Can You Eat Plums from a Tree with Black Knot?

Yes, you can eat plums from a tree infected with black knot, provided the fruit itself looks healthy and shows no signs of mold, rot, or unusual discoloration. The fungus that causes black knot ( Apiosporina morbosa ) affects only the woody parts of the tree, not the fruit directly, so plums from an infected tree are generally safe to eat as long as they appear normal.

However, there are important factors to consider. A tree stressed by black knot may produce fewer plums, and the fruit may be smaller or less sweet. More importantly, eating plums from a heavily infected tree means you should inspect each fruit carefully for secondary rot or insect damage that can occur when the tree is weakened. In this article, you will learn exactly what black knot is, how it affects your plum tree and its fruit, and what you should do to keep both your tree and your harvest safe.

What Is Black Knot and How Does It Affect Plum Trees?

Black knot is a fungal disease that attacks trees in the Prunus genus, which includes plums, cherries, peaches, and apricots. The fungus enters through wounds or natural openings in the bark and causes swollen, black, corky growths called galls to form on branches and twigs.

These galls start as small greenish swellings in spring and gradually turn black and hard over the growing season. If left untreated, they can girdle branches and kill them. The disease spreads through spores released during wet weather in spring.

How does the fungus affect the tree?

It blocks the flow of water and nutrients through infected branches.
It weakens the tree overall, making it more vulnerable to other pests and diseases.
It can reduce fruit production, sometimes severely if major branches are affected.

The fungus lives in the galls and produces spores for several years. Rain and wind carry the spores to healthy branches, starting new infections. This is why removing infected wood is critical.

Is It Safe to Eat Plums from an Infected Tree?

The short answer is yes, plums from a tree with black knot are safe to eat as long as they look and feel normal. The fungus does not live inside the plum fruit. It only grows in the bark and wood of the tree. There is no evidence that eating a plum from a knot-infected tree poses any health risk to humans.

That said, there are a few important guidelines to follow:

Inspect every plum before eating. A tree weakened by black knot may have fruit that is small, misshapen, or prone to rot. Discard any fruit with soft spots, mold, or cracks.
Wash the fruit thoroughly. This removes any spores, dirt, or debris that may have landed on the skin.
Taste test a small piece first. If the fruit tastes off, bitter, or fermented, do not eat it. This is not caused by black knot itself but by secondary rot or insects.

What about plums that fall to the ground?

Plums that fall prematurely from a black-knot-infected tree are more likely to be damaged or diseased. Do not eat ground-fallen fruit unless you are certain it is fresh and intact. Typically, it is safer to harvest only from the tree.

What Do Infected Plums Look Like?

Plums from a tree with black knot often look normal, but there are some common changes to watch for.

Smaller size: The tree diverts energy to fighting the infection, so fruit may not reach full size.
Fewer plums: A heavily infected tree may produce a much smaller harvest.
Slight skin blemishes: Some plums may have minor spots or uneven coloring, which is usually cosmetic.
Premature dropping: Fruit may fall early, especially if the tree is stressed.

If you see plums that are shriveled, rotting, covered in mold, or obviously insect-damaged, remove and discard them. These are signs of secondary problems, not black knot itself.

Can you tell if a plum is from a sick tree just by looking at it?

Not reliably. The best way to know is to examine the tree itself. If you see the characteristic black, rough galls on branches, the tree has black knot, and you should inspect its fruit more carefully. But remember, the fruit itself is not infected.

Does Black Knot Affect the Taste of Plums?

In most cases, the taste of plums from an infected tree is normal or slightly less sweet. The fungus reduces the tree's ability to photosynthesize and move nutrients, which can lower sugar content in the fruit. You may notice that plums from a healthy tree taste richer and sweeter.

However, many people report that plums from lightly infected trees taste perfectly fine. The flavor change is usually subtle unless the tree is severely stressed.

What about bitter plums?

A bitter taste in plums is not caused by black knot. Bitterness can occur if the plum is unripe, if the tree is suffering from other stressors like drought, or if the fruit has been damaged by insects or rot. Discard any bitter-tasting plums.

Should You Remove the Tree or Treat It?

You do not need to remove a plum tree just because it has black knot, unless the infection is extremely severe and the tree is dying. Most trees can be saved with proper pruning and care.

Remove the tree only if:

More than 50 percent of the branches have large galls.
The trunk itself has galls, which means the disease has reached the main structure.
The tree produces no edible fruit for two consecutive years.

For most home gardeners, treating black knot through pruning is the best option. It saves the tree and allows you to keep harvesting plums.

