Our trees form such important parts of our yards and home gardens. They give us privacy from neighbors, shade to make our yards tolerable on hot summer days, and sometimes even tasty fresh fruit. In this article, we will identify plum tree diseases, treatment options, and, of course, prevention of the diseases .
Plum Tree Diseases: Bacterial
Bacterial canker (Pseudomonas syringae)
Bacterial canker is easiest to spot in the spring when buds do not open and the nearby twigs die back. Cankers also occur and trunks and main branches and manifest as oozing spots, or sores, that produce a sour smell. The cankers additionally create spots on leaves that begins as small purple spots that transitions to black before finally turning into a shot hole as the leaf tissue dies and falls out.
Ultimately, the canker causes necrosis of the leaves and woody tissues until the branch, and perhaps the whole tree, dies.
Source and Treatment of Bacterial Canker
Bacterial canker affects all stone fruit trees and colonizes on the surface of healthy trees. It only becomes a problem for stressed trees or trees with an entry point of leaf scars or pruning wounds.
Unfortunately, treatment for bacterial canker is disappointingly inconsistent and ineffective. Some options are to apply a copper fungicide or a broad-system fungicide. In addition to trying a fungicide, containment will be your most important course of action as it can be spread from tree-to-tree.
Bacterial Spot (Xanthomonas campestris)
Bacterial spot also produces oozing cankers on the tree trunk, sometimes so significantly that the tree is left pitted and ridged from the ever-increasing cankers. Long before you notice the cankers, the bacterial spot will damage the leaves.
You will see angular, as opposed to round, spots on the underside of the leaves. Shot holes will appear. And then finally, the leaves will turn yellow and fall off the tree, which will then lead to sunburned fruit. Fruit may also have black sunken spot.
Source and Treatment of Bacterial Spot
Bacteria colonizes on healthy tree tissue then spreads with rain to the leaves, fruit, and twigs. Warm rain and temperatures above 65 degrees Fahrenheit permit the bacterial growth, so warm, wet springs year after year promote severe infections.
The oozing cankers can also spread the disease to uninfected trees.
Chemical applications can prevent additional or new infections but cannot cure an existing infection. Prevention will again be key.
Crown Gall (Agrobacterium tumefaciens)
Crown Gall creates burly gnarls on the root or trunk of trees, and it impacts many, many types of fruit and nut trees. On plum trees, the gnarls will be soft and sometimes hollow. The galls stunt the growth on young trees and can cause wood rot on older trees.
Source and Treatment of Crown Gall
Crown Gall is a bacterial, and it is particularly problematic because it can live independently in soil and in roots. This means that even if you get rid of a diseased tree, the crown gall can remain.
There are no effective treatments for Crown Gall, but botanists suggest just tolerating it for the life of the tree. Trees can continue to bear fruit.
Plum Tree Diseases: Oomecyte
Phytophthora Root and Crown Rot (Phytophthora spp.)
Root and crown rot is caused by Oomecytes, which are water molds. Plum tree leaves will have noticeable maladies – wilting, followed by discoloration and then premature drop. As the disease progresses, you will see branch and twig dieback. Brown lesions encircle feeder roots, and the feeder roots eventually disappear. Ones left behind will be woody and brittle. The tree may ooze black sap.
Root and crown disease can stunt growth, and eventually cause the death of the tree. Young trees are most susceptible.
Source and Treatment of Root and Crown Rot
Root and crown rot can survive in the soil for years, but it requires a high level of moisture to infect a tree. Gardeners are most likely to have the disease introduced to their property by the planting of an infected tree purchased elsewhere. It can then spread through the soil.
Phosphonates can help control the rot. Ensure your soil has proper drainage.
Plum Tree Diseases: Fungal
Armillaria Root Rot (Armillaria mellea)
Armillaria root rot is commonly known as oak root fungus and is ultimately fatal to infected frees. The soil-borne fungus infects the root and crown of the plum tree, and by the time you can see above-ground symptoms, it is likely too late to save the tree.
During wet seasons, mushrooms may appear at the base of the tree, but the small stand of mushrooms is a deceptive sign of the massive infection below the ground.
Source and Treatment of Root and Crown Rot
Containment of infected soil, wood, and root tissue is critical for preventing the spread of the fungus. The fungus continues to live in the dead root tissue long after the tree is cut down or dies back. Planting a new susceptible tree in the same soil is not recommended unless there is complete removal of the tree and and fumigation of the soil.
