Black-Eyed Susans are an extremely hardy plant, but even the hardiest of plants can succumb to disease. Knowing what diseases are possible is one of the easiest ways to treat or prevent disease from occurring in your Michigan garden of Black-Eyed Susans.
Different Types of Leaf Spot in Black-Eyed Susans
Angular Leaf Spot
In Black-Eyed Susan plants, angular brown spots that occur on the leaves are known as angular leaf spot. These spots can potentially cover the entire leaf and with this particular type of leaf spot, the spots generally affect the lower leaves of the plant prior to moving upward toward the top of the plant.
To treat angular leaf spot, during the late fall (following the first full freeze,) all of the Black-Eyed Susan that is present above the soil must be removed. Tools that are used to remove the infected plant should be thoroughly cleaned using a product specifically for this situation; as the tools can re-spread the infection when reused later in the spring or summer the following year.
Cylindrosporium and Ramularia Leaf Spot
These types of leaf spots are not as common as Septoria. However, when they do occur, they can produce small lesions that are often angular in appearance. Therefore, it can be difficult to differentiate from angular leaf spot.
These plants too, must be removed from the root up.
Septoria Leaf Spot
Septoria leaf spot is currently among one of the most common diseases affecting Black-Eyed Susans. Like with Cylindrosporium and Ramularia leaf spot, the lesions start out small in size. These lesions, which are dark brown in color, will double in size and most of the lesions are round in appearance. However, angular spots may occur around the leaf veins in the plant.
Again, removal of the infected portion is necessary and the tools used should be cleaned to prevent reinfection.
Aster Yellows in Black-Eyed Susans
Aster yellows is a phytoplasma, meaning it does not have a cell wall. It spreads via insects (also referred to as leafhopper vectors,) so there is little a gardener can truly do to prevent them, as any insect could potentially carry the disease.
Unfortunately, aster yellows can affect several plants within the daisy family including, but not limited to: marigolds, purple coneflowers and cosmos. Symptoms of aster yellows include deformed flowers of which additional leaves and plants grow outward from within the cone of the flower.
To treat aster yellows, the infected plants (including the roots) must be removed entirely from the garden.
Mildew affecting Black-Eyed Susans is generally more of a visual issue, though if left untreated it can result in a die-off of the leaves. With powdery mildew, the leaves will slowly turn a shade of yellow before dying and falling off.
Like with many diseases, the affected leaves and flowers should be removed from the plant. In severe cases, the entire plant (including the roots) may need to be removed.
Stem Rot in Black-Eyed Susans
Stem rot will begin by causing lower leaves to become discolored (often to a shade of yellow or yellow-brown.) Eventually, the entire plant will die. During the process, you may notice that there is a cotton-like mound growing at the base of the plant in your garden.
Plants with stem rot should be removed from the area, including the roots and the surrounding soil.
University of Minnesota: Diseases of Rudbeckia
Missouri Botanical Garden: Aster Yellows
Do It Yourself: Identifying Black-Eyed Susan Diseases