Leaf Spots of Rudbeckia

Overview

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-sized 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.

Life Cycle

Both organisms overwinter in infected debris in the garden. Initial infection occurs in late spring or early summer on the lower leaves and then spreads 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 helps 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 as needed starting in early spring. Apply at the first signs of disease to protect healthy yet unaffected foliage.

Organic Strategies

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.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against the specific leaf spot. Not following the pesticide label before usage is a violation of federal law.