The bristly, lumpy round galls produced by the grape phylloxera (Daktulosphaira vitifoliae, family Phylloxeridae) may dominate our perception of galls on grapes. Indeed, these peculiar plant structures are a common feature on the lower leaf surfaces of wild grapes (Vitis spp.) in Ohio.
The grape phylloxera is a native sucking insect that's related to aphids. It has a complex life cycle that involves both root and foliage forms. Since native grapes co-evolved with the grape phylloxera, our grapes developed defenses including the ability to ooze sticky sap from their roots that saps the vigor of the phylloxera.
This is not the case with the European grape, V. vinifera, which is the mainstay of the European wine industry. The grape phylloxera had a devastating impact on wine grapes after it was accidentally introduced into Europe in the mid-1800s. By 1889, 2/3rds of all vineyards in Europe had been destroyed. Total wine production was cut by 57% in France alone from 1875 to 1889.
I provide more details in a 2016 BYGL Alert including the fascinating but mostly forgotten story of how America saved the European wine industry: https://bygl.osu.edu/node/400
The Handiwork of Gall-Midge Flies
Grape phylloxera galls are relatively easy to identify because of their consistent appearance, their common occurrence, and of course, their notoriety. This is not the case for the galls produced by gall-midges (family Cecidomyiidae) on grapes. They are much less common and are seldom a significant problem on cultivated grapes. However, their odd appearance can draw attention and concern.
Currently on display are the highly variable galls produced under the direction of the gall-midge, Vitisiella (formerly Janetiella) brevicauda. They are sometimes called grape tumid galls, tomato galls, or blister galls.
Their capricious locations, shapes, and colors depend on the tissue the gall-midge hijacked to create the gall-abodes for their maggot offspring. I've commonly seen them appearing as ball-like or blister-like growths on wild grape leaves.
However, I came across a form on Monday that I'd never seen before with tiny green knobs sprouting from clusters of merged galls. The gall structure looked like something from the mind of a cartoon artist. Question #6 of our OSU Fact Sheet, 20 Question on Plant Diagnosis, asks, "What exactly do you see?" It wasn't until I looked at normal developing flower clusters that I realized the gall-midge had hijacked flower clusters!
Grape hazelnut galls, also called grape filbert galls, will become obvious around midsummer in southern Ohio. The galls are produced under the direction of the gall-midge, Ampelomyia (formerly Schizomyia) vitiscoryloides, with the scientific name reflecting the plant host (Vitis is the grape genus) and the resemblance of the galls to hazelnuts (Corylus is the hazelnut tree genus).
The bizarre-looking grape tube galls produced under the direction of the gall-midge, Ampelomyia (formerly Schizomyia) viticola (old name Cecidomyia viticola) will also reach their full devil-horned glory sometime in midsummer. The galls may arise from leaf blades, petioles, or the stems of grape clusters.
All of these midge galls are more of an oddity than a real concern. However, the tumid galls do have the potential for reducing fruit yield both indirectly by reducing leaf function and directly if large numbers of flower clusters are affected. The good news is that populations can be significantly reduced by pruning and destroying galled tissue before new gall-midge flies are produced.