Downy Mildew on Coleus

Downy mildew came in on the wind

Coleus has been one of our top selling categories for the last several years so we always make a point of planting them around the facility here. One of our favorite varieties, ‘Wasabi’, was not growing as vigorously as it should so we became concerned. Given the weather lately and looking at the display bed—particularly at the growth of the plant—we suspected downy mildew [DM] came in on the wind.

Beautiful, healthy coleus varieties

You probably remember this disease plaguing the impatiens a few years back but this is a different strain. While that type rated a ten on the devastation meter, this one comes in at about a two or three: a nuisance that can sometimes be a problem. Fortunately, it doesn’t mean the end for coleus—unlike the impatiens DM, this variety is not going to kill the plant or mutilate it. You’ll notice that the color will be a bit off, but Coleus DM stunts the growth more than anything else.

Telltale grayish fuzz on the underside

Now, you might confuse this fungus with powdery mildew [PM]; just remember that the two present themselves in different ways. PM is found on the upper surface of the leaf as a white dusting that you can see. DM appears on the underside of the leaf and is more of grayish fuzz, though it’s not always visible to the naked eye. It masks itself in other ways.

Scorched leaves

If the leaves look scorched or the pigmentation is not 100%, that could be a clue that DM has eaten on the bottom sides of the foliage. Likewise, if the growth is not what it should be it’s a sign that DM could be present. ‘Wasabi’ is a vigorous grower—it can get up to 42 inches tall when it’s happy. We just weren’t seeing it.

Look for flecks

Also, pay close attention to the weather conditions. DM needs just the right environment in order for it to come through, and that means warm, muggy, and stagnant. These recent thunderstorms set the stage perfectly so we had an idea right away about what we were dealing with and we were right.

‘Redhead’ is more resistant

As I mentioned, it’s not the end of the road for coleus though you might have to take a slight detour. Some cultivars appear to be less susceptible to the disease than others, so the problem can be avoided by choosing those varieties—and I don’t need to remind you of how many coleus varieties there are. Unfortunately, lighter colors like the trendy chartreuse (‘Electric Lime’, ‘Wasabi’, ‘Gay’s Delight’) appear to be the most vulnerable, with darker and more heavily patterned selections like ‘Redhead’ and ‘Kong’ resisting better.

If the chartreuse is a deal-breaker, go ahead and use it but be careful not to water late in the day and space the plants a bit further apart so there’s more airflow. DM can also be controlled successfully with a fungicide so if you notice the telltale signs, head to the local supply house and ask for a foliar application for DM for coleus. If the extra time spent is not going to fit in with your work schedule—use a different plant.

As always, where you plant is important. Keep in mind that this fungus exists in a microclimate—it only manifests under certain conditions. A courtyard that hardly ever gets a breeze is going to be more prone to an outbreak of DM than a naturally windy area. Coleus DM spores are wind-borne, unlike impatiens DM spores that are omnipresent.

‘Wasabi’ is looking great now

Poor ‘Wasabi’ was the canary in the coalmine, bringing the situation to our attention so we could bring it to yours. We’ve sprayed with a fungicide and we’re all good. Remember, DM does not affect every coleus, in every place, all the time—and a little knowledge now can save a lot of aggravation later. We’re happy to use Field Notes to give you a heads up and help keep you on top of your game. Please feel free to call or stop by if you have any additional questions.

Yes, plant disease is a bit of a downer to talk about in our weekly chat, but I thought it was important to bring this to your attention. On to more cheerful topics next time and until then—keep calm and coleus on.

We have a large selection of coleus cultivars in the 8-inch pot to suit a variety of growing situations. For the complete listing please refer to the Availability section of our web site.