Macadamia

Description

Macadamia trees belong to the genus Macadamia which contains four species of tree or shrub in the family Proteaceae which are grown for their edible seeds (nuts). Cultivated macadamia is a derivative of the subtropical species' Macadamia integrifoia and Macadamia tetraphylla and their hybrids. Macadamia plants naturally have multiple stems but are trained to a central leader system (single stem) in plantations. The leaves of the trees are oval or oblong and are arranged in whorls of 3 or 4 depending on variety. The tree produces creamy white or pink-red flowers on racemes of 100–300 flowers and a rounded fleshy fruit up to 27 mm (1 in) in diameter. The flesh covers a single spherical or elliptical seed (nut) with a white or gray kernel depending on the variety of tree. Macadamia can reach a height of 20 m (65.6 ft) and have a commercial lifespan of 40–60 years. Macadamia may also be referred to as Queensland nut or Australian nut and originates from Australia.


Uses

The kernels of the macadamia nut are mainly consumed as a snack food. The kernels from M. integrifolia and M. tetraphylla can be eaten raw from the shell while the remaining Macadamia species contain toxins which must be removed prior to consumption. Macadamia may also be used in baked goods and confectionery.


Propagation


Basic requirements
Macadamia trees originate from the sub-tropical rainforests of Australia and therefore tend to grow best in areas of high humidity and high rainfall. They will grow optimally at average annual temperatures between 20 and 25°C (68–77°F) in a deep, well draining loam and sandy loam with a pH between 5.0 and 5.5. Trees should not be planted in heavy clays. It is recommended that macadamia only be planted in frost free areas, young trees will be killed by temperatures of -6°C (21°F) although older, established trees can tolerate some light frost.

Transplanting
When planting numerous macadamia trees in a plantation, consideration must be given to drainage, irrigation, row alignment and wind protection. Planting distances tend to be site specific and are dependent on factors such as the macadamia variety, soil and rainfall. In general, macadamia trees are planted 4 to 8 m (13-26 ft) apart within a row, allowing 7 to 11 m (23-36 ft) between rows. Young trees are planted in Spring and Fall by digging holes which are 45 cm deep and 60 cm in diameter. Once the tree is in position, the hole is backfilled with soil. it is recommended that some compost and rock phosphate be added to the backfill at time of planting. The soil around the tree should then be gently tamped and the tree watered. Staking will help to protect the young trees from wind damage. Grafted trees usually begin to produce fruit 3 years after planting.

General care and maintenance
During the first four years after planting, animal manure and a thick layer of organic mulch can be applied around the trees year round. after this period, the amount of manure added should be decreased to prevent the trees being provided with too much nitrogen and being deficient in potassium. In the first 4 years following planting, vegetative growth should be stimulated through pruning. Once the leading shoot has reached a height of approximately 80 cm (31.5 in) it should be clipped to stimulate branching. Established trees are usually pruned in the Spring when old, unproductive and diseased branches are removed from the canopy. Any nuts remaining on the tree should also be removed. Mulches are best applied after nut harvest to allow the material to decompose before the next crop. Avoid piling mulch up against the trunks to prevent disease.

Harvesting
Grafted trees will begin to produce a crop 3 years after planting but it is not until the 7th year that the crop will be of any economic significance. Trees will give maximum yields 15 to 20 years after planting. Nuts should be collected when they fall from the tree and this should be done every 1-2 weeks during the nut producing periods. In commercial plantations, nuts can be collected mechanically or by hand.


References

Augstburger, F., Berger, J., Censkowsky, U., Heid, P., Milz, J. & Streit, C. (2000). Macadamia nuts. Naturland e.V. Available at: http://www.naturland.de/fileadmin/MDB.... [Accessed 16 February 15]. Free to access.

CABI Crop Protection Compendium. (2014). Macadamia integrifolia (macadamia nut) datasheet. Available at: http://www.cabi.org/cpc/datasheet/32791. [Accessed 16 February 15]. Paid subscription required.

Cooke, T., Persley, D. & House, S. (2009). Diseases of Fruit Crops in Australia. CSIRO Publishing Ltd. Preview available at: http://books.google.com/books?id=GepY.... [Accessed 16 February 15]. Free to access.

Duke, J.A. (1987). Macadamia integrifolia Maiden & Betche, Macadamia tetraphylla L. Johnson. In Handbook of Energy Crops. unpublished. Available at: http://www.hort.purdue.edu/newcrop/du.... [Accessed 16 February 15]. Free to access.

Teviotdale, B. L., Michailides, T. J. & Pscheidt, J. W. (eds) (2002). Compendium of Nut Crop Diseases in Temperate Zones. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopap.... Available for purchase from APS Press.

Quinlan, K. (2007). Organic macadamia growing. NSW Department of Primary Industries. Available at: http://www.dpi.nsw.gov.au/__data/asse.... [Accessed 16 February 15]. Free to access.


