How To Prevent and Treat Black Knot Fungus

Milosh Potikj | September 29, 2023 | 8 MIN READ

Nothing can be more frustrating to an avid gardener, agriculturalist, or horticulturist than seeing your beloved plants and trees damaged by black knot fungus. If left untreated, this fungal disease can cause significant damage and even the death of certain plant and tree species.

In this blog post, we’ll discuss how to recognize the signs of black knot fungus and the steps you can take to prevent it from occurring in the first place, as well as provide treatment options if prevention isn’t enough.

This comprehensive guide will help you ensure that your gardening successes aren’t met with disappointment due to a dreaded outbreak of black knot fungus!

Recognizing Black Knot Fungus on Plants

The first step in managing a disease is to identify it properly. The distinctive feature of black knot disease is the presence of hard, black, irregularly shaped galls or swellings that form on the branches and twigs of susceptible trees. These galls start as small, light-brown swellings in the spring and can reach several inches in length by the end of the growing season. Recognizing these signs early can be crucial in halting the disease's progression and saving your trees.

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Signs of Infection

As its name suggests, black knot fungus infection is characterized by black galls or knots on the branches and twigs of susceptible trees, like cherry trees and plum trees, including wild plums, Japanese plum, and even ornamental trees. These knots are initially soft and olive green in color, often starting as a small swelling on the new season's shoots. As the fungus matures, the galls harden, become darker, and reach several inches long. They resemble a black, woody swelling, thus the name black knot.

Conspicuous during the dormant season, these galls predominantly occur regularly on branches and trunks of infected trees and can even enwrap twigs entirely. You may also notice a sticky liquid oozing from the black knots during wet periods in the spring. This is a sign of active spore production and release, which can further spread the disease.

Understanding these signs of infection is the first step toward effectively managing this common disease. Early detection and intervention can help save your trees from significant damage and deter the disease cycle.

Understanding the Disease Cycle

Understanding the Disease Cycle

Black knot disease is caused by a fungal infection that follows a specific disease cycle. Once it infects a tree, the fungus induces extra plant cells to form the distinctive black galls or swellings. Understanding this cycle is key to effectively managing and preventing the disease's spread. Let's delve into how this fungal disease proliferates and the damage it causes to trees.

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Fungal Spores and Spore Production

Fungal spores play a critical role in the spread of black knot disease. These tiny entities are produced by mature galls and are released into the environment during the wet spring period, especially during the bloom period. The spores are carried by wind or rain to the young, green shoots of susceptible trees, where they initiate a new infection. The spores can infect the terminal shoots and fruit spur of trees, leading to initial swellings that signify disease onset.

It's important to realize that spore production and release mostly occur on wet spring days, making this period crucial for disease prevention efforts. The effectiveness of the fungicide sprays is highest when applied just before bud break in spring, as this is when the infective spores are most prevalent. By understanding this aspect of the disease cycle, gardeners can take strategic steps to prevent the spread of this detrimental fungal disease.

Growth and Development of the Fungus

The growth and development of the black knot fungus is a process that begins with the initial infection of a susceptible tree by the fungal spores. As these spores land on the young, green shoots and twigs, they germinate and penetrate the plant tissue. Once the infection has set in, the fungus triggers the plant to produce extra cells, resulting in the characteristic swellings or galls.

During the first season, these galls appear as small, light-brown swellings. By the end of the second season, they mature into the hard, black growths we typically associate with black knot disease. This persistent fungus can continue to produce spores for several years, leading to further spread of the disease. Therefore, it's crucial to manage the disease and prevent the conditions that favor its growth and development.

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Prevention and Treatment of Black Knot Fungus

Prevention and Treatment of Black Knot Fungus

Preventing and treating black knot fungus requires a combination of good cultural practices, effective pruning, and strategic use of fungicides. By understanding how the fungus operates, we can employ measures to interrupt its life cycle and stop the spread of disease. In this section, we'll guide you through the steps to protect your trees from this damaging disease and how to treat it if your trees are already infected.

Proper Pruning Techniques

Proper pruning is an essential step in preventing and managing black knot disease. It involves the removal of infected branches and twigs, which harbor the disease-causing fungus and its spores. Begin by inspecting your trees regularly, especially during the dormant season, for signs of infection such as black or brown swellings or galls. When you spot a gall, prune the branch or twig at least 2-3 inches below the infected area to ensure you remove all diseased wood.

Understand that larger branches with galls should also be pruned back to the point of healthy wood, but ensure that the pruning cuts do not damage the wood's collar to avoid causing extensive injury to the tree. Always remember to clean your pruning tools after each cut with a solution of bleach or alcohol. This helps to sterilize the blades and prevents the spread of fungal spores from the infected branches to healthy parts of the same tree or other trees.

