Spring 2023 has been an incredible year for azalea blooms in Lee County. Yards are awash in pinks, whites, reds, and purples from all of the different varieties. Our cool, mild spring and overall mild winter has created the perfect blooming conditions and all we have to do is sit back and enjoy (for now!). Yet, the recent uptick in temperature and humidity, and rash of cold, wet days have put a damper on our season. What you might not realize is there is a fungus among us.

Melting flowers versus skirt of petals

Have you ever noticed that instead of a beautiful skirt of blossoms on the ground surrounding your shrubs that the blossoms stay attached to the plant? Does it look like someone took a hairdryer and melted the flowers on the plant? If so, you most likely have Azalea Leaf Blight, Ovulinia azaleae. The fungus only affects the blossoms and does not affect the overall health of the plant. It does impact the aesthetic quality of the plant, though, and it does not allow for pollination and seed development. It is a common issue for azalea growers in greenhouses and in landscapes. It is fairly common in landscapes, especially where there is a large concentration of azaleas and there is poor airflow or high humidity.

The lifecycle of the fungus is straightforward. The fungus infects a flower by wind or splashing, and begins to grow on the petals, causing a tell-tale white spotting.

Rhododendron indica ‘Formosa’ flowers are typically bright magenta, but Ovulinia azaleae can affect floral performance after temperature fluctuations, especially in humid climates, for mid- to late-season blooming cultivars.

Eventually the fungus will cause the flower to completely deteriorate and will be slimy to the touch. Other fungal pathogens will not feel slimy like Ovulinia. Once the flower “melts”, the fungus is still growing, eating the sugars of the petals, and it will eventually dry out and create black, plate-like growths on the petals called sclerotinia. This is the overwintering structure of the fungus. The petals will fall to the ground and the sclerotinia will overwinter in the mulch and soil around the base of the azaleas to reinfect the plants next year!

Controlling the fungus

The first step in dealing with a fungal pathogen is breaking the disease triangle. For a disease to be pathogenic, the pathogen and the host have to be present, and the right growing conditions have to be present. Since most of us gardeners have no control over the weather, which is a huge factor in Ovulinia infestations, we have to use other tools in our arsenal.

The first step is planting your plants in locations where there is good airflow and to not group them all together. It is common practice to want to plant a lot of azaleas together, so if you choose to do this, it is important to know you are creating a conducive situation for fungal infections.

You should already be enjoying your azaleas anyway! Keep an eye on your blooms by scouting daily, especially after warm, wet weather. If you notice your blossoms melting on the plant, you probably have Ovulinia. You can bring a sample into your local Extension office for confirmation.

The next step is to remove diseased flowers and clean up flower litter on ground around the base of the plants after flowering is through. While it may not be possible to pick off all the flowers, it is easy enough to remove the mulch and change it out for fresh material! This reduces the amount of fungus present next year that could potentially re-inoculate the flowers.

It is critical when azaleas are flowering to reduce overhead irrigation or to irrigate at times when the water will evaporate quickly and not linger on the buds and blossoms.

Fungicides are an option, but timing for these applications is critical. Apply fungicides to flower buds and directly onto the ground around the plants in spring before flowering begins. As long as the flowers continue to open, repeat applications at intervals specified on the fungicide’s label. Test fungicide on a few flowers before spraying the entire plant; some fungicides may discolor flowers. Potassium bicarbonate can also be used as an easy fungal control option.

Enjoy!

All azaleas are in the genus Rhododendron, but not all species in the genus Rhododendron are azaleas. The genus Rhododendron has more than 1,000 species in it and there are so many varieties and cultivars to enjoy in your home landscape. Knowing what to look out for can guarantee you can keep your landscape healthy and looking gorgeous.

References

Azalea Society of America

American Rhododendron Society

Growing Azaleas – UGA

Pacific Northwest Pest Handbook guide to Ovulinia Petal Blight

University of Maryland Ovulinia Profile

University of Massachusetts at Amherst Ovulinia Profile

Breakdown of Classification of genus Rhododendron

Fungal Control from NC State Ag Chem 2023 Manual (page 544)