Diseases of rudbeckia
A profusion of bright yellow flowers on bushy plants with dark green, lanceolate foliage make black-eyed Susans (Rudbeckia spp.) popular annuals and perennials among Minnesota gardeners. Most of the disease problems listed below affect all members of the genus Rudbeckia to varying degrees. Accurate diagnosis of these diseases often requires the use of a microscope with exceptions for powdery mildew (which is clearly visible to the naked eye) and aster yellows (which requires laboratory diagnosis).
Fig 1. The fungus Septoria rudbeckiae is one of the most common foliar pathogens of Rudbeckia.
Photo: Janna Beckerman
Septoria leaf spot of rudebeckia
The fungus Septoria rudbeckiae is one of the most common foliar pathogens of Rudbeckia (Figure 1).
Symptoms begin as small, dark brown lesions that enlarge from 1/8 to 1/4 inch in diameter. Although the lesions are usually rounded, there may be angles where leaf veins limit the spread of the fungus (Fig. 2). Symptoms of this disease may be difficult to distinguish from those of angular leaf spot of Rudbeckia.
Fig 2. Angular lesions on leaves may be Septoria, but microscopic analysis is necessary for positive diagnosis.
Photo: Janna Beckerman
Microscopic examination of the lesion will reveal black, flask-shaped structures called pycnidia that contain thousands of thread-like spores. Spores are produced in late spring and early summer, causing leaf spots on the lower leaves. The spores of the fungus are dispersed by splashing water, with lesions first appearing on lower leaves and later developing on upper leaves as the season progresses.
To manage this disease, remove the infected leaves at the end of the growing season to reduce inoculum levels. Because leaf moisture is essential for infection to occur, increase air circulation around the foliage by properly spacing plants (and removing volunteer seedlings) to prevent over-crowding. As with other foliar disease problems, avoid overhead watering. Fungicides containing chlorothalonil or copper* may protect new growth and reduce the spread of the disease. Preventative applications of fungicides should begin in early to mid June prior to the onset of symptoms.
Fig 3. Less severe symptoms than Septoria result from infection in Cylindrosporium.
Photo: Chad Behrendt
Cylindrosporium and ramularia leaf spot of rudbeckia
Ramularia rudbeckiae and Cylindrocladium spp. have been reported as causing leaf spots on members of the genus Rudbeckia (Fig. 3).
Development of this disease is rarely as severe as Septoria leaf spot. Symptoms include small, scattered lesions that are often angular in appearance.
Microscopic identification of the acervulus (in the case of Cylindrocladium) or conidia and conidiophores (for Ramularia sp.) is required to distinguish from Septoria leaf spot.
Like Septoria leaf spot, frequent watering (due to rainfall or irrigation) that results in leaves that are wet for extended periods favors these diseases. Control consists of proper watering so that leaves are not wet for extended periods. Management is the same as for Septoria leaf spot.
Fig 4. Powdery Mildew of Rudbeckia. If only all diagnoses could be so easy!
Photo: Chad Behrendt
Powdery mildew of rudbeckia
Powdery mildew is more likely to appear in mid to late summer when cool evenings are followed by warm, humid days. In severe instances, if left untreated, powdery mildew can cause leaves to turn yellow, die and fall off. In most instances it is only an unsightly nuisance.
The presence of a white 'powder' on leaves makes diagnosing powdery mildew easy (Fig. 4). Erysiphe cichoracearum and Sphaerotheca fusca are the causal agents of this disease.
Management of powdery mildew is slightly different than leaf spot management. Provide adequate air circulation with proper thinning and spacing of plants. Remove and destroy infected leaves and flowers, and avoid fertilizers high in nitrogen that promote new growth, which is more susceptible to infection. Control may also be achieved with the use of fungicides applied as soon as symptoms are visible. Among the compounds registered for use are potassium bicarbonate*, ultra fine oil*, sulfur*, neem oil*, triadimefon, or thiophanate-methyl fungicides. As always, consult the label for application instructions.
Angular leaf spot of rudbeckia cv. 'goldsturm'
Symptoms appear as angular, brown spots on the leaves that may cover the entire leaf. Leaf spots may appear water soaked. Symptoms begin on the lower leaves first and progress up the stem.
Bacterial streaming from lesions and an absence of fungal pathogens.
Control strategies are aimed at prevention, namely, using other 'generic' cultivars of Rudbeckia, which are less susceptible to this problem. In the fall after the first hard freeze, remove all above ground plant tissue. Tools should be disinfected with 10% household bleach, 70% alcohol, or one of the commercially available compounds, like trisodium phosphate (TSP) or Phyton 27. In the spring, apply a copper-based* bactericide (Bordeaux, Kocide or Phyton 27, to name a few) to minimize the possibility and severity of infection. Follow label instructions for subsequent applications. It is important to remember to avoid overhead watering since these bacteria are easily spread in splashing water. If you wish to plant susceptible cultivars, starting with clean seed or healthy transplants it may prevent infection from becoming established.
Aster yellows of rudbeckia
Aster yellows has an exceptionally large host range that includes purple coneflower, aster, marigold, goldenrod, cosmos and other members of the daisy family (Compositae). The symptoms that result from this disease are witches' brooms, flowers appearing out of the 'cone', leaves sprouting from flowers, dwarfing and yellows.
The pathogen, a phytoplasma, is a bacterium-like organism without the cell wall. It is an obligate pathogen and can be conclusively identified only by laboratory analysis.
The phytoplasma is spread by a leafhopper vector.
Management options are limited to 'search and destroy'. Infected plants should be removed and thrown away. Early season control of the leafhopper vector and removal of weed hosts may help prevent re-infection.
* Denotes organically accepted pesticide.