Pathological growths and swellings of leaves, buds, stems, flowers, roots and fruits of a host plant caused by bacteria, fungi or animal pests. On the vine they can be caused by aphids, gall midges, gall wasps, gall mites and grapevine aphids (see picture). The insects and/or also the deposited eggs or the hatched larvae release substances with a high content of gallotannin (gall- tannic acid) to the infested plant site, which characteristically alter the surrounding plant tissue. Gall is available in a wide variety of forms, each pest causes a characteristic bile shape that allows identification. Depending on the pest causing it, the pattern of damage manifests itself very differently in the form of pox-like elevations on the upper side of the leaf in green, reddish, brown or sulphur yellow (in the case of the gall mite), lenticular bile (in the case of the gall midge) and prey-like bile (in the case of phylloxera). In phylloxera, the gall body is located on the underside of the leaf and the opening on the upper side
The bile consists of several layers of small, thin-walled cells, which are rich in proteins, starch and oil and are used for nutrition. Larger, thick-walled cells, which tend to lignify, give the bile its firmness. The growth of the bile is due either to the enormous swelling of individual cells or to accelerated cell division near the site of attack. The bile also provides a protective cover for the pest while it feeds on the plant. The most conspicuous bile is caused by insects that invade plant tissue with their eggs. The enlargement of the surrounding tissue is then caused by secretions from the eggs or from the hatched larvae. The phylloxera that appear at the roots also cause reactions of the vine by biting on them, in the form of growths called nodosities (on young roots) and tuberosities (on older roots).
Images: LWG Bavaria