A hitherto undescribed leaf spot of balsam poplar (Populus balsamifera L.) associated with a gall midge (Cecidomyiidae) was first observed in Waterton Lakes National Park, Alberta, in 1963. Subsequent collections indicated its wide distribution in Alberta. Only a minor portion of the foliage of a tree was affected and there was no indication of premature leaf fall. Observations and culture studies were undertaken because, although this spot resembled those caused by pathogenic fungi, it did not fit an existing description

The first conspicuous symptom of the leaf spot was observed near the end of June as a tiny chlorotic papilla, approximately 1.5 mm in dia and 1 mm high, which protruded adaxially with an abaxial concavity. The concavity was partially filled with a clear sticky fluid within which lay a cecidomyiid larva. Papillae occurred singly or in groups. Three to 4 days later each papilla or group of papillae was surrounded by a chlorotic zone with a brown to black perimeter (Fig. I-B). On dried specimens the papilla was less chlorotic than the surrounding tissue within the spot, thus resulting in a green island (Fig. 3).

[Several black and white photos of the gall and larva appear in the paper]

Cecidomyia sp.

Globose leaf gall on Balm-of-Gilead [Populus balsamifera], gall midge