Black Knot (Apiosporina morbosa)
Black knot is a blissfully accurate and descriptive name for fungal diseases. On plum trees, you will see black, swollen masses on the twigs and branches. The masses will start as subtle green or light brown soft spots that grow over multiple seasons to black tumor-like growths. They can be up to a foot long and encircle an entire branch.
When the branch is encircled by the knot, the branch will die. The tree may suffer from decreased fruit production, structural damage, and ultimately death if the infection is severe. Mature trees are more resilient and may survive without any noticeable ill effects.
Source and Treatment of Black Knot
The knots house the fungus, and the fungus spreads throughout the tree and even to other trees by spores that settle on new green growth. Without moisture, the spores will not develop into a larger colony that creates the knot.
Unfortunately, fungicides alone will not remedy black knot, and removal is much more laborious. Botanists recommend pruning the tree in the winter to remove any visible knots. Remember, the spores will be released from those knots in spring. Applying a copper fungicide can help prevent the creation and spread of the stores.
Dip your shears in a 10% bleach mixture between cuts, and burn the infected branches. As it takes a few seasons for the knot to materialize, it may take a few years of heavy winter pruning to fully control the infection.
Brown Rot (Monolinia spp.)
Brown rot impacts stone fruit like plum trees the world over, but it will actually be most noticeable as fruit rot rather than abnormal tree growth. Like black rot, brown rot is a fungus spread by springtime spores. The fungus overwinters in infected fruit and twigs and then settles on the blossoms in spring after being transported by wind and rain.
The fruits develop brow spots that expand into larger spots or rings of spores. The fruit rots and shrivels, known as a mummy.
Cankers will also appear on the twigs and branches, creating cankers that can disrupt the vascular system in the tree and cause damage or death.
Source and Treatment of Brown Rot
Because the spores overwinter in the infected fruit and twigs, you must be sure to clear infected fruit as it appears and at the end of the growing season. Prune the infected twigs and branches to remove the source of the infection, and treat with a fungicide.
Apply a fungicide during bloom and prior to harvest for maximum effect
Powdery Mildew (Sphaerotheca pannosa and Podosphaera tridactyla)
These two fungi manifest as a white, powdery substance that rapidly spreads throughout the tree. Sphaerotheca pannosa effects the fruit while Podosphaera tridactyla attacks the leaves. Depending on which type, it may look stringy or like a powder.
Source and Treatment of Powdery Mildew
The spores overwinter on fallen leaves, bark, and on shoots of the tree. The powdery mildew easily spreads from roses and other stone fruit trees, so any treatment must treat the collective impacted plants. Apply fungicide starting at full bloom at 10-14 day intervals until fruit pits harden. Rotate fungicides to prevent resistance.
Rust (Tranzschelia discolor)
Plum rust creates tiny, rust-colored spores on plum tree (and other fruit tree)leaves. It can cause leaf drop and a reduced plum yield, but it is unlikely to kill the tree. Plum trees are usually impacted at the end of the growing season, making it more resistant to the mal effects of the fungus.
Source and Treatment of Plum Rust
The spores spread easily on the wind, but the fugus can be treated with fungicides. Rotate your chemicals to manage any resistant fungi.
Plum Tree Diseases: Viral
Plum Pox Virus
The USDA identifies plum pox virus (PPV) as a serious viral disease impacting plum and other fruit trees. Fortunately, it does not kill the tree, but it does reduce the fruit yields of impacted trees and it mottles and severely deforms the fruit, rendering it unmarketable. PPV does not cause harm to humans, but it can devastate commercial orchards.
Source and Treatment of Plum Pox Virus
PPV is highly contagious and is spread by the infected plant or fruit.
Once an infection is identified, the source tree and all trees within a 50-meter radius should be removed and destroyed.
Plum Tree Disease Prevention
Because many of the above diseases are easily or effectively healed, prevention will be the best bet to maintain the beauty, fruit production, and ultimate survival of your plum trees. Here are some key tips to avoid infection of your plum trees
- Correct pruning is the number one thing you can do to keep your trees safe. Prune flowering trees when they are blooming because wounds heal the fastest. By eliminating weak, damaged, or brittle branches, you are removing possible entry points into the tree. The increased air circulation promotes a dryer environment to impede fungal growth
- Apply sealant to any wounds to prevent diseases from entering the tree, and not jus pruning cuts. Did you nick the tree with the weed whacker? Did your teenager tie a hammock and cause a break in the bark? Seal it.
