It may not be flashy and sexy but if you grow dahlias you are going to come across powdery mildew sooner or later. This is meant to be the layman’s guide to powdery mildew. If you want to get really geeky, there’s a Google rabbit hole you can go down, I’m sure!

What is it?

First off, what is powdery mildew (PM)? It’s a fungus that is characterized by a white powdery growth on the upper leaves of plants. Newer growth and succulent tissues are more susceptible to PM. It can affect pretty much any type of plant however it is host specific. This means that if you see PM on your rose bush, that variety of PM isn’t going to affect your dahlia plant. Why? Because they are in different plant families. Roses are part of the Rosaceae family and Dahlias are in the Asteraceae family. This is handy information to know and helps you figure out how to arrange plants in your garden— what you can plant next to what. A quick google will tell you the plant family for a species.

PM does overwinter in soil so once you have it, you’ll be dealing with it again and again. If you grow dahlias in a hot climate, it’s a given. That’s why prevention is important. I’ve developed some ways to hold it off for a large part of my season. It can kill a plant but I’ve found that if you hold it off long enough, you can usually make it through the season without losing your dahlia. It’s not a death sentence.

What causes it?

There are environmental triggers that cause PM to show it’s ugly face. The biggest factor in a hot climate is hot and dry conditions. Often in early summer, my area goes through a week’s worth of rain (where we all worry about tuber rot) and then we will go dry for 2-3 weeks with very little rain. This period is a perfect example of conditions that powdery mildew loves! Now PM does need humidity in the air to form spores on our leaves however in my hot southern climate, humidity is still a given even during a hot dry period. Our air is naturally humid enough.

Another environmental issue is airflow. PM has to have humidity to form but the humidity around your plants may be higher than what’s in the air above because your plants are closer together. Airflow is insanely important!! This is why you often see hot weather dahlia growers remove the bottom foliage off their plants. Increasing your airflow is key.

Prevention

Once you understand the conditions PM thrives in you can begin to thwart it. A few years ago, I began to use sprinklers in my field as a method of cooling the plants during extremely hot days. During that summer I noticed that my plants got hardly any powdery mildew until later in the season when it was cooler and I didn’t use the sprinklers as much. This led to a bit of research and I learned what I just shared above about PM environmental triggers. Keeping a “rinsing” once a day on the leaves kept the spores from having time to form. So now I set up sprinklers that cover my field in late May. I run them for 20-30 minutes daily. Sometimes a bit longer if we are extremely hot and dry. The sprinklers are the best method I’ve found for PM prevention and it’s rather organic!

Another aspect to note is that certain varieties are more susceptible to PM. Dark leaf dahlias always get it first! Which is a shame because I love dark leaf varieties. I’ve also noticed some varieties have great resistance to it. Looking at you Ice Tea!

Treatment

The old saying of “an ounce of prevention is worth a pound of cure” is most definitely true when it comes to PM. However, you may get to the point where water is not cutting it. There are some tried and true ways to deal with it. Sprayed fungicides are the surest way to deal with PM. However, you do have to spray regularly once you start. I hate having to spray 2000+ plants which is why I’m big on prevention. Copper fungicide is my go to. Whatever you use, apply it according to the directions on the label. Also remember- Don’t spray during the middle of the day. Only spray when it’s not windy and the sun is not directly on your dahlias. I prefer evenings so that the spray has time to absorb into the leaves without any risk of burning.

A few other things- If you see PM on a few leaves, remove them from your plant. Do Not Compost. Dispose of in your trash or remove them from your property. Even with prevention methods, you’ll eventually see a few spots but if you can remove them before they spread, it’ll keep the disease down longer. Scouting your plants regularly is important. That is one side benefit to spraying— you get time to look at your plants!

Also be careful of late season nitrogen fertilizers. You shouldn’t be fertilizing dahlias with nitrogen late in the season anyways, but this applies to other types of plants too. The new growth is especially attractive to PM. If you live in a climate where you cut dahlias back in mid summer, pay close attention to removing your plant debris from your field if you’ve already seen PM that season.

Based purely on observation, I do feel like growing dahlias in landscape fabric has slowed the spread of PM too. The fabric blocks the migration of the spores from the soil. However, I do think it’s a good idea to clean your fabric if you re-use it.

Winter is a great time for thinking about disease prevention and how to deal with pests. Take this time to get your game plan for a successful dahlia season!