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Plum Tree Diseases – What to Watch for and How to Protect Your Plum Tree

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.

Plum Tree Diseases: bacterial canker
photo shows gummosis, a bacterial canker causes decline of fruit trees

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.

Plum Diseases: Sunken canker on trunk of tree
Sunken canker on trunk of tree, a common symptom of bacterial

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.

Tree Burr Knot
Tree Burr Knot (Burl) or Crown Gall close-up, plant disease that cause abnormal growths of galls

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)

Fungal mycelium on the surface of a tree stump

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)

fungus growth on a tree branch
Closeup of a fungus growth on a tree branch

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.)

Fungal Disease Monilia cinerea in Orchard
Plum Fruit Infected by Fungal Disease Monilia cinerea in Orchard.

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.

Rotten mummified plums on the fruit tree
Rotten mummified plums on the fruit tree, Monilia laxa (Monilinia laxa) infestation, plant disease

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)

Powdery mildew on leaf
Powdery mildew on leaf of apple tree

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
Plum rust (Tranzschelia pruni-spinosae) on green leaf of Plum or Prunus domestica

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

 chlorotic mottling and mosaic
Virus-triggered symptoms of chlorotic mottling and mosaic on green leaves of bird cherry.

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

Ripe Plums
Beautiful ripe purple plum on a branch in autumn
  • 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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