Hello Lynn,

Thank you for reaching out to the UNH Yard and Garden Infoline with questions about your dahlias. 

Several things come to mind.  If your dahlia’s leaves have turned brown, first check that they’re receiving adequate water and the right amount of sunlight. Then, evaluate them for signs of disease to identify the dahlia problem.

Too much sun can cause leaves to become dry and crunchy. If your dahlias are getting more than 8 hours of direct sunlight per day, moisture may be evaporating faster than the plant can replace it. Try offering some protection from the late afternoon sun and water them regularly.

Other reasons leaves are browning are too much fertilizer. Use only the recommended amount and no more than once a month. Dahlias need regular watering in the mid-summer sun. I see you have drip irrigation, which is excellent. If the leaves are yellowing or have crispy brown edges it is a sign of under watering. I also see some white spots on the lower leaves where the brown spots are noted. This could be a sign of insects, or under-watering as well. It is hard to tell by a photo.  You could consider sending a sample to the UNH plant diagnostic center for positive identification of the symptoms.

There are several reasons your dahlia’s leaves may develop white spots, most often due to a fungal issue or insect damage.

Before powdery mildew spreads into a white fuzz, it begins as a scattering of white spots that gradually grow and merge. Catching this fungal disease in its early stages makes it easier to manage. Prune off affected leaves and remove them from the garden to prevent further spread.

If you notice pale yellow to white spots with dark centers, your dahlias may be showing signs of smut, a fungal condition that typically appears after prolonged rain, high humidity, and overcast skies. Though unsightly, smut is not usually fatal. Simply prune off and dispose of the affected leaves.

Thrip damage can also result in white, papery-looking scars on dahlia leaves. Thrips are especially active during dry weather, so consider this a likely cause if conditions have been hot and arid. Keep your dahlias evenly watered, spray the leaves with a strong blast from the hose to dislodge thrips.

Finally, in hot dry conditions, dahlias are susceptible to pests. Spider mites, slugs, etc. I see grass clippings under your plants which can provide a positive environment for insects. Just something to consider. I have attached the link below if you chose to go that route.

https://extension.psu.edu/dahlia-diseases

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://extension.unh.edu/agriculture-gardens/pest-disease-growing-tools/plant-diagnostic-lab&ved=2ahUKEwix6-ndodOOAxWGlYkEHcM_KSMQFnoECAoQAQ&usg=AOvVaw0pZQWFq0yL-ezDrtpCoeJJ

I hope this information is helpful. Please reach out if you have any further questions. Good luck and happy gardening.

Hello Lynn,

Thank you for contacting me with your question about yellow versus brown leaves on you dahlias.

I see both colors in your photos.  The link below may answer your question about yellowing leaves. Most likely, it still remains a deficiency in nutrients, insects or soil composition.  Unfortunately, it is hard to make a recommendation from a photo as there are so many variables.

https://extension.illinois.edu/blogs/extensions-greatest-hits/2007-08-28-yellow-leaves-can-indicate-plant-problems

The best option for a solution would be from a diagnostic test result through the UNH lab.

Best, Eileen