Reddish swellings on a leaf of a Nipplewort seedling.
Caution: Do NOT use iSpot to identify fungi to eat!
Some fungi are very poisonous so a mistaken ID could have serious consequences.
No interactions present.
There are no galls recorded on L. Communis only a rust on the stems and a Puccinia on the leaves. If you suspect it is a gall send it to the British plant Gall Society. But it still looks like a Puccinia fungus probably Puccinia lapsanae common on leaves.
Wild Flower Society member
Have a look at Redfern, Shirley, and Bloxham (2002), P. 240 =
"One rust (Uredinales: Pucciniacea)galls nipplewort, L communis"
P. lapsanae is also listed here:
Galls will have a hard shell but this looks like a fungus as mentioned.
Again, to quote from R, S and B
"A gall is an abnormal growth produced by a plant or other host under the influence of another organism. It involves enlargement and/or proliferation of host cells..."
Your "hard shell" test is perhaps a little restrictive; consider, for example, the squishiness of a new oak apple compared to the hardness of a marble gall - both galls.
Lat/Lng: 51.9, -2.1
OS grid ref: SO9324