The damage caused by the broad mite (Polyphagotarsonemus latus) can look similar to the damage caused by viruses. The typical pattern of damage consists of malformation and distortion of the above-ground growth of the plant. The mites show a preference for young, developing plant tissue, like the growing tips, young leaves and flower buds. The mechanism causing the deformation is not yet fully understood, possibly the mites, whilst sucking out the contents of plant cells, secrete substances that disturb local growth.
Leaf feeding is mainly concentrated on the underside near the leaf stalk, which tends to cause the leaf to turn brown and curl up. A typical indication of an attack of broad mite (Polyphagotarsonemus latus), is the appearance of dark brown edges at the base of young leaves. In case of a mild infestation, one can often see brown, frequently collapsed spots, or brown stripes forming a fine network on the leaves. In case of a more serious infestation, this network becomes so dense that there is no more green tissue visible. In most cases, the main veins are untouched, causing them to stand out as a green pattern against the brown leaf tissue. Brown, corky patches can appear on the leaf stalks and main stems.
The growing tips of affected plants acquire a misshapen appearance, with contorted leaves and sporadic brown discolouration caused by cork formation. When plants are severely attacked, the growing tip can be killed, plant growth stops and, in time, the whole plant dies off.
Cork tissue can also develop on fruit. Where pierced cells are killed, deformed corky patches frequently appear causing misshapen fruits that often crack open at the site of deformation. Flowers are often discoloured, and with a severe attack, deformed. Extensive damage can be caused by relatively low populations.
Frequently, the lower leaves of a plant remain unaffected while the younger leaves are badly damaged. Most broad mites (Polyphagotarsonemus latus) are found on the undersides of young, expanding leaves. Symptoms of an attack remain visible several weeks after the mites have been removed.