Chrysanthemum stunt viroid

The main hosts of chrysanthemum stunt viroid are chrysanthemum and related ornamentals including   Tanacetum. Susceptibility varies between cultivars, but generally all-the-year-round cultivars are more susceptible.

Chrysanthemum
About 30% of plants are symptomless carriers. Infected plants bloom earlier than normal plants of the same cultivar and this effect increases with time; on plants coming from infected mother plants, it is usually shorter in the first year of infection (a few days) than in the following year (up to more than 3 weeks). Flowers are fewer and smaller and the colour, particularly of the bronze and red ones, may be bleached to a lighter shade. Plants infected the previous summer produce far fewer laterals the following spring.

Leaves are reduced in number and size; a striking crinkle symptom is found in cultivars Blanche and Yellow Garza, the leaf surface being wavy or crumpled with associated yellowish-green patches. Stems become very brittle and break easily where they branch.

Tanacetum
Tanacetum parthenium cv. Matricaria Golden Ball may show a dwarfing with associated pale leaves and shortened, crowded inflorescences.

Control
Control of the disease is extremely difficult due to its highly contagious nature and long latent period. Viroid-free plants may be obtained by meristem-tip culture, which may be combined with heat treatment. However, the proportion of viroid-free plants which can be obtained is only about 5%.