diseases – an overview

Dahlias are undoubtedly beloved for their beauty and diversity, making it disheartening when issues arise in the garden. Symptoms such as stunted growth, twisted stems, yellow or black foliage leaves, or wilting and browning petals can indicate a problem. To address these concerns, it is crucial to identify the specific disease affecting your dahlias and take immediate action.

We will explore 11 common diseases that can infect dahlias, understand their causes, and discuss the most effective approaches for managing them. By delving into the root of the problem and implementing appropriate measures, you can help restore the health and vibrancy of your beloved dahlias.

Bacterial Disease

Not all bacteria are harmful, as some can be beneficial. However, in the case of dahlias, bacterial diseases are typically fatal and require immediate action. Two prevalent bacterial diseases that affect dahlias are bacterial wilt and soft rot.

Bacterial Wilt

If your dahlias are affected by this disease, you’ll observe darker and wilted leaves during the day. There might be a temporary improvement at night, but eventually, the leaves will turn brown and the plants will collapse. When you cut the stems with a pruning tool, you may notice strands of bacterial ooze.

Regrettably, the bacterial infection has obstructed the dahlia’s vascular system, leaving no option but to take preventive measures to avoid infecting other plants in your garden. Remove the infected dahlia, along with as much soil around the affected tubers as possible, and dispose of it in the garbage can.

Soft Rot

Bacterial soft rot is a highly damaging bacterial disease, causing significant crop loss worldwide. Unfortunately, dahlias, with their tuberous roots, are particularly susceptible to this disease.

If you observe yellow, water-soaked spots on the outer leaves of your dahlias, it is likely they are affected by soft rot. The presence of a foul odor and mushy stems further confirms this diagnosis as the bacteria continue breaking down plant tissue.

As bacterial diseases are typically fatal, the best course of action is to promptly remove the infected dahlias and excavate as much surrounding soil as possible. Ensure proper watering and spacing, and consider planting dahlias (or any other tubers) in a new location next year. Using containers for dahlia tubers can also be beneficial, as bacteria can overwinter in various climates.

Aster Yellows

Aster yellows infection in dahlias can result in light green, yellow, or white leaves that are stunted or deformed. The blooms may also appear small, have abnormal shapes, and display pale colors. This disease is caused by phytoplasma, a small bacteria that can reside in the veins of a plant or the body of an insect.

Aster yellows is typically transmitted by leafhoppers, so preventing this disease involves both pest control and bacterial management. Using reflective mulches and light mesh covers can help exclude leafhoppers from your dahlia beds, although these methods may not be visually appealing. Another option is to apply neem oil to the leaf surfaces, which can deter leafhoppers without compromising the natural beauty of your dahlias.

Fungal Disease

Fungus, constantly present in the soil, can cause significant damage in dahlia gardens. Often not visible until the issue has advanced, fungal infections come in various forms and can be challenging to treat. Let’s explore some of the common fungal diseases that affect dahlias and discuss how to manage them.

Powdery Mildew

Powdery mildew can be identified by the presence of grayish or white, powder-like spots. Initially, these spots appear on the leaves but can also spread to the stems and blooms. As the season progresses, the affected areas may turn black. Keep an eye out for powdery mildew when the climate is characterized by high humidity levels and moderate day and night temperatures.

Although dahlias are highly susceptible to powdery mildew, especially in spring and fall, this disease does not have to be fatal. If you notice infected leaves, stems, or blooms, you can prune out the affected parts while leaving the healthy growth intact. If the appearance is tolerable, and the fungal growth is not too severe, you can leave mildly affected plants in place until the end of the season. At that point, cut the stems to the ground and remove them to prevent the fungus from overwintering.

Botrytis (Gray Mold)

Botrytis can be identified by the presence of fuzzy gray spores and petals that turn brown from the center outward. This fungus tends to attack dahlias especially after prolonged periods of rain, so it is important to remain vigilant for its presence when conditions are favorable.

Similar to powdery mildew, botrytis is a surface disease and not systemic. This implies that the damage is primarily aesthetic, and you can prune off the infected parts. To prevent the spread of the disease across multiple seasons, it is advisable to cut down and remove all above-ground plant parts at the end of the season.

Verticillium & Fusarium Wilt

These fungi thrive in the xylem of dahlias and obstruct the transportation of water. Early signs may include yellowing leaves, but common symptoms of this disease are darkened stem tissue near the soil level and roots with streaky brown or black discoloration.

Both fungi flourish when temperatures experience abrupt shifts from cool to warm, so be observant for signs of wilt during such conditions. As wilt damage impacts the entire feeding system of the dahlia, it is advisable to eliminate and dispose of the infected plants.


