Karnal bunt is caused by Tilletia indica, which is a floret-infecting fungal smut fungus. Unlike systemic smut fungi, only a few florets of the inflorescence become infected; with little or no swelling of the kernels, which may be only partially destroyed. In kernels partially infected, the sorus destroys the embryo tissue on the suture side; in advanced infection the tissues along the suture and adjacent endosperm are replaced with smut spores contained within the pericarp. The spores are powdery, brownish black in mass and produce a foetid, decaying fish-like odour (trimethylamine). Point infections are most common, but infection may also spread down the adaxial groove and, in severe cases the whole grain may be infected (OEPP/EPPO 2004).
Sterile cells are very variable, globose, subglobose, frequently lacrimiform, yellowish brown, LINK49>LINK48>10?28 µm at their widest point, up to 48 µm in length, with a small or robust appendage; walls laminated, up to 7 µm thick.
Spores mostly globose to subglobose, occasionally with a mycelial fragment attached, subopaque, dark reddish to coppery brown, 24?47 µm diam.; exospore adorned with thick, truncate, compact projections (1.4?4.9 µm high) as seen in median view (Duran and Fischer, 1961).
If only a few spores are present for examination then morphological characteristics are not considered totally reliable for confident discrimination between Tilletia indica and morphologically similar species that are known as contaminants of wheat grain e.g. Tilletia ehrhartae (Pascoe et al., 2005) and Tilletia w