Macadamia Tree Problems: Premature Nut Drop, Husk Spot, Rats and the Risk to Dogs

Start below ground

When a macadamia declines slowly, thins at the top, produces small pale leaves and drops its crop early, the cause is usually in the root zone rather than anything you can see on the branches. The two usual culprits are physical damage from digging, trenching or cultivating under the canopy, and root rot in soil that stays wet.

Root rot shows as a general dullness in the foliage, dieback starting at the outer tips, and sometimes a dark stain in the bark at the base of the trunk. Scrape the bark just above soil level with a knife: healthy tissue underneath is pale green and moist, while rotted tissue is brown and smells sour.

There is no spray that fixes this. The response is to improve drainage, stop watering, pull the mulch back from the trunk so the collar dries, and cut nothing until you can see which wood is genuinely dead. A tree that has lost part of its root system also needs its top reduced, because it can no longer support the canopy it has.

Sucking bugs and premature nut drop

Some early drop is normal: a macadamia sets far more nuts than it can carry and sheds the surplus within weeks of flowering. What is not normal is a steady rain of small immature nuts through the filling period. That is almost always stink bugs or fruitspotting bugs feeding through the young shell.

The evidence is on the nut, not the tree. Husk a few of the fallen ones and look for tiny dark puncture points on the shell and a matching sunken brown spot on the kernel underneath. Shield-shaped bugs on the racemes at dusk confirm it.

Nut borer damage looks different: a neat round hole through the husk and shell, and a tunneled kernel with frass. Both problems are managed the same way at garden scale.

1. Gather and destroy every fallen nut Larvae complete their life cycle in nuts left lying on the ground. Weekly collection through the season does more to break the cycle than any spray.
2. Check the racemes at dusk Sucking bugs feed in the cool of evening. Look on the underside of young nut clusters with a torch and pick them off into soapy water.
3. Clear alternative hosts nearby Fruitspotting bugs breed on other soft-fruited trees and shrubs. Removing neglected fruit and heavy weed cover next to the tree cuts the pressure noticeably.
4. Spray only if you must, and not at flowering Any insecticide applied while the racemes are open kills the bees doing your pollination. If a treatment is unavoidable, apply it after petal fall, in the evening.

Husk spot, anthracnose and the fungal problems

In humid weather you will see circular yellow blotches on the green husks that darken and become sunken. Affected nuts drop early with the husk still gripping the shell, and the husks tend to hang on the tree afterwards. Those clinging husks carry the fungus through to next season.

Anthracnose shows as black sunken lesions on husks and dieback on twigs, again after long wet spells. Both respond to the same housekeeping: pick off and bin hanging husks after harvest, gather fallen nuts promptly, keep the skirt of the tree clear of the ground so air moves underneath, and avoid overhead watering.

Do not compost infected husks in a heap that goes back under the tree. Put them out with the rubbish or burn them where that is allowed.

Rats, squirrels and other nut thieves

Rats are the most persistent problem for a home macadamia. They gnaw a neat round hole in the shell and clean out the kernel, and they will work a tree nightly through the whole drop season. Squirrels and cockatoos take whole nuts, often before they are ripe.

The defenses are physical. Keep the lowest branches two feet (60 cm) clear of the ground and cut back anything that bridges from a fence, shed roof or neighboring tree, so the only way up is the open trunk. Collect fallen nuts weekly rather than leaving a food supply on the ground, and store the drying nuts in a sealed metal or rigid plastic container, never a sack in an open shed.

A smooth metal band fitted around the trunk stops rats climbing on an isolated tree, but only if no branch touches anything else. Check that condition again after every storm.

Raw macadamias are toxic to dogs

This is worth stating plainly because a fruiting macadamia puts food on the ground where a dog will find it. Macadamia nuts poison dogs. A dog that eats them, raw or roasted, in or out of the shell, can show weakness, vomiting, tremors, an unsteady or collapsing back end and a raised temperature within about twelve hours.

It is rarely fatal and most dogs recover within a day or two, but they are genuinely unwell and it warrants a call to a vet, especially for a small dog or where a large quantity was eaten. Take the shells into account too, which are a choking and obstruction risk in their own right.

In practice: fence the dog out of the tree during the drop season or sweep the ground every day, keep drying trays off the floor and out of reach, and dispose of cracking waste in a closed bin rather than the garden.

The nuts are a problem for dogs specifically. They are not a known hazard to cats, and they are safe for people, so this is about where the fallen crop and the shell waste end up rather than about growing the tree at all.

Keeping a problem log

Note the date you first see early nut drop each year, along with the proportion of fallen nuts showing puncture marks or borer holes. Bug pressure builds over several seasons and a dated record shows the trend long before the crop makes it obvious.

Record any wet spell longer than a few days during the season, and whether husk spot followed. Once you have seen the pattern twice, you will know which years to gather more often rather than reacting after the damage is done.