseases to develop.

Infection is usually scattered within the tree and stops when the weather becomes drier. Phytophthora blight is more common in closed—in orchards, attacking all flower stages and young developing nuts, whereas Botrytis blight is more common on opened flowers.


Fungicides are available but seldom improve yield. Dry weather or reduced rainfall generally ends the infection. If problem persists, opening the orchard by pruning or tree removal to improve air circulation may help. Don’t plant closer than 25’ by 25’ in high rainfall areas.

Broad mite

This mite, Polyphagotarsonemus latus damages leaves, flowers, and nuts, Flower buds and unopened flowers may turn brown and young nuts drop. Husks of large nuts can be russetted or bronzed but this doesn’t affect the nut. This pest has a wide host range, which includes many weeds and doesn’t appear to affect Macadamia yield under normal mite populations.


Monitor flower buds for damage every week or two and spray wettable sulfur if you see a rapid increase in mite damage. Eliminating weed host around the orchard should also help. See also premature nut drop.

Katydid damage

Two long—horned grasshoppers, Conocephalus saltator and Elimaea punctifera occasionally cause damage to Macadamia blossoms. Normally, these katydids eat other insect pests but they can damage unopened Macadamia flowers, young shoots and leaves. Though only the tip of flowers looks damaged, it won’t set a nut.

Katydids eat at night and hide in the Macadamia leaves during the day. If you rustle the branches, they will fly out.


Weed control around the trees will help. Damage is usually not serious enough to warrant spraying but Malathion is effective. Follow the label.

Problems Seen on Nuts and Kernels

Premature nut drop

This is a natural phenomenon of almost all fruit and nut trees that set more than can be ripened. Most including Macadamia have the tendency to drop fruits at almost any stage of development. Soon after nut set is the most common time for premature nut drop, these nuts are uniform size generally pea size or smaller. In Hawaii, this is from March through June. In temperate fruit and nut growing areas of the Northern Hemisphere, it occurs in June; hence the term ‘June drop’.

Nuts dropped prematurely so close to harvest that they appear normal are usually harvested. During processing these premature kernels called ''immature'' or ''shriveled nuts'' are discarded. They may be wrinkled, spongy, or discolored. These are poor quality; they sink in water. Low oil and high sugar content make the kernels dark colored when roasted.

Many different factors influence how many nuts are dropped. Drought, cold, heat, waterlogging, cloudiness, nutritional imbalance, injuries, and disease and pest infestation can increase premature drop. Simply by dropping nuts, each tree regulates the size of the crop it can bear without damaging the tree, even without any pests or disease present.


Most premature drop is normal. Regularly, check development of the crop. Pest problems such as stink bug and koa seed worm should be controlled if possible. If you suspect that stress, such as poor nutrition or drought is reducing yield try to correct the condition.

Southern green stink bug

This sucking insect, Nezara viridula is a serious pest that attacks young and matures nuts. Symptoms aren’t easily detected until the nuts are dried and cracked. Pits on the kernel are typical signs of stinkbug damage. Damaged kernels might become moldy before harvest. Early heavy infestation may increase premature nut drop.


Remove host plants from the orchard particularly members of the bean family. Plant rattle pod crotolaria on the borders of the orchard. This provides a favorite food for the stink bugs and encourages stinkbug parasites.

However don’t let the rattle pod die in the dry season, Stink bugs will move from the dead rattle pod into the orchard and feed on the nuts, If stink bug parasites are available, releasing them in the border areas and orchard is effective provided this is done before major damage occurs. Macadamia trees can be sprayed with endosulfan or Malathion. Follow the label. See also, Insect Control chapter in 4th Macadamia Industry Analysis, 1987.

Koa seed worm

The caterpillar (larva) of this moth Cryptophlebia illepida and a related species C. ombrodelta, the litchi fruit moth (in Australia called the Macadamia nut borer) can seriously damage husks and kernels, as it eats through the shell or enters through a natural hole (micropyle) in the shell. Damaged kernels may become moldy before harvest. Early infestation may increase premature nut drop. See also Insect control, in 4th Macadamia Industry Analysis, 1987.


Currently, no effective control measures are available. Spraying with endosulfan must be timed accurately with the increasing population. An insect attractant (Isomate—M?) may be registered for use soon. Parasites from other countries are being studied. Preventive measures include using cultivars that have a short harvest season (in your area), and better sanitation, such as removing infested nuts, husks and premature from the orchard.

