Insect–fungus blister galls on Solidago graminifolia and S. rugosa. I. A macroscopic and light microscopic study of the host–parasite relationship

The host–parasite relations of an insect–fungus blister gall on Solidago graminifolia and a related gall on S. rugosa were studied. The midge, Asteromyia carbonifera, and fungus, Sclerotium asteris, were associated with both gall types.Externally, young galls of both types had a black center surrounded by a yellow halo. During growth in diameter the external coloration did not change in S. graminifolia; however, in S. rugosa a beige – ashy grey area developed in the center of the black region and expanded outward. In both species the leading edge of intercellular fungal hyphae was present in the halo region. Behind this front, hyphae sequestered to form an undifferentiated subepidermal stroma. The stroma differentiated into a black cortex and underlying white medulla at the margin of the black region. Differentiated cells remained subepidermal in both species except for the cortex that became subcuticular in S. graminifolia. During cortical proliferation in S. graminifolia the cuticle remained intact; however, epidermal cells became isolated and collapsed. In S. rugosa the epidermis separated extensively from the mesophyll but remained intact. Mesophyll cells within the halo and black region exhibited a reduction in number and size of chloroplasts and starch grains in both species.