Pruning should ideally be done in late winter or early spring before bud break, when trees are still dormant, and the risk of fungal spore release is minimal. Pruning during the active growth period in spring can spread spores and increase the risk of infection. If you must prune during this time, apply a fungicide treatment to pruning wounds to protect them from infection.

Proper pruning is not just about removing unhealthy branches; it also involves promoting the tree's overall health. Maintaining the tree's natural shape, encouraging good air circulation, and ensuring adequate sun exposure can all help reduce the tree's susceptibility to black knot and other diseases.

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Fungicide Sprays

The strategic use of fungicide sprays is essential to treating and preventing black knot disease. Fungicides, such as copper-based products, inhibit fungal growth and spore production, halting the disease's progression. Applying fungicides at the right time is crucial to achieve maximum effectiveness.

As mentioned earlier, the most effective time to apply fungicides is in the spring, just before bud break, when the infective spores are most prevalent. The fungicide should be sprayed regularly until the end of the wet period. Moreover, remember to thoroughly coat all tree parts, including the trunk, branches, and twigs.

It's worth noting that fungicides are primarily preventative and most effective when used with pruning. They are less effective at curing an already infected tree. Therefore, a comprehensive approach that includes regular inspections, proper pruning, and the strategic use of fungicides must be adopted to maintain healthy trees.

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Management and Resistance

Effective management of black knot disease requires a two-pronged approach: active management of infected trees and fostering resistance to healthy product. Active management involves regular inspections, proper pruning of infected branches, and strategic use of fungicides. It's important to remember that even after removing the visible signs of the disease, the tree still needs regular inspections as it can re-occur.

Creating an environment that fosters resistance in trees is crucial in preventing black knot. Give preference to resistant varieties when planting new products or replacing diseased ones. Some plum and cherry varieties are less susceptible to black knot. For instance, the Japanese plum and Santa Rosa varieties show some resistance, while wild plums and sour cherry are most susceptible.

It's also essential to maintain the tree's overall health to enhance its resistance to diseases. This means ensuring the tree has proper nutrition, adequate water, and the right pruning to maintain good air circulation and sunlight penetration. Dense foliage can create a moist, shaded environment that favors the growth of fungi, so keep your trees well-pruned.

Implementing these management and resistance strategies will protect your trees from black knot disease and ensure their healthy growth and development. Remember that the keys to successful disease management are early detection and quick response. Therefore, regular inspections and prompt action at the first sign of infection are essential.

Cultivating Resistant Species

Cultivating resistant species is a highly effective method of preventing black knot disease, particularly if establishing a new orchard or replacing diseased trees. Certain species and cultivars of plums and cherries have demonstrated increased resistance to black knot. For instance, the Japanese plum variety 'Santa Rosa' and P. armeniaca have shown some resistance to the disease. Conversely, sour cherry, black cherry, and wild plum species are more susceptible to black knot. Therefore, opting for resistant varieties can significantly reduce the potential for disease in your orchard or landscape.

Dealing with Advanced Infections

Dealing with Advanced Infections

Advanced infections of black knot disease, characterized by extensive black galls or swellings on branches, the trunk, and even twigs, require more aggressive management strategies. In such cases, pruning alone may not control the disease.

Where possible, removal of the entire infected tree may be necessary to prevent the spread of the disease to surrounding healthy trees. If removing the tree is not an option, you can still manage the disease by pruning more heavily, removing branches with galls, and cutting several inches into the healthy wood to ensure complete removal.

Additionally, fungicide treatments should be applied more frequently and thoroughly, ensuring that the entire tree, including the trunk and larger branches, is well-coated. Fungicides like copper-based products can help inhibit the growth of the fungus and prevent further spread of the disease.

Furthermore, a spray of lime sulfur during the dormant period can effectively manage advanced infections. The product helps desiccate the gall's fungal tissue, reducing spore production and subsequent infection in the coming season. Remember, it is key to maintain regular application intervals, especially during wet periods when the fungus is most active.

Lastly, consider seeking advice from your local extension service or a professional arborist, as they can provide additional insights and guidance on managing advanced infections of black knot disease.

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Frequently Asked Questions

How do you prevent black knot fungus?

Preventing black knot fungus involves proactive strategies such as cultivating disease-resistant species and conducting regular tree inspections. It also includes proper pruning and fungicide applications to inhibit the growth and spread of the disease.

What causes black knot fungus?

Black knot fungus is caused by the fungal pathogen Apiosporina morbosa. This fungus infects trees during the spring, spreading its spores through wind and rain to new susceptible hosts.

Is black knot fungus bad?

Yes, black knot fungus is harmful as it causes a disease that can severely damage or even kill susceptible trees. This fungal disease forms distinct black, knot-like galls on branches and twigs, affecting the tree's health and appearance.

How does a black knot spread?

Black knot disease spreads when infective spores, released during the bloom period in spring, land on susceptible trees, particularly during wet periods. The spore release from black knot galls is facilitated by rain, and the wind carries these spores from tree to tree.


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