Caused by Puccinia horiana, this fungal disease is highly host-specific, affecting only Chrysanthemums. This fungus prefers cool, wet weather, making it proliferate in late summer and fall. It first originated in China and Japan, and it spread worldwide due to it taking a while for plants to manifest symptoms. This is also known as “latent periods” and was the reason why infected plants that seemed healthy were transported from garden to garden across countries. It spreads by spore through the air and primarily affects the plant’s foliage. It grows within the leaf tissue, extracting nutrients from the cells. This is why heavy attacks can cause collapse and premature death. Other types of rust affect Chrysanthemums, but white rust is the most dangerous.
Chrysanthemum White Rust On Plants
Signs of damage
- Sunken spots. Upper leaf surfaces will display tallow to brown spotting.
- White pustules. These are what give this disease its name. Lumpy white pustules form on the lower leaf surface.
- Leaves shrivel and turn brown.
- Petal pustules. White pustules can also occur in petals, although this is less common.
- Stunted growth and reduced vigor.
How to prevent
Prevention starts with good practices, such as plant selection, overall tidiness and hygiene, and optimal plant growing conditions. Plants should be regularly checked, and affected leaves should be removed, while heavily damaged plants should be fully destroyed, ideally by burning.
New plants should be monitored and quarantined. Different cultivars have different susceptibility, so choosing resistant cultivars is advised. Don’t take cuttings from infected plants.
The presence of natural enemies is encouraged, as well as weed control and adequate spacing between plants to promote airflow. Prolonged leaf wetness should be avoided.
Heal
You can choose non-chemical treatments, such as the hot water treatment, where foliage is removed, roots are trimmed, and the plant is submerged for 4 minutes in water at 115 ºF (46 ºC) followed by cold water. Allow the plant to air-dry.
Chemical treatments are also available, such as fungicides with tebuconazole and triticonazole, known to help control rust diseases, although fungi might show resistance.
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