Rudbeckias have always been one of my favorite perennials, but they have taken a beating in the last few years. One particularly unappealing disease of this species is Septoria leaf spot, caused by a fungus, Septoria rudbeckiae. I have not yet seen it in the landscape this year, but I want to remind growers of this disease so they can prepare for the battle. The dark brown leaf spots are hard to miss, starting as 1/8-inch-diameter spots that quickly merge to cause large, brown areas on otherwise dark green leaves. Disease begins on lower leaves and progresses up the plant.
One of the few problems that might look like Septoria leaf spot is a bacterial disease called angular leaf spot. A plant lab can easily distinguish between the two. Angular leaf spot produces bacterial streaming from sections observed with a microscope. Septoria leaf spot produces fruiting bodies that are embedded in the spots. Diagnostic, long, narrow spores are produced in the fruiting bodies.
Disease spread depends on leaf moisture. Rainfall splashes spores and helps spread the disease. You can help prevent further disease spread by watering the soil, as opposed to syringing the foliage. Also try to prevent overcrowding of plants and keep weeds under control. Preventive fungicide applications protect new growth from Septoria leaf spot. These sprays should be initiated before symptoms begin. Copper-based fungicides have some effect against both Septoria and the bacterium causing angular leaf spot. The copper products have protective-contact activity. A systemic product registered for this use for commercial applications is Heritage. The active ingredient is azoxystrobin. Consult the Home, Yard & Garden Pest Guide or the Illinois Commercial Landscape & Turfgrass Pest Management Handbook for details. The University of Minnesota has a fact sheet about rudbeckia diseases online at http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/p154rudbeckiadisease.html. This fact sheet has images that can help you diagnose the problem with your rudbeckias.