How to Manage Silver Leaf Disease in Plum Trees

There’s something magical about plum trees, with their beautiful blossoms and abundant, delicious fruit.

I’ve always been fascinated by them after growing up with a plum tree in my backyard. I remember lazy days in the summer swinging from the strong branches of our ‘Victoria’ plum. Not to mention all those jams and puddings my mum used to make.

It can take up to six years to produce a bounty of those tasty fruits, so it can be soul-destroying when something goes wrong. Silver leaf disease is an extremely common and devastating plum disease. We want to enjoy our trees, so let’s talk about silver leaf.

What is Silver Leaf Disease?

Silver leaf disease, is also known as silver leaf fungus, is caused by the fungal pathogen Chondrostereum purpureum (formerly Stereum purpureum). This fungus can grow as large, silvery-purple clusters of mushrooms.

This fungal infection affects plum trees and other stone fruit trees, such as apricots, apples, and cherries as well as ornamentals like maples and rhododendrons. The pathogen can infect almost anything in the Rosaceae family, but those in the Prunus genus are most impacted.

Characteristics

The fungus primarily infects trees through wounds or damaged bark and once infected, the disease is characterized by the foliage turning silvery-gray. This is caused by the fungus disrupting the movement of water and nutrients within the plant.

Up close, the fungus is distinctive as the fruiting bodies are typically thin, leathery, and fan-shaped. They grow up to about four inches wide. The upper surface of the fungus is smooth and has a violet or purple hue.

The underside of the fungus’s fruiting body is pale and smooth in texture. Hidden within this is a hymenium layer which contains the fungus spores.

When the spores find a host plant, the foliage will first turn silver, sometimes with brown patches. A tree might become asymptomatic after the initial infection, but it will still carry the disease.

Lifecycle and Habitat

Silver leaf disease is predominantly spread through the movement of spores, which are dispersed by wind or water.

The disease is most active in areas with damp conditions in fall and the start of winter when temperatures are above freezing.

Once spores land on a desirable host tree, the fungus infects it and begins to replicate itself to spread further. You might see large clusters of mushrooms on the wood, or you will only see the symptoms on the wood and leaves.

The fungus releases chemicals into the host tree, which are carried to the foliage via the sap, which normally takes nutrients for tree growth and fruit development.

The toxin acts as a blocker, splitting leaf cell particles, rendering the absorption of nutrients and photosynthesis impossible.

The disease can be fast-acting and might take three or so years to progress to the point where it destroys the host tree.

The disease can have a significant impact on the health and plum tree productivity.

The fungus spores attach themselves to broken, split, or damaged bark, branches, and wood. They then utilize the tree’s vascular system to travel and further penetrate the host.

If left untreated, it can lead to branch dieback, and early leaf and fruit expulsion. The plum tree cannot absorb the nutrients it needs to develop, grow, and remain healthy.

5 Signs and Symptoms

This disease may not be visible on the foliage until spring when leaves and blossoms emerge. This is because most of the trees susceptible to this disease, including plums, are deciduous, meaning they lose their foliage in fall.

The disease can be present, but you won’t know it until the leaves develop and take on the characteristic silvery hue.

Various factors can influence the rate of disease progression and the signs and symptoms. The cultivar of plum tree, age, health, position in the garden, and tree condition will all influence the disease.

1. Leaf Discoloration

The most obvious sign is when the leaves develop a silver-gray tone as they lose the ability to take up nutrients.

Generally, the discoloration occurs first on the leaves higher up in the canopy and may not be detectable if the tree is large until the lower leaves become symptomatic. A more visible warning sign is when you see premature defoliation on the upper branches.

As well as discoloring, the leaves may start to curl and wilt before dropping.

2. Branch Dieback

in addition to leaf discoloration, the branches might die. Fallen branches reveal black or dark brown markings inside the wood. Splashing water on the wood may make them more noticeable.

Silver leaf disease is highly likely if this discoloration under the bark is present in the tree.

Other dieback symptoms include the peeling back of bark and tree branches dying at the tips.

3. Reduced Fruit Production

As silver leaf disease impacts the host tree’s ability to take up food, water, and minerals, and to photosynthesize, the tree will struggle to produce blossoms and fruit.

Depending on the degree and stage of the silver leaf infection, the fruit might be highly impacted. Or, it might be relatively unaffected. Left unchecked, the disease spreads, and poor fruit production becomes more prevalent.

What fruit is produced may show evidence of the tree’s inability to absorb the nutrients it needs. Fruit may be smaller, lack sweetness, be less juicy, or abort before it is fully developed.

