Landscape and Nursery - Downy Mildew on Rudbeckia

Rudbeckia, especially the cultivar "Goldsturm" has been showing susceptiblity to a number of diseases in recent years. The following is information on Downy Mildew of Rudbeckia.

Downy mildew is becoming an increasingly important problem in nurseries. The disease is caused by the fungus Plasmopara halstedii. A recent report detailed a severe outbreak of downy mildew on Rudbeckia ‘Goldsturm’ in Virginia Beach nurseries in June 2005 and 2006, when most of the crop was lost, though other Rudbeckia species and cultivars adjacent to Goldsturm were not affected.

The symptoms begin as light green spots on leaves’ upper surfaces. These mature into dark necrotic blotches, with fuzzy gray-white hyphae and spores appearing on the lower surfaces. Leaf and shoot distortions may follow. In warmer areas, the fungus can overwinter as oospores in dead plant material or soil. If it is not warm enough for inoculum to survive in infected plant debris or soil, spores will arrive by wind or through introduction of infected plant material from southern areas. New infections begin as water splashes spores to the surfaces of lower leaves. Additional spores produced in these leaves will travel by wind or water to spread the disease. Favorable temperatures for disease development are cool – 58 to 72 °F – with humidity higher than 85% at the leaf surface.

Management: Growers should scout incoming plant material carefully for signs of downy mildew. Infected plants must be removed and destroyed immediately – do not compost these. Keep nighttime temperatures in greenhouses high, and try to maintain humidity lower than 85% by proper spacing and pruning. Avoid overhead watering; keep leaves as dry as possible in the morning and early afternoon as this is when spore release and dissemination are most active. Fungicides should be used preventively. Downy mildews can develop resistance, so a combination of systemic and protectant fungicides will provide best control and avoid development of resistance. A partial listing of fungicides includes Subdue, Heritage, Phyton 27, Kocide, Dithane, and liquid copper. Professional applicators may be required.

Information from the current edition of the TPM/IPM Weekly Report for Arborists, Landscape Managers & Nursery Managers from the University of Maryland Cooperative Extension.