A malformation of plant stems commonly appearing as enlargement and flattening as if several stems were fused is called fasciation. Unusual growth, such as a spur on cherry fruit, can also be considered fasciation.
An abnormal, swollen flattening and twisting of stems is a common manifestation of fasciation. Small stems or leaves growing from the distorted stems can be abnormally abundant and undersized. Terminal growth can be atypically dense and tuftlike, called a witches' broom. For more photographs see Fascinating Fasciation .
The bacterium Rhodococcus fascians is the cause of at least some infectious fasciations. Fasciation bacteria survive on infected plants and debris and spread in water to infect plants through wounds.
Certain viruses may be the cause of some infectious fasciations. Some fasciations are noninfectious, possibly caused by a genetic disorder. The cause of many fasciations has not been identified.
Fasciation causes stems to be atypically flat, thick, and twisted. Foliage terminal to the abnormal stem may have atypically short internodes causing foliage to be dense and tuftlike. Hosts more commonly affected by fasciation include alyssum, aster, carnation, Chinese pistache, chrysanthemum, geranium, impatiens, Marguerite daisy, nasturtium, petunia, primrose, smoke tree, and snapdragon.
Fasciation rarely threatens the survival of established woody plants. Most fasciations consist only of aesthetic damage and the malady is relatively uncommon. However, plant parts fasciated because of infection by the R. fascians bacterium may die prematurely.
Obtain new plants from a quality supplier known to provide pathogen-free plants. To help prevent the spread and damage of the fasciation bacterium, avoid injuring plants such as by excessive pruning especially when plants are wet. Keep the basal trunk as dry as feasible; for example, keep mulch back from the basal stem or trunk and redirect or shield irrigation sprinklers so trunk bases are not wetted. Use good sanitation, such as dispose of diseased plants and plant debris in covered containers. Keep plant-care equipment, hands, and tools clean.
To control fasciation due to all likely causes (genetic and microbial), prune off and dispose of distorted tissue. Do not propagate or graft symptomatic plants. Clean cutting tools after clipping off fasciated plant parts.
Adapted from the publication above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Fasciated (flattened) snapdragon stem.
Fasciated (twisted) smoke tree stem.
Flattened, thickened Chinese pistache stem.
Fasciation (spur growth) on cherry fruit (left).