How to identify rust diseases by host

Rust diseases are caused by a large group of obligate fungal parasites (order Pucciniales) that produce distinctive spore masses on infected plant tissue. The orange to yellow, powdery or pustule-like appearance is unmistakable once you know what to look for. Many rust fungi have complex life.

Rust diseases are caused by a large group of obligate fungal parasites (order Pucciniales) that produce distinctive spore masses on infected plant tissue. The orange to yellow, powdery or pustule-like appearance is unmistakable once you know what to look for. Many rust fungi have complex life cycles requiring two different plant hosts — identifying both hosts helps explain where infections come from and how to interrupt the cycle.

How to recognize rust disease

Per Penn State Extension, the characteristic features:

• Pustules: Small (1–3 mm), blister-like raised spots on leaf surfaces or stems that rupture to release spore masses
• Powder: The released spores are powdery and orange-yellow, rusty-brown, or bright yellow, depending on the rust species and spore type
• Underside preference: Most rusts produce their visible spore masses on the underside of leaves, while the upper surface shows a yellow, pale, or chlorotic area above each pustule
• Upper surface symptoms: Often a yellow-orange spotting on the upper leaf surface, with the corresponding pustule below