What Fungicides Can I Use to Control Rust Spots on the Leaves of Black-Eyed Susans?
Black-eyed Susan (Rudbeckia hirta), which grows in U.S. Department of Agriculture plant hardiness zones 2 through 11, is a cheerful, sunny addition to the backyard wildflower garden. Boasting yellow, daisylike blooms with dark, black centers in the summer and autumn, black-eyed Susans have become a staple for many home gardeners. Unfortunately, these perennials are prone to a number of fungal diseases, including rust. Numerous fungicides are available, although an integrated management plan can help curb the spread of the fungal spores.
Types of Fungicides
There are two main types of fungicides: systemic and contact. Systemic fungicides work by moving up through the plant, protecting it from the inside out. Contact fungicides, as the term implies, are sprayed directly onto the affected plant parts to kill the fungus by contact. Both are relatively simple to use, but your decision will likely depend on how far the rust has spread.
Some of the more common fungicides for homeowner use include systemic products with the active ingredient thiophanate-methyl. Typically, you can apply these products via overhead or ground applications. Mix 2 1/4 to 3 teaspoons of concentrate per gallon of water in a sprayer and spray in seven- to 14-day intervals in late spring or when you first notice rust on your Rudbeckia. Always double-check the label for application and mixing instructions as products may vary. Other products, such as disease- and insect-control products, have proven effective against rust. A common product containing the active ingredient tebuconazole requires applications at least three times per year, seven to 14 days apart.
Ingredients and Considerations
Numerous active ingredients are known fungicides. A stroll through your hardware store or garden center disease-management section may show fungicide products with the following ingredients: copper, chlorothalonil, lime sulfur and neem. Many of these are general-use fungicides for ornamental plants, but not all of them are labeled for Rudbeckia use. Read the label before purchasing to ensure Rudbeckia or black-eyed Susan is listed as an acceptable plant. Most of these products come as either concentrate or ready-to-use products. RTU products are the simplest to use and usually require spraying the affected plants on the tops and bottoms of the leaves on a seven- to 14-day schedule.
Before jumping into using a chemical fungicide, take a few steps to curb the start or spread of rust on your black-eyed Susan patch. Avoid overhead watering when you suspect or notice rust symptoms; the disease requires water for infection, like many other fungal diseases. Remove any stems or leaves that show signs of rust and burn them, bury them or compost them if you have a rapid compost bin.
With a professional background in gardening, landscapes, pests and natural ecosystems, Jasey Kelly has been sharing her knowledge through writing since 2009 and has served as an expert writer in these fields. Kelly's background also includes childcare, and animal rescue and care.