Pale leaf spots with dark centers, initially appearing on lower branches and progressing upwards, can be indicative of dahlia smut disease. Similar to other fungi, smut tends to emerge during wet periods or when irrigation is focused on leaves rather than roots.

If you suspect smut, it is recommended to prune off the affected leaves and promptly remove them from the garden. Additionally, pruning some of the foliage to improve light and air circulation can be beneficial, as crowding contributes to smut transmission.

Keep in mind that smut can overwinter in the tuberous roots of dahlias. If you are planning to bring them indoors for overwintering, treat the tubers with a fungicide and ensure they are thoroughly dried before wrapping and storing them.

Southern Blight

In warmer temperatures, this fungus manifests as a ring in the soil surrounding the stems of your dahlias. Initially, small white spores will transform into black clusters, eventually suffocating the plants. The dahlias will suddenly wilt and eventually die. If you suspect an infection, it is crucial to remove the infected plants entirely as a control measure.

To prevent the spread of blight, it is important to maintain a tidy garden bed, as this fungus thrives on decaying leaves and blooms. Keep a close eye on your dahlias after periods of drought, as the first heavy rain following a dry spell creates optimal conditions for blight growth. Consider planting dahlias in a raised bed, as this can help prevent debris from accumulating near the bases of the plants.

Sclerotinia Disease

Early signs of this fungus include browning and wilting stems. If you slice open the stems laterally, the presence of a string of tiny black spores can confirm a diagnosis of Sclerotinia. These spores can spread easily through the air and attach to petals, which in turn cling to the stems, perpetuating the cycle of infection.

To manage Sclerotinia infection, prune off the infected stems while being cautious not to shake spores onto the soil surface. Remove and dispose of the infected plant material. Regularly deadhead and remove all petals from your garden beds. When digging up tubers at the end of the season, cut the stems down to the roots and inspect them for Sclerotinia spores. If spores are detected, treat the tubers with a fungicide and ensure they are thoroughly dried before wrapping them for winter storage.


Symptoms of viral diseases in dahlias can vary, with some plants showing visible signs while others appear healthy but decline gradually over time. Look for stripes and splotchy patterns on the leaves, with variations between light and dark green, or twisted and yellow leaves. It is crucial to be aware that viral diseases can spread among dahlias, so it is important to know and trust the source of your dahlia stock and take proactive measures to prevent outbreaks. Mosaic and ring spot are among the most common viruses encountered in dahlia gardens.

Dahlia Mosaic

To identify the presence of dahlia mosaic virus, look for leaves with irregular colors and pale green bands near the veins. In certain cultivars, the virus may show minimal symptoms, but you may notice reduced bloom intensity and slow tuber growth as indications of its presence.

Mosaic virus can eventually lead to the death of your dahlia, albeit it may take a season or two. If you suspect your plant is infected with this disease, it is crucial to remove it promptly and dispose of it. Thoroughly disinfect your tools to prevent further spread, and diligently control aphid populations, as they can transmit the virus.

Ring Spot

Mottled leaf patterns and wavy or concentric rings are distinct signs of ring spot disease. This disease is primarily spread by thrips and can also be transmitted through infected plant cuttings or root divisions. To minimize the risk of unintentionally spreading ring spot, it is advisable to take dahlia cuttings from the tops of vigorously growing and visually healthy plants. Additionally, it is important to effectively manage thrips populations in your garden to prevent the transmission of the disease.

If ring spot disease is confirmed in your dahlia, it is recommended to destroy the infected plant to prevent further spread. Additionally, it is crucial to closely monitor the remaining dahlia plants in the vicinity for any early symptoms of the disease. Taking proactive measures and promptly addressing any potential outbreaks can help protect your dahlia garden.

Indeed, differentiating between various ailments that affect dahlias can be challenging, as the symptoms of pests and diseases can overlap and be interconnected. While laboratory testing can provide accurate diagnoses, it may not be practical for every case. Understanding the distinction between fungal, bacterial, and viral diseases is crucial, as fungal infections can often be treated and are not always lethal. However, when in doubt, it is important to prioritize the health of your other dahlias and prevent the transmission of diseases by removing and disposing of infected plants. Vigilance, good hygiene practices, and proactive measures can go a long way in maintaining the overall health of your dahlia garden.


In conclusion, being able to identify common diseases that affect dahlias is essential for maintaining their health and vitality. By promptly recognizing the symptoms and taking appropriate measures such as pruning infected parts, removing diseased plants, practicing good garden hygiene, and implementing preventive strategies, you can help prevent the spread of diseases and promote the well-being of your dahlias. Regular monitoring, proper care, and trusted sourcing of plant stock are key to ensuring a thriving dahlia garden.