Fasciation (or cresting) is a widespread phenomena reported in more than 100 vascular plant species (Tang and Knap, 1998) affecting dicots and monocots in 39 plant families and 86 genera.
Every once in a while the normal growth pattern of a plant goes awry, producing a physiological disorder, called fasciation. It can occur in almost any plant part. In a normal plant, growth in the apical meristem occurs at a single point, producing essentially cylindrical growth. A meristem is the tissue found in most plants containing undifferentiated cells found in zones of the plant where growth can take place.
Fasciation causes an elongation of the apical meristem so that flattened; ribbon-like growth is produced instead. This abnormal activity in the growing tip often produces much flattened stems with a fan-like enlargement on the end. It may appear like several stems have been fused so that the stem looks like a wide, ribbed ribbon (fasciation comes from the Latin word fascia which means “a band” and refers to anything which resembles a wide band in shape); stems may be bent or coiled in abnormal directions; numerous growing points may develop to produce a witches-broom effect; flowers and leaves may appear at odd angles to the stems and the leaves growing from distorted stems are usually smaller and more numerous than normal; or flower heads may be elongated, deformed, or misshapen with more flowers than normal.
Any occurrence of fasciation has several possible causes, including hormonal, genetic, bacterial, fungal, viral and environmental.
Photos below are of a fasciation effect on a Kahikatea sapling (Dacrycarpus dacrydioides). The stem has become broad and flattened as well as twisted. The external "fur" are scale-like leaves.
Fasciation (Witches broom gall) effect caused by a mite (Eriophyes paratrophis) on Streblus heterophyllus (Turepo).