Chrysanthemum rusts—Puccinia spp.
Chrysanthemums and daisy, Chrysanthemum spp., are susceptible to brown rust, Puccinia chrysanthemi, and white rust, Puccinia horiana. Brown rust is common where chrysanthemums are grown outdoors. White rust has been the target of eradication and quarantine programs, which may require the destruction of infected Chrysanthemum spp.
Chrysanthemum white rust fungus causes brownish, pinkish, pale buff, or white spore-producing pustules on the underside of leaves. Brown rust does not produce raised pustules on the leaf underside; its dark brown spores occur in powdery masses.
Both brown and white rusts distort, discolor, and cause premature drop of leaves in chrysanthemums. Infection causes pale green, yellow, or white spots on the upper surface of leaves, and both of these rusts and other causes can produce similar leaf spotting. Chrysanthemum white rust is distinguished by the presence of its pustules.
Chrysanthemum white rust has a very short life cycle and only two spore stages. The pathogen reproduces only on living chrysanthemum foliage. Teliospores develop on the underside of infected leaves and germinate in the leaf pustules, producing basidiospores. The basidiospores move with air and if they land on chrysanthemum foliage that is wet, cause a new infection. Both brown rust and white rust of chrysanthemum are favored by the same conditions (cool temperatures and damp weather or high humidity).
Where white rust occurs, it causes damage primarily in greenhouses. White rust usually is not damaging outdoors because direct sunlight and low humidity destroy the white rust basidiospores. However, white rust can survive outdoors where temperatures are mild and humidity is high. Residential and outdoor chrysanthemum plantings in coastal areas of California can serve as a reservoir of infection that is difficult to eliminate. When infection spreads from outdoors to chrysanthemums in greenhouses, crops can be devastated.
Report any suspected white rust infections to the county agricultural commissioner. To avoid the disease, plant nonhost species, including pyrethrum, or painted daisy, Chrysanthemum coccineum, and Shasta daisy, C. maximum.
Where brown rust is a problem, avoid overhead watering. Collect fallen infected leaves and dispose of them away from host plants. Fungicides applied in the spring can prevent or reduce brown rust disease. However, the frequent applications required to provide good rust control may not be warranted in many landscape situations.
Rust spotting of foliage
Rust pustules on leaf underside
Pale, raised spots on leaf upper side