Plum Tree Problems: Silver Leaf, Brown Rot, Sawfly and Split Fruit

Sort problems by whether they threaten the tree or the crop

It is worth being clear about which category a problem falls into, because the response differs completely. Silver leaf and bacterial canker attack the wood and can kill the tree, so they justify cutting out large limbs and losing several years of crop from that side. Brown rot, sawfly and splitting cost you fruit but leave the tree standing, and are managed by hygiene and timing rather than surgery.

The other useful distinction is between what you can still fix this season and what you are too late for. Sawfly, once the larvae are inside the fruit, is finished business for that crop and your action aims at next year. Brown rot spreading through a tree is live and urgent, and an hour spent removing infected fruit today saves half the crop.

Silver leaf: the reason you never prune in winter

Silver leaf is the disease that shapes the whole plum calendar. The fungus enters through wounds, colonises the wood, and produces a toxin that makes the leaves on affected branches look metallic and silvery. The silvering shows up on one limb first, then spreads. Leaves on that limb fall early, the branch dies back, and over a few seasons the infection reaches the trunk.

The diagnostic test is in the wood, not the leaves, because several kinds of stress produce silvery-looking foliage. Cut through a suspect branch. Genuine silver leaf leaves a dark brown or purplish stain in the center of the cross section. No stain means no silver leaf, and you are looking at something else, usually drought or root damage.

There is no cure. Cut affected limbs back to at least 6 inches (15 cm) beyond the last visible internal staining, checking each cut face as you go, and burn or remove the wood entirely. Do this in summer, and never stack infected plum wood in the garden: the fungus fruits on dead wood and sends spores from the log pile straight back into the tree.

Prevention is the whole game. Prune only in late spring and summer, keep cuts small, and deal with bark wounds and torn branches promptly rather than leaving them open through winter.

Trees under stress from drought or waterlogging are markedly more susceptible. Consistent watering in the first years and a mulched, grass-free circle at the base do more for silver leaf resistance than anything you can spray.

Brown rot spreads from the fruit you did not pick off

Brown rot starts as a small brown patch on one fruit, usually where the skin was broken by a split, a bird peck or a sawfly hole. The patch spreads within days, and concentric rings of buff-gray pustules appear. Those pustules are spores, and they travel to any fruit touching the infected one, then to fruit nearby in wet weather.

The fruit then dries and shrivels on the branch into a hard, dark mummy, and this is the part that matters. That mummy does not fall. It hangs on through winter and releases a fresh wave of spores in spring, directly onto the blossom of the same tree. One mummified fruit left on a branch restarts the disease the following year. Brown rot does not come back on its own; it never left.

The disease can also move from the fruit back into the wood through the stalk, killing the spur and sometimes producing blossom wilt, where flowers and the shoot behind them brown and die suddenly in spring. Control is entirely mechanical and entirely about removal; there is no useful spray for a garden plum.

1. Remove infected fruit the day you see it Pick off any fruit with a spreading brown patch, along with anything touching it. Put it in the bin or burn it. Do not compost it and do not drop it on the ground under the tree.
2. Clear fallen fruit weekly through the season Fruit on the ground under the tree is a spore source and also feeds the next generation of sawfly. A weekly walk round with a bucket during the fruiting season is the single highest value job in the plum year.
3. Strip every mummy in autumn After leaf fall, when the branches are bare, the mummified fruit is obvious. Go over the whole tree and pull off every one. This is the reason for keeping the tree at a height you can reach: an unreachable canopy full of mummies reinfects itself indefinitely.
4. Thin so fruit does not touch Spacing fruit 3 inches (8 cm) apart means one infected fruit cannot pass the disease on by contact. Touching fruit in a cluster is how a single rot becomes a whole branch.
5. Handle fruit gently at picking Bruises and skin breaks are entry points. Fruit picked carefully and stored in a single layer rots far less in the kitchen as well as on the tree.

Plum sawfly, and telling it apart from moth damage

The adult sawfly lays its eggs in the plum blossom. The larvae hatch as the fruitlets form and tunnel straight into them. From outside, the first sign is a small hole in the skin of a young fruit with a sticky black wet mass of frass around it, often with a smell. Affected fruit drops in early summer, well before ripening, which is why a heavy sawfly year looks like an unusually severe natural drop.

Cut a dropped fruit open and you will find a tunnel running towards the stone, sometimes with a pale larva with a dark head. This distinguishes sawfly from the fruit moth larvae that arrive later. Sawfly damage is early, wet and black around the entry hole, with fruit falling before it colors; moth damage is later, drier, and found in full sized fruit.

The larvae leave the fallen fruit and go into the soil under the tree, where they overwinter and emerge as adults the following spring. That life cycle is the weak point. Collecting and destroying fallen fruit promptly, before the larvae have left it, removes that generation, and lightly cultivating the top inch or two (3 to 5 cm) of soil under the canopy in late autumn exposes the overwintering stage to birds and cold. Sawfly is managed across years rather than solved in one: clear the fallen fruit and the problem shrinks noticeably by the second season.

Canker, splitting, and the things not worth worrying about

Bacterial canker shows as sunken, dead patches of bark, often with amber gum oozing out, and shoots above the canker die back in spring while the rest of the tree leafs out normally. Leaves may develop small holes as dead spots fall out. As with silver leaf, cut affected wood out in summer, back into clean tissue. Gum on its own, without sunken bark or dieback, is usually just a response to injury and is not a diagnosis.

Split fruit is a water problem, not a disease. Dry soil during fruit swelling followed by heavy rain near ripeness is the classic sequence: the skin has finished growing, the flesh takes on water, and it bursts. Consistent moisture is the answer, meaning a good mulch and a thorough soak in dry spells. Use split fruit quickly, because it goes to brown rot fast.

Aphids curl the growing tips in spring and leave leaves sticky, with ants running up the trunk. On an established plum this costs almost nothing and predators clear it within a few weeks. Spraying near blossom does more damage than the aphids, by removing the insects you need for pollination. On a newly planted tree, wash them off with water or squash them by hand.

Keep a note each season of what you found and when: the date the first brown rot appeared, the date sawfly-damaged fruit started dropping, and how many mummies you pulled off in autumn. That mummy count is a good year-on-year score of how well the hygiene is working, and it falls sharply once you have done a thorough autumn strip twice.

Any plum branch you cut out for disease should leave the garden. Burn it or take it away. A plum log pile within reach of the tree is a spore factory for both silver leaf and brown rot.