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Two of the problems on this list can kill a limb or the whole tree, and they need a response even in a year when you do not care about the fruit. The rest cost you part of a crop and are managed by routine rather than by intervention. Knowing which is which stops you spraying for things that do not matter and ignoring the two that do.
The pattern across almost all of them is the same, and it is worth stating once. Fruit hanging for weeks in autumn conditions is fruit that gets wet repeatedly. Wet fruit rots. Rotting fruit is a spore source that infects sound fruit next to it, and fruit that falls and is left on the ground is a reservoir that carries the problem into the following year. Almost every control that actually works is a form of removing fruit from the ground and from the canopy.
| Problem | What you see | Threat level | First response |
|---|---|---|---|
| Brown rot | Brown patch spreading on fruit, rings of buff pustules, fruit shriveling to a black mummy | Crop, and next year's crop | Remove every affected fruit from tree and ground, twice a week |
| Split fruit | Fruit cracked along the seam after rain, flesh exposed | Crop only | Pick and use immediately; it becomes brown rot within days |
| Silver leaf | Metallic sheen on leaves of one branch, dieback, dark stain in cut wood | The tree | Cut the limb out in summer, well below the staining |
| Bacterial canker | Sunken patch on a limb, amber gum oozing, shoots dying above it | The tree | Cut back to clean wood in summer; do not cut in winter |
| Wasps | Ripe fruit hollowed out, wasps working the tree | Crop only | Remove split and rotting fruit; they are the attractant |
| Plum sawfly | Early fruit drop with a sticky black hole and a grub inside | Part of the crop | Collect and destroy dropped fruit in early summer |
Brown rot is the defining problem of a long hang
Brown rot begins as a small soft brown patch on the fruit, usually where the skin has been damaged by a split, a wasp, a bird peck or simple bruising from touching a neighbor. It spreads outwards, and within a couple of days concentric rings of buff-colored pustules appear on the brown area. Those pustules are producing spores. The fruit then shrivels and hardens into a black mummy which may hang on the tree right through winter, and each mummy is a source of infection for the following spring.
This is a worse problem for a prune than for a dessert plum for one simple reason: the fruit is on the tree for several more weeks, in later and wetter conditions, at a stage when the sugar-rich flesh is exactly what the fungus wants. A dessert plum crop is off the tree before the worst of it. A prune crop sits through it, which is why the routine below is not optional maintenance but the main work of the season.
There is no spray that will save a crop once brown rot is running through a wet canopy, and there is no need for one if the routine is kept up. Removal works because the fungus spreads fruit to fruit over short distances. Break that chain twice a week and it never builds momentum.
- Walk the tree twice a week from first color Look for any fruit with a brown patch, and take it off with its stalk. Ten minutes, twice a week, from color through to the end of picking. This single habit is worth more than everything else combined.
- Clear every fallen fruit Fruit on the ground under the canopy is a spore reservoir and a wasp magnet. Pick it up at the same time you go over the tree. Keep the grass short so you can see it.
- Take infected fruit out of the garden Do not drop it at the base of the tree and do not put it in an open potting mix heap. Bin it, burn it, or bury it deep. Fruit left anywhere nearby carries the infection into next spring.
- Strip the mummies at the summer pruning Every shriveled black fruit still hanging in the canopy is next year's starting inoculum. Pick them all off when you prune, and again at the end of the season if any are left.
- Prune for air, every summer An open canopy dries after rain in hours rather than overnight. Wet hours are what the fungus needs, so cutting for air movement is a direct control measure, not just a shaping job.
Split fruit, and the rain that causes it
Splitting is not a disease and there is nothing wrong with the tree when it happens. Late in the hang the skin of the fruit has stopped expanding while the flesh is at its most concentrated. Heavy rain, or heavy watering, sends water up through the roots and into the fruit faster than the skin can accommodate, and it cracks, generally along the natural seam. It affects the ripest fruit first, which is to say the fruit you most wanted.