4. Fungal Growth

In some cases, and those that are more advanced, the presence of the fungus itself is visible. The pathogen can be observed as a purple-colored growth on the affected areas of the host tree.

5. Decline in Tree Vigor

Severely affected trees may exhibit a general decline in vigor, with reduced growth, poor foliage, and an overall weakened appearance.

Depending on the age and size of the tree, it may show stunted growth or might even die.

Prevention

As with most diseases, it’s best to prevent it in the first place if you can.

Because this is a progressive disease that continues to spread during fall and winter, preventative care must occur throughout the year.

The risk of this disease taking hold can be significantly reduced by implementing the following strategies.

Sanitation

When you work on your trees, ensure all pruning tools are clean and disinfected. Use a targeted cleaning solution, a 10% bleach solution, or rubbing alcohol.

Sanitize the tools before tackling each tree and before storing the equipment away.

Pruning

When it comes to avoiding silver leaf in plums, timing is everything. When you prune, you open up the tree’s protective bark and leave it exposed to pathogens.

Pruning during the dormant season when the risk of infection is lower, but only if the weather is dry for a period of at least two weeks. Prune right in the middle of the dry period.

Never prune during wet weather, as this can increase the chances of spore dispersal and infection. If you don’t have a dry patch of weather during the winter, it’s best to wait until the summer to prune.

Always use proper pruning tools that are sharp and clean.

Improper equipment or poorly maintained cutting tools can lead to excessive wounds and damage to the plum tree. A wound leaves the tree vulnerable to the Chondrostereum purpureum spores.

After any pruning or in early spring use a copper fungicide spray as a preventative measure to deter any fungal spores that might be present. Grab a bottle of Bonide Captan Jack copper at Amazon.

Wound Management

Be vigilant when trimming, mowing, or working around plum trees or other susceptible trees. Any damage to the branches, trunk, or bark of a tree makes it vulnerable to contracting silver leaf disease.

Maintain Health

Always promote tree health, no matter what area you live in or what season it is.

Maintain the overall health and vigor of your plum trees through proper nutrient management, irrigation, and pest control.

Use a good-quality fertilizer when the tree is starting to bud and when the tree is most active. A 10-10-10 feed with equal parts nitrogen, phosphorus, and potassium (NPK) works well for most situations, but you should do a soil test before feeding.

Healthy and nurtured trees are better equipped to withstand and recover from fungal infections.

You should also treat for any pests the moment you see them. Pests damage the tree and leave it open to pathogens.

Also, remember to water at the soil level and not on the trunk or canopy.

Improve Air Circulation

Ensure there is adequate air circulation around your plum trees by planting with future sizing and space in mind.

You also need to maintain air circulation within the canopy of the tree, which you do, of course, through regular pruning. Always prune out any crossing or crowded branches and any that touch each other. Remove one of any pair of branches with a tight crotch.

Good airflow can help reduce the humidity within the tree’s canopy. Remember, silver leaf needs water to reproduce and spread.

Preventative Treatment

A dormant spray can be used when the plum tree has gone dormant, and all the leaves have dropped in the fall. Pick up a dormant spray at Amazon and follow the manufacturer’s directions.

You can also treat preventatively for pests to keep them from damaging the tree and exposing it to problems. You can use a product that contains malathion or neem oil.

For a more natural approach, use a product that contains the naturally occurring bacterium Bacillus thuringiensis. These bacteria kill the soft-bodied pests that can damage the plum tree. Bt won’t kill adult hard-bodied insects, so it’s best used as a preventative to kill the larval stage.

Routine use of fungicide treatments may be warranted, particularly if silver leaf disease has previously been an issue on the land.

Disease Resistant Plum Varieties

There are some varieties of plum trees that are more resistant to the silver leaf pathogen.

Using modern technology, there are increasing efforts amongst scientists and horticulturalists to try and hybridize stronger plants that can withstand diseases, environmental challenges, and pests, all of which threaten food production worldwide.

The following are known to have at least some resistance to silver leaf:

• ‘Farleigh’ or ‘Farleigh Prolific’ (damson)
• ‘Pershore’ (European)
• ‘Honeysweet’ (European)
• ‘Au Rosa’ (Japanese)

Reach out to your local extension office for local options.

Management

If the worst happens despite all your best efforts and you still get silver leaf disease, you’ll need to address it.

If you catch it early, you can prune off infected branches and then double up your prevention efforts. This is why regular inspections are necessary. Be sure to include monthly applications of a broad fungicide.

Sadly, if a tree is symptomatic on more than half of the tree, there’s nothing you can do. Completely remove all trees with significant infections and dispose of them.

Discard these materials away from any planting areas or compost piles. Better yet, bag and trash them or burn the wood.