Brown rot

What you are looking at

A brown patch appears on an apple, pear or plum and spreads until the whole fruit is brown and soft. Within days, small raised pustules the colour of clotted cream push through the surface, and they are arranged in rings around the point where the rot started. That concentric pattern is the diagnosis.

The fruit then dries rather than falling apart. It shrivels, darkens to blackish-brown, hardens, and stays hanging in the branches through the winter as a mummy.

On cherry, plum and apricot the same group of fungi does something different in spring. Whole trusses of blossom brown and die within a day or two, the dead flowers stay attached, and the spur behind them dies back with a small canker at its base.

The mummies are the disease

Everything about brown rot runs through the fruit that stayed on the tree.

Through winter, a mummy sits a few centimetres from the buds that will open in March. When they do, it produces spores that land straight into the blossom or onto the developing fruitlets. In summer the pustules on any rotting fruit release spores that splash and blow onto neighbours, and the rot moves through a touching cluster from one to the next.

Sound, unbroken skin resists infection well. What lets the fungus in is a wound. A wasp hole, a bird peck, a codling moth entry, a crack over an apple scab and a bruise from careless picking will all do it, which makes brown rot the disease that follows other problems, and it is why controlling wasps and moths pays off twice.

None of this is reached by anything sold to a gardener. There is no amateur fungicide for brown rot in Britain. What there is instead is an hour in January with a pair of secateurs and a bucket, which works.

What to do about it

Go over the tree in December or January and pick off every mummified fruit, including the ones lodged in a fork or lying in the grass. Bin or burn them and keep them out of the compost.

Prune out spurs and shoots that died back at blossom time, cutting 10 to 15 cm into sound wood. On plums and cherries do that in summer, never in winter.

Thin the fruitlets in June so no two touch.

Pick off any fruit that starts to rot, twice a week through late summer, and carry it away from the tree.

Collect windfalls weekly.

Pick the crop gently into a lined container and store only unmarked fruit, spaced out so nothing touches.

Deal with wasps and codling moth, since most infections start in the holes they make.

What it gets mistaken for

Codling moth leaves a round entry hole plugged with dry brown crumbs and a tunnel to the core. Brown rot has no tunnel, and the two are found together because the hole is how the fungus got in.

Apple scab makes dry corky patches that crack the skin over sound firm flesh.

Grey mould covers a fruit in a loose grey fuzz that releases a cloud of spores when knocked, and its growth is not arranged in rings.

Bitter rot and other storage rots produce a sunken brown patch without the ringed pustules. If in doubt, the rings settle it.

Bird and wasp damage leaves open ragged craters with the flesh eaten away rather than rotted, and brown rot usually follows into them a few days later.