First report of dwarf bunt caused by Tilletia controversa in Latvia

In August 2006, an unreported bunt disease was observed on winter wheat (Triticum aestivum) ears and in grain samples collected from commercial fields in different geographical regions in Latvia.

The visible symptoms on ear included glumes pushed apart laterally, giving them characteristic disheveled appearance. Sori were spherical, containing a blackish mass of teliospores surrounded by a thin gray-brown tegument (Fig. 1). Consistency of teliospore mass was hard to pulverulent. Hardness of sori is a typical characteristic of dwarf bunt whereas T. tritici is normally pulverulent.

The initial identification was made using light microscopy to compare teliospores with those of the morphologically similar species in Latvia – Tilletia tritici (syn. T. caries) (Priekule, 1997). Teliospores are yellow-brown, globose or subglobose, in average 19.6 ± 0.58 µm (16 – 24 µm) in diameter, reticulated (Fig. 2). Exospores have relatively wide and deep polygonal meshes (areolae); areolae are irregular. The polygonal reticulations are in average 1.81 ± 0.04 µm (1-3 µm) deep. Sterile cells are fewer and smaller than the teliospores, globose, with smooth walls. These morphological characteristics are similar to those previously reported for t. controversa (Wilcoxson, Saari, eds., 1996).

Epifluorescence microscopy method was used to confirm the identity as T. controversa (Stockwell, Trione, 1986). Teliospores of T. controversa appeared spherical when mounted in immersion oil and the reticulated wall layer fluoresced yellow-orange and appeared as spike-like protrusions when viewed at the median plane (Fig. 3) or appeared netlike when focused on the upper surface of the teliospores (Fig. 4). The endospore wall layer fluoresced yellow and the cytoplasm was yellow-green in color. By contrast, mature teliospores of T. tritici collapse in immersion oil and the reticulated walls do not autofluoresce.

The environmental conditions that favor dwarf bunt development (a long period with stable low temperatures and persistent snow cover in winter) occur only sporadically in Latvia. However, because of the longevity of teliospores in the soil, there is potential for localized agronomic and economic losses in winter wheat when those disease-conducive conditions do occur.

This is the first report of T. controversa on winter wheat in Latvia.