You cannot control the rain, but you can control everything else about the fruit's water status. Keep watering even and moderate through the growing period rather than swinging between dry and soaked, and taper it off deliberately as the fruit colors. A tree kept steadily on the dry side through the final weeks takes a downpour far better than one that was already full of water when the rain arrived. Mulch, which evens out soil moisture, does more for splitting than any other single measure.
The other half of the answer is the calendar. Watch the forecast once the fruit is close, and when a wet spell is coming, pick everything that is anywhere near ready rather than gambling on another week of sugar. Fruit that has already split should come off the tree the same day you see it: it will not dry, it will rot within about two days, and while it hangs there it is infecting its neighbors. Cook it or freeze it straight away and it is not a loss at all.
Silver leaf and canker: the two that can kill the tree
Silver leaf shows as a metallic, silvered sheen on the leaves of a single branch or one side of the tree, usually appearing a season or more after the infection took hold. The branch then dies back, and if you cut across the dead wood you find a dark stain running through the center. The fungus enters through wounds and its spores are abroad in cool damp weather, which is precisely why dormant-season pruning on these trees is a mistake. Cut affected limbs out in summer, taking them back well below the visible staining into clean wood, and remove the material from the garden.
Bacterial canker appears as a sunken, discolored patch on a limb or the trunk, often with amber gum oozing from it, and the shoots above it die back. On leaves it can show first as small round spots that dry and drop out, leaving a shot-hole appearance. As with silver leaf, cut back to clean wood in the growing season and never in winter, and sterilize the blade afterwards.
For both, the underlying defense is the same. Keep wounds to a minimum and make them in summer. Do not leave stubs, which die back and become entry points. Keep mulch and long grass off the trunk so bark stays dry. And avoid mechanical damage entirely, especially from string trimmers at the base of the trunk, which is where a surprising number of canker infections start.
Some things on a prune tree are not worth treating. A scattering of shot-holes in the leaves, a little gum at an old pruning cut, or some leaf yellowing and drop late in the season after picking are all normal and need no response. Aphid curling on the tips of young shoots in spring looks alarming and is almost always cleared up by predators within a few weeks.
If a whole young tree silvers rather than one branch, the infection is in the trunk and the tree will not recover. Take it out and burn or remove the wood rather than leaving the stump, and do not replant a plum in the same spot for several years.
Wasps, birds and the record that tells you what actually cost you
Wasps are drawn by sugar and by damage, not by sound fruit. They almost never break intact skin first; they arrive at fruit already split, pecked or rotting, and then they enlarge the hole until the fruit is hollow. Because a prune hangs so long and so sweet, the tree is attractive to them for weeks. The control is the same removal routine as for brown rot: take off every split and rotting fruit twice a week and the wasps have far less to work with. Hanging sweet traps near the tree tends to bring more wasps into the area rather than fewer.
Birds go for conspicuously ripe fruit, and a tree carrying dark fruit for a month is conspicuous. Netting works but only if it is held out on a frame clear of the fruit, because anything pressed against the mesh bruises and then rots. On a large tree, netting is usually not worth the effort, and it is better to accept a share and concentrate on picking promptly.
Keep a short failure log each autumn and it will change what you do the following year. Write down roughly what proportion of the crop you lost, to what, and when. Most gardeners are convinced their losses are to wasps and birds; the log usually shows that far more went to splitting after a specific rain and to brown rot that spread from fruit nobody had time to pick off that week.
Note the first date you saw brown rot, and where in the tree. Year after year it tends to start in the same place, which is the shaded, still part of the canopy that dries slowest, and that consistent starting point is the clearest possible instruction about which branch to remove at the next summer pruning.
Record the date of any rain event during the picking weeks alongside how much fruit split in the following days. After two or three seasons you will know exactly how much rain your fruit can take at that stage, and the decision to strip the tree ahead of a forecast becomes straightforward instead of agonising.