Macadamia shothole borer

This very small beetle, Hypothenemus obscurus, is a new pest and a potentially serious one in dry areas. The Macadamia shothole borer attacks Macadamia stick—tights in Costa Rica. It eats a small pin-size hole (smaller than the Koa seed worm) through the shell and lays eggs in the kernel. The larvae feed and become adult beetles then eat pinhole size tunnels in the kernel and through the shell. Nuts can be attacked on the ground or during in-shell storage. Stick—tights are frequently infested.

The damaged kernel might become moldy. Little is known of its history in Hawaii.


Sanitation and frequent harvesting is important to prevent MSB attack. Take nuts to processor as soon as possible. Don’t sort and leave MSB infested nuts in the orchard or near nut storage areas, damaged nuts may contain MSB. It would be better to take all harvested nuts to the processor. Just because a nut has MSB hole doesn’t mean the kernel was damaged. If you do sort out damaged nuts either bury them at least 3 feet below ground or burn them. No pesticides are recommended at this time.

Rat and pig damage

Rats can attack nuts left on the ground. They chew quarter to half-inch diameter holes through the shell and eat the kernel completely.

Pigs consume the entire nut, leaving only small pieces of shells, and husks. They’re a problem in areas with large number of wild pigs.


For rats, remove brush piles in orchards and borders; use rat bait throughout the year. Follow the label.

For pigs, keeping dogs is helpful. Electric fencing and trapping may be necessary.


Nuts that don’t drop when mature but hang on until knocked down are called stick—tights. This is caused because a small layer of cells called the abscission zone isn’t produced as the nut matures.

Why some nuts don’t produce an abscission zone is unknown. Drought, environmental stresses even koa seed worm, anthracnose, red and black flat mites have been suggested as possible factors.

‘Kakea’ (508) produces more stick-tights than other cultivars. Despite the stick-tights, 508 have high yields of excellent quality kernels. In areas where Macadamia shothole borer becomes common, stick-tights may lead to higher levels of damage.


Unknown, it doesn’t appear to have an economic effect on yield. If you want to knock ‘em down, do it at end of harvest before they become too old and damaged by the MSB.

Poor postharvest handling

Nuts will spoil or start to germinate if they’re not picked up within a reasonable length of time after dropping. There are several stages and ways that nuts spoil. Late and early season nuts stay on the ground longest before harvest. Every day a nut is on the ground increases the chances that a pig, rat, or shothole borer will attack. Nuts lying on the ground in wet shaded areas will eventually germinate, begin to rot or become moldy. The fungus and bacteria enter the nut through natural hole called the micropyle. Germinating nuts become bitter and are undesirable for processing.

Another common mistake is to store the unhusked nuts in bags or boxes for days even a week, before taking them to the processor. The husks become moldy and the kernels ferment. Although, the kernels don’t look moldy they’ll begin to stink. Moldy and stinking kernels can’t be eaten. Stale kernels may look ok, but they won’t taste good.


Keep spoilage records each year. Each grower must decide when losses resulting from moldy nuts cost more than the extra labor cost of picking up a few nuts. The rule of thumb is pick every 4 weeks in rainy weather and less often in dry weather.

Never store unhusked nuts more than 1 day in bag or box. If you don’t want to take a small load to the processor and the rats and pigs aren’t too bad, it's better to leave the nuts in the orchard. If you’ve picked but can’t go to the processor and can’t husk the nuts, then dry them, Spread the in—husk nuts to dry on a wire or slotted rack out of the rain and direct sun. It's best to immediately husk nuts and air-dry or take them to the processor the next day.

Don’t deliver old unhusked or husked nuts with freshly harvested nuts together, unless the two loads are sampled separately. The poor quality of a single bad bag might ruin the sampling for an entire load. Remember when you lose quality you lose money.

1. From the Hawaii Macadamia Nut Association 29th Annual Meeting Proceedings, May 6, 1989

2. Extension Specialist, Department of Horticulture, College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, 3190 Maile Way, Honolulu, Hawaii 96822

3. Hawaii Extension Agent, University of Hawaii, Komohana Agricultural Complex, 875 Komohana Street, Hilo, Hawaii 96720