Today’s MG Series: Leaf Spot Diseases of Rudbeckia.
Rudbeckia, Black Eyed Susan, is in the Asteraceae family, and their common name is coneflower or black eyed susan.
A beloved cottage and cutting garden staples, with their marvelous blends of gold, orange, reddish, and mahogany petals on daisies up to 6 inches across.
They are a very hardy plant, but are very susceptible to a a few fungal problems.
One fungal problem is powdery mildew can be an issue on the foliage, so avoid overhead watering and don’t crowd the plants. Plants that are “crowed” together prevents air circulation which encourages powdery mildew.
Another fungal problem are the leaf spot diseases of Rudbeckia. It is a very easy to spot the disease, the leafs develop black spots…
It’s ugly!!! This is the enemy to Rudbeckia, Rudbeckia Leaf Spot, (RLS). It is caused by the fungus Septoria and/or Cercospora.
They both begin and causes purplish brown spots on the leaves. At first the leafs veins will keep it restricted, but as the weather heats up and air moisture builds up and stays on the leaves for long periods of time causes the leaf blight to accelerates.
The disease starts slowly on the bottom leaves and you don’t really see it. When it heats up is when you’ll notice it on top of the leaves like the photo above shows. The lower leaves gives the blight a launching pad when the conditions are prime for infecting all the leaves in late July/August.
Even through the fungus (Septoria) is unsightly, the damage is primarily cosmetic, the plant will still flower/bloom. Infected leaves may die a little earlier in the fall than uninfected leaves.
It is important to remove the infected leaves, the plant itself will flower as normal, it just looks funny with fewer or no leaves.
How Does It Get The Fungus?
The fungus overwinters in infected plant residue. Spores are produced in late spring and early summer, causing leaf spots on the lower leaves. As the season progresses, lesions develop on upper leaves as well. The spores of the fungus are dispersed by splashing water (either irrigation or rainfall), and can cause lesions throughout the growing season. Like most fungal leaf spot diseases, the spores require moisture to germinate and cause infection.
What Can We Do About It?
One could use a general-purpose garden fungicide may help reduce the spread of the disease, but these chemicals are protectants and do not cure infected leaves. Application in early to mid June may help reduce initial infection, and result in a slower onset of disease symptoms.
I don’t like to use chemicals, so it’s important to remove the infected leaves at the end of the growing season to reduce the amount of spores available the following year. Proper plant spacing will increase air circulation around the foliage and allow leaves to dry off quickly after dew or rainfall events.
Avoid overhead irrigation, which will promote leaf wetness and also splash spores from plant to plant.
We want our Rudbeckia to look like Black Eyed Susan’s, NOT Black Leaf Susan’s!