How to Treat Black Knot on Plum Trees (Step-by-Step)

Treating black knot requires patience and consistency. Here is a reliable step-by-step plan.

What you will need

Pruning shears or loppers for small branches. Look for a pair that is sharp and easy to clean, like bypass pruners.
A pruning saw for thicker branches. A folding pruning saw works well for this job.
Rubbing alcohol or a bleach solution to sterilize tools between cuts.
Fungicide (optional, for severe cases). A copper-based fungicide is commonly used.

Steps

Wait for the right season. Prune during late winter or very early spring, while the tree is dormant and before new growth begins. The galls are easiest to see when there are no leaves.
Cut at least 15 to 20 centimeters (6 to 8 inches) below each gall on the branch. The fungus spreads inside the wood beyond the visible gall, so cutting well below it is essential.
Sterilize your pruning tool between every cut. Dip the blades in rubbing alcohol or a 10 percent bleach solution to avoid spreading the fungus to healthy wood.
Remove all infected branches from the area. Do not leave them on the ground near the tree. Burn them or bag them and throw them in the trash. Do not add them to your compost pile.
Check for new galls each year. Black knot often returns if any infected wood remains. Inspect the tree every spring and repeat the pruning process as needed.

When to use fungicide

Fungicide is not always necessary, but it can help in areas with heavy rainfall or in orchards with a history of black knot. Apply a fungicide labeled for stone fruit trees according to the package instructions, usually during spring when buds break and again just before bloom.

When Is the Best Time to Remove Black Knot Galls

The best time to remove black knot galls is late winter or early spring, before the tree leafs out. Why?

The galls are most visible against bare branches.
The fungus is dormant, so you are less likely to spread spores.
The tree heals faster from pruning cuts made in cool weather.

Do not prune in wet weather. Rain can spread fungal spores to fresh cuts. Choose a dry, calm day for pruning.

Can you prune in summer?

Summer pruning is less ideal but can be done if you spot a new gall. Cut it out immediately and dispose of it. However, summer cuts heal more slowly and the tree is more vulnerable to infection during active growth.

Common Mistakes When Dealing with Black Knot

Avoid these errors that many home gardeners make.

Cutting too close to the gall. This almost guarantees the infection will return because the fungus spreads inside the branch beyond what you can see.
Not sterilizing tools. This spreads the fungus to healthy branches and possibly to other trees in your yard.
Leaving infected wood on the ground. Spores can still release from cut galls and infect nearby trees.
Ignoring the problem. Black knot does not go away on its own. The galls grow larger each year and the tree weakens.
Applying fungicide at the wrong time. Fungicide must be applied during the spore-release period in spring, not during dormant season.

Can You Prevent Black Knot on Plum Trees?

Prevention is possible, though not always guaranteed. Here are effective strategies.

Choose resistant varieties

Some plum varieties are less susceptible to black knot. Look for resistant types like:

Stanley
President
Methley
Shiro

Maintain tree health

A healthy tree is better able to resist infection and recover from minor outbreaks.

Water deeply during dry spells.
Mulch around the base to keep roots cool and reduce stress.
Avoid wounding the bark with lawn equipment.

Remove wild host trees

Black knot also infects wild plum and cherry trees. If you have wild Prunus trees within 150 meters of your garden, they can act as a source of spores. Removing them can significantly reduce the risk.

Inspect regularly

Walk around your tree every few weeks during the growing season. Look for new swellings on branches. Early detection makes treatment much easier.

Can You Eat Plums from a Tree with Black Knot and Still Save the Tree

Yes, you can absolutely eat the plums while you treat and save the tree. In fact, you do not need to stop harvesting just because your tree has black knot. The fruit is still usable as long as it looks and tastes healthy.

To make the most of your harvest while managing the disease:

Pick plums as soon as they are ripe. Do not leave overripe fruit on the tree because it attracts insects and can rot.
Remove any fallen fruit from the ground promptly. Rotting fruit can lead to other fungal problems and attract pests.
Continue with annual pruning of galls. It may take two or three seasons to fully control the disease, but your harvest will improve as the tree recovers.

If you still feel uncertain about eating the fruit, you can also use the plums for cooking, baking, or making preserves. The heat from cooking further reduces any theoretical risk and the fruit is still perfectly good for jams, sauces, and desserts.

A final note on safety

Your health is not at risk from black knot itself. The real concern is the overall condition of the fruit. As long as you harvest healthy-looking plums, wash them well, and taste before eating large amounts, you can continue to enjoy your tree's fruit while you work on controlling the disease.

With regular pruning and good tree care, you can keep your plum tree productive for many years, even with black knot present in your area.