Thursday, May 11, 2017

Featured Diagnosis: Heuchera Rust

small brown fungus pustules on the underside of a leaf
Rust pustules on the underside of the leaf
(Photo: J. French, NMSU-PDC)
Rust on coral bells is caused by Puccinia heucherae, which only infects members of the genera Heuchera and Saxifraga. A wide range of plants can be infected by rust fungi, but individual rust fungi have a very limited host range. The most common symptom of rust infection is the production of powdery pustules on the leaves, stems, twigs, flowers and fruit of susceptible plants. Pustules are most common on the lower leaf surface and stem. Pustules may be bright yellow, orange, orange-red, reddish brown, chocolate brown, or black in color. These spots enlarge and may coalesce as the infection builds. Rusted leaves often turn yellow, die, and drop prematurely. Rust fungi infect only the plant’s aboveground parts, and while rust fungi generally do not directly kill their host plants, severe infections ultimately may lead to death by other factors (winter-kill or other diseases). Spores are moved short distances by wind, insects, rain, and animals. Water on the plant surface (aka leaf wetness) is required for spore germination and infection. After the plant is infected, leaf wetness is not needed for continued disease development. Disease builds quickly during periods of high humidity. 

small yellow spots on the upper leaf surface
Rust on the upper leaf surface of
coral bells (Photo: Jason French)

A microscopic photograph of rust spores
Teliospores of Puccinia sp. (Photo: Jason French, NMSU)