Sunflowers, Helianthus annuus, are annuals in the Asteraceae family suitable for cultivation in Zones 2 to 11.
Whether you grow them for their ornamental value or the edible seeds, these flowers are a bright and sunny addition to your garden.
When provided with optimal growing conditions, these beautiful and vibrant flowers aren’t prone to many plant diseases, but you’ll need to know what to look for if pathogens do come to visit.
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In our guide to growing sunflowers, we cover how to plant and care for Helianthus.
Whether you’re growing cute little teddy bear types or black oil sunflowers for the birds, this article will discuss seven common diseases that can ruin the floral display.
Here’s the lineup:
7 Common Sunflower Diseases
1. Alternaria Leaf Blight
Caused by the fungal species Alternaria helianthi or A. zinniae, Alternaria leaf blight is most likely to appear in warm, humid conditions, especially after rain as moisture is a prime driver of disease development.
Infected specimens will exhibit small, dark, sometimes yellow-ringed spots on the foliage, typically along the margins or towards the leaf tips.
The spots will eventually merge and grow in size, eventually covering the majority of a leaf. In time, infected specimens will lose their leaves – generally the lower leaves first – and eventually die.
The pathogens can spread via wind or splashing water, be seedborne, survive in weed hosts, and overwinter in crop detritus or the soil. And even if an infected sunflower doesn’t die, having blighted foliage won’t help a sunflower to develop its lovely blooms.
Maintaining appropriate spacing to promote adequate air circulation, pruning symptomatic foliage, and watering at soil level can help curb the spread.
If a majority of your plants are infected, you’ll need to pull them up and dispose of them in the trash, not on the compost pile.
If you’re growing sunflowers as a cover crop, in the case of infection make sure you rotate your crops and don’t plant Helianthus in the same spot for two to three years.
2. Downy Mildew
Many different species can suffer from a variety of different mildews, but this flavor of downy mildew – caused by the fungus Plasmopara halstedi – is actually unique to Helianthus.
Capable of surviving in soil for up to a decade, P. halstedi thrives in cool temperatures, high humidity, and waterlogged conditions.
If these conditions are present during seed germination and seedling emergence, young specimens become infected and usually die quickly.
If these juveniles make it to adulthood, they’ll lack vigor and generally exhibit chlorotic leaves with white cottony growth on the undersides. If infected plants don’t die, they’ll suffer from stunted growth and reduced seed set.
Growing resistant sunflower varieties and purchasing certified disease-free seed can help you avoid infection.
Foliar fungicides won’t do much, though. If your plants are infected, remove and dispose of those specimens in the trash.
3. Phoma Black Stem
Also known as Phoma blight, Phoma black stem has one very obvious symptom – I’m sure you can figure out what it is.
This disease is caused by the fungus Phoma macdonadlii, which survives in seeds or on infected crop residue and is spread by insects such as sunflower stem weevils, Apion occidentale.
And as with many fungal conditions, wet weather promotes the development of this disease.
Once infected, the plant will exhibit one- to two-inch black lesions on the stems. Additional dark spots can show up on the foliage and flowers, the entire plant can droop and dry up, and flowers can die.
Cleaning up plant residue at season’s end, growing hybrid species, and rotating your sunflowers every three years can help avoid this disease, along with managing the local pest populations.
Fungicides won’t do much for control, so it’s best to remove and pitch infected specimens.
Avoid planting sunflowers in the same location the following season.
4. Phomopsis Stem Canker
This disease can be easily confused with Phoma black stem, for a few reasons.
Caused by fungal species in the Diaporthe genus, the pathogens overwinter in plant debris. Infection usually occurs during moist conditions like wet weather and high humidity.
And it also produces gnarly stem lesions – although they’re larger than those of Phoma black stem, and brown in color rather than black.
Infected foliage will develop small brown spots, which eventually merge into large brown sections of foliar decay. In addition to looking nasty, infected specimens are quick to wilt and die.
To prevent this disease, ensure adequate airflow between plants, clean up crop detritus, rotate your plantings, and if you’ve had a problem in the past, choose resistant hybrids.
For control, you can use copper fungicide when the flower heads have just formed. Otherwise, remove and destroy severely symptomatic plants.
Once again, avoid planting sunflowers in the same spot the following season and choose something that doesn’t play host to these pathogens, like wheat or corn.
5. Powdery Mildew
Surviving as mycelium in plant detritus, the fungal pathogen Erysiphe cichoracearum spreads via wind or splashing water to new, uninfected foliage.
Infection and disease development is most likely in humid weather, yet while the foliage remains dry.
The main symptom is leaves coated with a whitish-gray powder. Additionally, foliage could develop small, black specks of spore-producing cleistothecia, which will only pass the problem onto other specimens.
Thankfully, this condition is mostly a cosmetic issue, as infections aren’t usually fatal.
To prevent powdery mildew, ensure your sunflowers have full sun exposure and be sure to space them apart to allow for sufficient air circulation.
Trim away symptomatic leaves as you notice them, and consult our guide to learn more about how to manage powdery mildew.
6. Rust
Puccinia helianthi is the fungal species that causes rust in sunflowers.
The pathogen survives in dead and decaying plant matter over the winter and attacks your sunflowers when they are wet from precipitation or irrigation.
Infected sunflowers will exhibit rust-colored, yellow-ringed growths on the foliage which produce spores that spread via wind to other plants. If infection is severe enough, defoliation can occur.
If symptoms are severe, pulling up and disposing of infected plants may be your best option.
7. Septoria Leaf Spot
Septoria leaf spot disease is caused by the fungus Septoria helianthi.
This disease tends to develop quickly in warm conditions and in times of high rainfall. The pathogen is generally spread via splashing water, and overwinters in plant residue.
Symptoms include water-soaked, rounded greenish-gray lesions on the foliage, which are usually ringed with chlorotic tissue.
In time, small spore-producing spots will also form on the undersides of leaves, and infected plants could develop a sad-looking, watery wilt before they die.
Diseased foliage leads to reduced photosynthesis, which results in reduced growth.
To prevent Septoria leaf spot, rotate your plantings every three years at least, avoid splashing the leaves with water when you irrigate, and purchase certified disease-free seed.
Cleaning up plant detritus at the end of the season denies the pathogen a place to overwinter. If your plants are infected, pull them up and dispose of them in the trash.
Health Is Wealth, Y’all
It’s true for people, and it’s true for sunflowers.
Most of these diseases can be avoided by growing these annuals in a full sun location, maintaining adequate airflow between plants, and watering at soil level instead of from overhead.
At season’s end, clean up beds and borders to prevent disease-causing pathogens from overwintering in dead plant tissue.
Are you growing sunflowers and have yours suffered from any of these diseases? Let us know in the comments section below!
And for more information about growing sunflowers in your garden, read these guides next:
