||Angular leaf spot is a common bacterial disease of Goldsturm coneflower (Rudbeckia fulgida var. sullivantii 'Goldsturm')
Two common leaf spots affect rudbeckia in the St. Louis area. Both rarely endanger the survival of the plant but both can cause depreciation of the foliage. One is caused by a fungus and the other by a bacterium.
Symptoms and Diagnosis
Septoria leaf spot is caused by the fungus Septoria rudbeckiae and causes small 1/8 - 1/4 inch spots on the leaves. Small, black, pinpoint size fruiting bodies (pycnidia) form in the center of the spots. Lower leaves are affected first. The disease then moves up the plant by splashing water. Angular leaf spot of rudbeckia is most common on the cultivar 'Goldsturm'. It begins as small, brown, angular spots on lower leaves, which may appear water-soaked but can quickly expand to affect the whole leaf. It begins at the base of the plant and moves up. Angular leaf spot is caused by a bacterium. The absence of fungal pycnidia and the presence of bacterial streaming under microscopic examination can be used to differentiate angular leaf spot from septoria leaf spot.
Both organisms overwinter in infected debris in the garden. Initial infection occurs in late spring or early summer on the lower leaves and then spread up the plant. Plants are rarely killed, but plants can look unsightly by early fall.
Integrated Pest Management Strategies
1. Clean up old foliage each fall and then live with the disease.
Sanitation can go a long way to helping control both diseases. Collect and dispose of old foliage in the fall or by late winter to help prevent early infection.
2. Provide good air circulation and avoid overhead watering. Keeping the leaves dry and help prevent movement of the fungi or bacteria by not watering overhead. If irrigation is necessary, use soaker hoses or at least water early in the day so the foliage can dry before nightfall.
3. Apply chemical protectants. Septoria leaf spot can be controlled with fungicides containing chlorothalonil or copper. Angular leaf spot can be controlled by applying a copper-based fungicide such as Bordeaux mixture as needed started in early spring. Kocide is also effective. Apply at the first signs of disease to protect healthy yet unaffected foliage.
Strategies 1 and 2 are strictly organic approaches. Of the fungicides listed in Strategy 3, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper products.
|Closeup of angular leaf spot on Goldsturm coneflower (Rudbeckia fulgida var. sullivantii 'Goldsturm'); leaf spot is bacterial