LEAF SPOTS ON HYDRANGEA

HYDRANGEA MACROPHYLLA WITH DISEASED FOLIAGE

On my recent garden visits, I’ve noted an abundance of leaf spots and unsightly foliage on hydrangeas. They can be bacterial leaf spot from an infection by Xanthomonas campestris, or Cercospora which grows from the pathogen Cercospora hydrangeae. I’m also seeing powdery mildew on many plants. You can thank Erysiphe friesii var. friesii (formerly Microsphaera friesii). If you’ve got blemished foliage, a trip to your local extension office can diagnose which it is so you can apply the proper treatment, if at all.

WHERE DO LEAF SPOTS COME FROM

The first thing to recognize is that these conditions are predictable. All of them need specific moisture and temperature levels to allow the pathogens to develop and infect your plant. But WHERE DO THEY COME FROM? 

There are two main sources of these infections: pathogens that wintered over, or the wind. Know, however, that the infection occurred long before the damage appeared. Those pathogens were hiding in old, diseased leaves that weren’t removed last year, either on your property or a neighbor’s. They could also have been in the mulch, just waiting for conditions to be right to activate and inoculate your plant. Then the wind blew in to help spread the pathogens. Splashing water from rain or overhead irrigation sealed the deal.

BACTERIAL LEAF SPOT ON HYDRANGEA

CERCOSPORA LEAF SPOT ON HYDRANGEA

WHAT TO DO ABOUT LEAF SPOT

As with all fungal infections, you can only treat for future impact. You can’t reverse what has already happened. Sometimes you can remove the infected leaves if it doesn’t make your plant look emaciated. But remember that the leaves are your plant’s food factory so don’t take off too many. Whatever foliage you remove needs to go in the trash, not the compost. You don’t want to have those spores around for subsequent infections when you use your compost.  

MEND YOUR WAYS

You can’t change the weather (humid periods, rain, wind), but you can change your watering habits. Soaker hoses or drip irrigation are a good improvement as they will reduce leaf wetness and water splash on your plant. 

Garden sanitation is your next step. Winter temps don’t kill these pathogens so it is imperative that you remove all diseased leaves this year. Don’t slack off  at the end of the season when you are least enthusiastic about being outdoors. Make this leaf removal de rigeur each fall. A fastidious clean-up is integral to keeping these infections under control.  

TREATING DISEASED PLANTS

Although it’s late in the season, you may still need to treat your diseased plant. Bacillus subtilis will protect any emerging foliage from becoming infected. The other product I go for contains Bacillus amyloliquefaciens strain D747. It is sold in my area and online as Monterey Disease Control but you may identify alternative products with the same active ingredient.  You can treat powdery mildew with a multitude of products. A long-standing favorite of mine for all powdery mildew is bicarbonate-based Green Cure.

As with most fungicides if you decide to treat, plan on reapplying every 7 to 10 days. Follow the instructions on the label for full info on how to use any product you choose. 

REFRESH THE MULCH

If there are any spores in the old mulch, you want to prevent them from re-activating. Replacing it now after clean-up can only yield desirable results and reduce future infections.

THIN OUT YOUR PLANT

Last, consider thinning out your plant at pruning time next season. That’s right, next season. Any pruning you do now will be unfavorable to your plant so just don’t do it. You will want to open it up which will allow air to circulate and leaves to dry. I’ll write about that next year when the time is right. 

HYDRANGEA MACROPHYLLA 'ROSABILLO' FALL COLORS

In the meantime, revel in this glorious fall season. Steamy hot days are gone for the most part. The panicle hydrangeas are blushed pink and rosy red now that night temps are cooler. The big leaf hydrangea flowers are transforming into green, lavender and other hues while the surrounding maples are going crimson. It all makes working in the garden a very pleasant activity.