White rust of chrysanthemum
Puccinia horiana P. Hennings
Puccinia horiana, the causal agent of white chrysanthemum rust, is a non-host-changing rust fungus. The spores can penetrate the leaf within two hours at favorable temperatures (optimum 17 °C) and at least five hours of leaf wetness. The incubation period is usually seven to ten days, but at temperatures above 30 °C (short-term) it can be up to eight weeks.
The first signs of infection with white chrysanthemum rust are small pale green to yellow spots visible on both sides of the leaf. As the disease progresses, creamy-white, sometimes waxy-looking pustules develop on the undersides of the leaves in these spots. These rust pustules are most commonly found on young leaves and flower bracts, but they can be found on any part of the foliage including flowers. These symptoms occur primarily during cooler and wetter weather.
Infected plants may also be asymptomatic during hot and dry weather. Under these conditions, it can take up to eight weeks for infected plants to show symptoms.
Propagation and transmission
Spread occurs mainly through trade in latently infested chrysanthemum cuttings. Since the spores of white chrysanthemum rust are very sensitive to drought, spread by wind over longer distances can only occur during very wet weather periods. Within a stand, infections usually happen through water droplets, contaminated soil, dead leaves, tools, clothing, shoes, or cultivation work.
Prevention and control
- Use of healthy cuttings
- Immediate inspection of purchased plants for symptoms on arrival at the farm
- Regular infestation checks during cultivation
- Avoid leaf wetness: do not irrigate from above, observe dew point in greenhouse
- Avoid excessive nitrogen fertilization
Measures in case of disease occurrence
- Remove and destroy without damage all plants/plant parts with visually visible symptoms
- Treat the remaining crop with effective, approved plant protection products (see list of plant protection products approved in Austria)
Puccinia horiana is a non-host-changing rust fungus with a shortened development cycle. Only teleuto- and basidiospores are formed. The teleutospores are on no stalks, which are about 45 µm long. The two-celled, thin-walled teleutospores themselves are pale yellow in color and elongated to oblong-cusped in shape. They are slightly constricted at the septa. The spore dimensions are 30-45 x 13-17 µm. The teleutospores germinate with delicate-walled small basidiospores that spread the disease in the stand. These are hyaline, slightly curved, broadly ellipsoidal to spindle-shaped, and measure 7-14 x 5-9 µm. Germination of teliospores can be observed in situ .
Last updated: 15.12.2021