Armillaria root rot

Its leaves turn pale and yellow, the canopy thins out, new growth is stunted and small, and branches begin dying back from the tips. Some trees fade slowly over a season or two; others suddenly wilt and collapse in a dry spell with the dull, dead leaves still hanging on. Because the fungus travels root-to-root, the dying trees form a patch or gap that creeps down the row — that spreading patch is the biggest giveaway.

Scrape the soil away from the root collar (the base of the trunk where it meets the ground) and peel back a little bark: you will find a flat, creamy-white sheet of fungus (called mycelium) shaped like a fan or a piece of wet leather, and it smells strongly of mushrooms. No other rot gives you this white leathery fan.

Rake the topsoil around the base and over the main roots and look for black or dark-brown strands like shoelaces or string running through the soil and across the roots. These are the fungus's travelling cords (called rhizomorphs — the fungal 'ropes' it uses to reach new roots). Snap one open and it is white inside.

In wet weather, clumps of honey-coloured or tan toadstools — the fungus's fruiting bodies, where it makes its spores — sprout at the base of a dying or dead tree, often with a small ring around each stalk.

drought or waterlogging, which wilts and yellows many trees evenly across a block rather than in a creeping patch, and gives no white fan and no black bootlaces (always dig and check); other collar and root rots, which can ooze dark, wet lesions but lack the white leathery fan, black bootlaces and honey mushrooms together; and termites or borers, which leave tunnels, mud sheeting or sawdust, not a white fungal fan. If you find the white fan under the bark and the black bootlaces, you are sure it is Armillaria.

Walk the orchard at least once a month, and more often right after heavy rain and through the wet season, when the fungus is most active in the moist soil and the honey mushrooms come out. Check any block you cleared from bush, forest or an old orchard most often — that ground carries the highest risk.

so you pass through every part, not just the edge by the path. Look along the rows for any tree that stands out as sicker than its neighbours — pale, thin, dying back — or a gap where a tree has already died. Give extra attention to trees next to a stump, an old dead tree, or a spot where a tree died before, because that is the direction the disease travels.

scrape the soil off the root collar and peel a little bark to look for the white fungal fan and the mushroom smell; rake the topsoil around the base and over the main roots for the black bootlace strands; and after rain look for honey mushrooms clumped at the base.

You do not wait for a percentage of trees. The moment a single tree wilts or declines for no clear reason, dig at its collar that same week, and if you find the white fans or black bootlaces, act on that one tree straight away. Waiting for a second and third tree to show only means the fungus has already reached their roots underground.

Tie a ribbon or drive a painted stake at every confirmed tree, and on your next walk check its neighbours on all sides. The disease moves tree-to-tree along touching roots, so the healthy-looking neighbours are exactly where it strikes next.

Dig out and burn old stumps, big dead roots and buried wood before you establish an orchard. That rotting wood is the fungus's food store; leave it in the ground and you feed the disease for years. This single step does more than anything else you can do.

— that soil is full of infected roots. If you have no other land, clear every scrap of wood first and, better still, rest the land or grow an annual crop on it for a few years so the old roots rot away before you plant trees.

The fungus hits stressed, wet, injured trees hardest, so give the orchard good drainage (never let water pond around the trunk base), mulch and feed the trees, and avoid nicking the roots and collar with hoes or machinery — every wound is a doorway for the fungus.

Where the disease has a history, plant a tolerant rootstock (the root part your fruit tree is grafted onto) if one is available for your crop, and avoid the most susceptible species on known-infested land. Ask your extension officer or nursery which rootstock stands up best locally.

Grub out the whole infected tree, roots and all, and burn it; lift as much of the big roots and collar as you can. Never just cut it off at ground level — the stump and roots left behind keep the fungus alive and spreading to the next tree.

Dig a barrier trench about 50–60 cm deep (deeper than most feeder roots) between the sick patch and the healthy trees. This breaks the root-to-root contact and the black bootlace strands the fungus travels along. Keep the trench open.

Because the fungus hides deep in the roots and dead wood, there is no fungicide spray or soil drench that reliably saves an infected tree — digging it out is the only real cure. Burn the wood you remove (don't stack it near living trees), and knock the soil off your tools after working an infected spot so you don't carry bootlace strands to a clean block.

If trees keep dying down the row despite removal, the fungus owns that ground. Stop fighting it one tree at a time: take out the whole affected patch, dig out every piece of root, and rest the land — fallow it (grow nothing, or only a short annual or grass crop) for several years to starve the fungus. When you replant, shift the new trees off the old infected line and use a tolerant rootstock; on badly infested ground, think about switching that block to a non-woody crop rather than losing tree after tree.