Common Pests and Diseases

Slow and quick tree decline
Xylaria arbuscula
Phellinus gilvus
Acremonium recifei
Nectria rugulosa

Symptoms
Yellowing or browning of some leaves in the tree canopy; subtle change in color of tree canopy from dark to light green; entire tree turning brown; in final stages of disease, whole tree is brown and distinct from green trees around it; tree death may occur in a few months after initial symptoms have appeared
Cause
FungI
Comments
Management
Remove any dead or diseased trees from the orchard; avoid wounding tree trunks

Anthracnose
Colletotrichum gloeosporioides

Symptoms
Black lesions on leaves and fruit; soft black lesions on husks of nuts followed by decay of nuts on the ground; senescent mature nuts may remain attached to the tree; shells of infected husks may turn brown-gray in color but the kernel inside remains unaffected
Cause
Fungus
Comments
Disease emergence favors high humidity; fungus survives on plant debris; spores are spread by splashing water
Management
Avoid stressing trees by providing them with adequate irrigation and fertilization; prune out dead or dying limbs from tree canopy to reduce levels of disease inoculum; control insect pests to prevent stress to trees

Raceme blight
Botrytis cinerea

Symptoms
Small brown spots on flower petals which spread to racemes (flower stalks); infection may affect small flower buds, florets and rachis; racemes turning black and dying; entire raceme may become blighted within a few days; necrotic flowers remain on tree and become covered in fuzzy gray fungal growth
Cause
Fungus
Comments
Disease emergence favors wet weather and cool temperatures
Management
Fungicide application is not usually warranted unless infection occurs during wet weather which can cause severe infections

Husk spot
Pseudocercospora macadamiae

Symptoms
Initial symptoms of the disease are chlorotic to yellow flecks on the husks which enlarge and develop tan brown centers; lesions coalesce and develop diffuse bright yellow halos; lesions may be covered in powdery gray spore masses during periods of wet weather; lesions mature into tough, woody spots; circular tan spots may develop inside the husk but shells and kernels remain unaffected
Cause
Fungus
Comments
Yield losses may result in susceptible varieties due to nuts dropping from tree prematurely; spores are spread by water splash from infected husks; diseased husks may produce spores for up to 2 years following infection
Management
If variety of macadamia being grown is susceptible to the disease then an appropriate protective fungicide should be applied to the fruits when they are approximately the size of a match head; remove and old and diseased husks from the tree to reduce inoculum levels

Macadamia nut borer
Cryptophlebia ombrodelta

Symptoms
Entry holes in husks of nuts, usually close to the panicle; holes usually have insect frass protruding from them and husks may have shallow, dimple-like depressions; infested nuts drop from tree prematurely; insect eggs are scale-like and are laid on the surfaces of green husks; larvae are legless grubs which are pinkish in color when mature with dark green spots; adult insect is a red-brown moth
Cause
Insect
Comments
Larvae pupate in damaged nuts; entire lifecycle of the moth is completed in approximately 5 weeks
Management
Control of macadamia nut borer can be difficult as insecticides are ineffective once the larvae have entered the nuts; appropriate insecticides should be applied if gren fallen nuts exhibit damage or live, unparasitized eggs are present on the tre; old nuts should be removed from tree and the orchard ground at harvest

Tropical nut borer
Hypothenemus obscurus

Symptoms
Numerous round holes approximately 0.5 mm in diameter on husks; extensive tunnelling through husk and shell visible when nut is cut open; in late stage infestations the entire kernel may be consumed
Cause
Insect
Comments
Economic damage is usually incurred if the nuts remain on the ground for a period of three weeks or more before harvest
Management
Control of the tropical nut borer relies on the use of resistant macadamia varieties and cultural control methods including harvesting nuts at three week intervals immediately removing husks, avoiding varieties prone to sticktight and removing damaged nuts from the orchard; applications of appropriate insecticides may be necessary

Phytophthora trunk and stem canker
Phytophthora cinnamomi

Symptoms
Nursery trees and young plantations trees are stunted and chlorotic and may be partially defoliated; if lesions girdle the stem then the tree will die; in mature trees, dark discoloration of wood is visible by removing the bark from the trunk of the tree; dark cankers may extend from trunk at soil line to the lower branches of the tree
Cause
Oomycete
Comments
Fungus survives in plant debris in soil; spread by water splash
Management
Plant only disease-free nursery stock; plant trees in well-draining soils on a slight mound to promote drainage and prevent water-logging; avoid wounds to the trunk of the tree which allow fungi to enter; incorporate organic matter into the soil around the tree; apply appropriate protective fungicides to tree trunks prior to wet season;

Rats
Rattus rattus
Rattus norvegicus
Rattus fuscipes

Symptoms
Extensive loss and damage of developing nuts or nuts that have dropped from tree; nuts have holes approximately 1 cm in diameter gnawed through shell to gain access to kernels
Cause
Rodents
Comments
Encourage predatory birds such as owls by providing nesting boxes in the trees
Management
All food sources for rats should be removed from the orchard, this includes old nuts, nut waste and wild tobacco; remove any rat nests from trees; keep grass around trees mown short and, if possible, maintain a clear mown area around the orchard to deter rats from entering and to aid predators such as owls in detecting and hunting rats