Garden Detective

Brown buds on dahlia signal gray mold

What’s causing these dahlias to turn brown before they open? It’s likely botrytis blight, also known as gray mold.
What’s causing these dahlias to turn brown before they open? It’s likely botrytis blight, also known as gray mold.

Experts tackle readers’ garden questions.

Q: We have an issue after planting our dahlia starters. The flowers before bloom look almost burnt or frozen. After planting the starters, the plants grew very healthy, looking very strong. We had a few weeks of beautiful flowers and then this fungus issue began. After pulling all flowers, a few volunteers popped up in their place and the same issue; a week of nice flowers and then this. I did treat the flowers with fungicide, but I wasn’t sure if it was too late, or if I need to amend the soil in some way. We were hoping it wasn’t going to transfer to any other of the flowers, which it didn’t. Now we are just trying to solve the problem before next summer’s blooms.

Jessica Allen, Newcastle

Bee garden writer Debbie Arrington: That’s botrytis blight, a common problem on roses – and dahlias. It often attacks dahlia starters and transplants such as yours.

Also known as gray mold, this fungal problem can attack almost any ornamental shrub and many popular perennials. It’s also common in strawberries, turning fruit to mush, and wine grapes. It tends to appear after extended periods of rain, drizzle, fog or cloudy days in spring, early summer or late fall.

According to University of California’s Integrated Pest Management research, “gray mold is one of the more destructive plant pathogens and attacks a wide variety of plants. Flower petals and ripening fruits and vegetables are particularly susceptible to infection, but leaves and stem tissues also may be infected, and young seedlings of several crops can be killed.”

This blight is caused by Botrytis cinerea, a fungus that attacks tender parts of plants. It needs high humidity for the spores to germinate and infect new tissue. Besides moisture in the air, overhead watering can prompt its appearance. It loves temperatures in the mid-70s.

Botrytis symptoms on flowers make them old before their time. Flower buds turn brown, fade and wilt before they open. In addition to the discoloration, the buds may not open at all or turn mushy and fall off.

Leaves and shoots infected by this blight develop brown lesions and gray edges or patches; those gray spots contain potentially thousands of spores that could infect other plants.

That makes cleanup essential. Remove and destroy infected flowers, fruit and foliage – and if necessary, whole plants – as soon as the gray mold appears. Clean up any fallen leaves or plant debris under the infected plant, too.

To help cut down on botrytis blight potential, maintain good air circulation around the plants. That means don’t overcrowd transplants.

Also, don’t water your dahlias from overhead; in particular, try not to get the flowers wet.

Fungicides – such as Decree (fenhexamid) or Heritage (azoxystrobin) – may be effective against botrytis but they must be applied at the first sign of trouble or before any gray mold is evident, according to the UC Integrated Pest Management website. Also, some fungicides may actually damage different plant varieties, so read the label carefully and follow directions.

The Bee’s Debbie Arrington is a consulting rosarian and lifetime gardener. Reach her at darrington@sacbee.com, 916-321-1075, @debarrington.

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