The Invisible Threats: Understanding and Addressing Diseases in Dahlias

The vibrant and splendid dahlias gracing our gardens with their diverse colours and forms are indeed a sight to behold. Unfortunately, these delightful blooms are not immune to the trials and tribulations of the natural world. They can fall prey to a variety of diseases, primarily viruses, that can mar their beauty and vigour.

Common Viruses Affecting Dahlias

DAHLIA MOSAIC VIRUS (DMV)

DMV is the most common virus affecting dahlias. Over the past five years, a commercial testing lab reported a positivity rate of 84% for DMV. DMV only infects dahlias, and its vectors include thrips, aphids, pollen, and cutting tools. One notable strain, DMV-EPRV (Endogenous Pararetrovirus), embeds itself in the dahlia genome. Ongoing research suggests that DMV-EPRV can be activated and infect dahlias. Symptoms of DMV can include stunted growth, yellow mottling on leaves, vein clearing, and color breaking or lighter streaking on petals.

Tomato Spotted Wilt Virus (TSWV)

TSWV in US dahlia stock rose to prominence in 2022 when many dahlia cuttings infected with TSWV were widely distributed in the US. TSWV is known to infect over 1000 plant species and is spread by thrips and cutting tools. Testing the petiole might yield the best results, as TSWV is unevenly distributed in dahlia plants. The most common symptom of TSWV is yellow mottled leaves, which are relatively easy to spot.

Tobacco Streak Virus (TSV)

TSV affects approximately half of the dahlias tested by vendors. It spreads via thrips and floral snips/pruners, and can infect more than 200 other plant species. The symptoms of TSV are not clearly defined, but infected dahlias can be asymptomatic or mildly symptomatic, often resulting in the virus going undetected.

Other Viruses

Other viruses, such as the Cucumber mosaic virus, Chrysanthemum stunt viroid, and Potato virus Y, can affect dahlias, but these are less common. Regular testing and monitoring of dahlias can help identify these infections early and prevent their spread.

Bacterial and Fungal Diseases

Bacterial disease symptoms can vary. For instance, bacterial wilt can cause symptoms nearly identical to those of Verticillium wilt, while Crown gall is characterized by overgrowths or galls on stems and roots. Fungal diseases also present various symptoms. Flower blight can cause brown, water-soaked spots on petals, and a woolly grey fungus may form on decayed tissues. Powdery mildew typically appears on older leaves and stems, leaving behind a white, powdery fungus. In severe cases, affected leaves may dry up and fall off. Other fungal diseases, like soft rot, can cause plants to wilt and die suddenly, with water-soaked stem cankers appearing near the soil line. Additionally, diseases like smut and stem and tuber rot can cause yellowish, circular spots on leaves and the wilting and yellowing of basal leaves, respectively.

Do Not Despair

While it might seem like a multitude of pathogens are threatening our dahlias, remember: forewarned is forearmed. With regular monitoring, testing, and good gardening practices, we can keep our dahlias blooming beautifully. Don’t let the threat of disease deter you from enjoying these wonderful flowers. Here are some protection and prevention tips:

Avoid introducing diseases: Purchase plants from reputable sources, and inspect new plants for signs of disease before introducing them to your garden.
Provide proper plant care: Many diseases are more likely to occur if the plant is not properly cared for. This includes providing the right amount of water, ensuring the plant has good soil and nutrients, and avoiding crowding, which can create conditions favourable to disease.
Maintain good garden hygiene: Regularly clean your garden tools, especially if you’ve been working with diseased plants. Some diseases can spread through soil, so avoid transferring soil from around a diseased plant to other areas of your garden.
Monitor regularly: Check your plants frequently for signs of disease. Early detection can make disease control much easier.
Use disease-resistant varieties: Some plant varieties are bred to be resistant to certain diseases. Using these varieties can help reduce the likelihood of disease in your garden.
Practice crop rotation and intercropping: These farming techniques can help prevent the buildup of diseases in the soil. Crop rotation involves changing the type of plant grown in a particular area each year, while intercropping involves growing different types of plants together.

Take advantage of modern technology, such as the PictureThis App. This remarkable app can diagnose plant diseases simply by analysing a photo. Snap a picture of your dahlia and let the app identify any potential issues.

Remember, it’s normal for plants to occasionally contract diseases, just like it’s normal for humans to fall ill. The goal isn’t to create a completely disease-free garden (which is nearly impossible), but to cultivate a healthy garden where disease is the exception, not the norm. And when diseases do occur, the goal is to manage them effectively to minimize damage.