Rust Diseases: Identification and Management

Many rust fungi exist, each host-specific (hollyhock rust won't affect roses). Other susceptible plants: pine, fir, hawthorn, fuchsia, geranium, snapdragon, asparagus, beans, onion, blackberry, fig, pear, lawn grasses.

Easy to identify: flip a leaf—powdery orange/brown pustules (sometimes purple, red, white, black). Untreated: pustules cover undersides; upper surfaces show yellow mottling; leaves yellow and drop.

Management

• Buy resistant varieties.
• Minimize overhead watering (leaves must be wet 4–5 hrs for spore germination).
• Remove/discard rusty leaves; clean up fallen debris in fall.
• Sulfur and summer oil may help—never apply together (toxic combo).

Chemical Controls

Chlorothalonil, triadimefon, triforine (especially on roses).