- Select disease-resistant varieties. Many species of plums are resistant to some diseases but not others. Consider the most pressing risks in your climate, and be sure to choose a strain that is resistant to that issue.
- Apply NEEM oil to fungus-infected trees. NEEM oil is a natural infection control medium that smothers insects and spores alike. For trees struggling with a fungicide-resistant fungus, the NEEM oil can inhibit the spread of spores and give you a leg-up in controlling the fungus through pruning and condition control.
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Sunday 5th of March 2023
Hi. Our wild plums scattered around the farm are developing a leaf growth that I cannot find info on. They do have the problem of black knot which I have been trimming back and burning as I find them, but the leaves have this series of small raised tubes that extend up 3-5mm in groupings of maybe a dozen or more on each leaf. How do I approach this? the worst if a small grouping of trees in the original yardsite, (60+ years old) and is sheltered by maple and carrigana. There are also some gooseberry and rose bushes.
Monday 6th of March 2023
Galls perhaps? https://www.canr.msu.edu/news/what_are_these_ugly_bumps_on_my_trees_leaves
Tuesday 2nd of August 2022
can someone advise? i have a maybe two or three year old methley plum that a dear cat used as a scratching post until i found out it was oozing sap in maybe six or seven spots so i recently pulled away the balls of sap (no insects in it, a pale amber color) and treated with a wound paste which has dried. but at the bottom, the base, of the tree i notcied it appears to be dark and rotting all the way around the very bottom of the trunk. i pulled away some sap from there and also what seemed a gob of snot or something does this sound like a fungus? there are also lightly peeling areas in the bark of the trunk, not bad and i thought it might be scalded form the horrific texas heat this year
the crown of the tree looks very healthy still, lots of growth this year, and the leaves look fine. so i am not sure what to do. i have neem oil but am not sure if i should spray the base heavily with the neem or if someone could take an educated guess as to what is happening to my poor love of a tree as i may need to purchase something else for it? i would hate to lose it so i want to do my best thank you for any response as i feel quite alone in this
Saturday 6th of August 2022
Trees can do an amazing job of healing themselves, so all may not be lost. That said, the bark is a pretty critical layer of defense! Glad to hear the crown looks good; that's your #1 sign it's doing ok (for now).
Until the tree heals up, I would treat liberally for insects & disease. You might put some stickyfoot / tanglefoot around the base of the tree to keep crawling insects off the tree.
Sunday 17th of July 2022
I have a Victoria plum tree which is now 7yrs old. Every year the poor tree gets so infested with aphids it makes all the leaves sticky, curly and deformed, everything underneath the tree is also sticky, dead aphids everywhere and so many ants. This is the first year it has actually fruited but fruit appears deformed too. I’m an organic gardener and have tried all types of natural mixtures to make home sprays. I’ve recently discovered neem oil (haven’t used it yet). However, the tree has so many ladybird larva (which are soft bodied). I note that neem oil is safe for beneficial pollinating insects and includes ladybirds. But it says harmful to soft bodied insects… which is what ladybird larva is. The dilemma is if I spray my tree with neem oil solution will I be killing the many ladybird larva too? I don’t want to do that. So to spray of not to spray that is the question? Should I leave the tree alone and next spring be better equipped and treat tree before the horrendous infestation takes hold?
Monday 25th of July 2022
I would do everything you can to get rid of the infestation this year. Fewer larvae larvae this year will mean fewer adults laying eggs this year that hatch next year. NOW is always the best time to start fighting an infestation.
Regarding your dilemma - could you run a test? Like put some in a container and spray and see what happens to them?
Thursday 2nd of June 2022
My new small plum trees leaves has holes in them and I’ve just found a tiny caterpillar like thing on it only one even though most of the leaves are affected. How do I get rid of them please?
Friday 3rd of June 2022
Read our posts about plum tree pests.
There is almost certainly more than one caterpillar on the tree! They can do a lot of damage quickly.
Wednesday 1st of June 2022
My trees have wave-shriveled leaves on the new growth parts. Slight reddish discoloring? I prefer bio-fungicides/pesticides. What is it and how can I remedy it? Thanks
Monday 27th of June 2022
@Mark Harvey, I have same issue with small holes on leaves. Small leaves that are starting to grow are shriveling
Friday 3rd of June 2022
Look closely at the shriveled parts... do you see any spider webbing or other insect debris. Bugs LOVE new growth, so without seeing it, I'd suspect bugs over disease.
Read this post about Plum Tree Pests.