New report of loose smut (Ustilago syntherismae) on Digitaria sanguinalis in Spain

M.T. Mas¹*, A.M.C. Verdú¹ and J. Girbal²

¹ Escola Superior d’Agricultura de Barcelona (DEAB-UPC), Campus del Baix Llobregat, Avinguda del Canal Olímpic s/n, 08860 Castelldefels, Barcelona, Spain
² D. Biol. Animal, Vegetal i Ecologia, F. Ciències, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain

*maite.mas@upc.edu

Accepted for publication 04/07/05

In September 2004, smutted inflorescences of large crabgrass (Digitaria sanguinalis) were observed in an uncropped edge of an arable field at Torre Marimon (near Barcelona, NE Spain, 46º 12’ N and 4º 33’ E). Groups of infected plants (4-5 individuals) of Digitaria sanguinalis were detected forming small patches among apparently healthy individuals. The first visible disease symptom was a slight separation of the leaf-sheath from the stem (at the flag leaf level). An examination with a stereo microscope at this stage showed that the inflorescence was initially infected by the rachis of racemes, where sori (irregular to confluent) could be observed (Fig. 1). As the disease developed further it extended into the spikelets from the rachis, and the inflorescence became entirely replaced by spore mass. At this stage, the inflorescence appeared partially hidden by enveloping leaf-sheath (Fig. 2).

Figure 1: Beginning of the sporulation of Ustilago syntherismae on the rachis of Digitaria sanguinalis racemes	Figure 2: Ustilago syntherismae on Digitaria sanguinalis inflorescence. Note, spikelets replaced by a blackish-brown spore mass

To determine the morphological characteristics, spores, were removed with small forceps and mounted in water for examination by light microscopy. Ustilospores were globose or subglobose to ovoid, mean 10 x 12.5 µm diameter (n=50, range 7.5-12.5 x 9-16 µm), echinulate and brown in colour (Figs 3 & 4), with a thick wall about 0.5 µm. Based on these morphological features, the organism was determined as U. syntherismae (Vánky, 1994). A sample was deposited in the BCB - Mycotheca of the Universitat Autònoma de Barcelona (ref. BCB-Mycotheca Ust034).

Figure 3: Spores of Ustilago syntherismae

There is only one previous citation of this smut species for Spain (Almaraz, 2002). However, this relates to herbarium material collected in 1875 in Málaga and determined by Thuemen (1880) as Ustilago rabenhorstiana on Panicum sanguinale. Vánky (1999) indicates U. rabenhorstiana is a synonym of U. syntherismae, just as Panicum sanguinale is synonymous to D. sanguinalis. This is believed to be the first report of U. syntherismae on Digitaria sanguinalis in Spain since the 19th Century. Further research is required to investigate the current distribution of the smut in Spain.

Figure 4: Scanning electron micrograph of Ustilago syntherismae on Digitaria sanguinalis showing globose echinulate spores

The authors would like to thank the Servei de Microscòpia Electrònica (UAB) for technical assistance.

Almaraz T, 2002. Bases corológicas de Flora Micológica Ibérica. Números 1766-1932. In: F Pando, ed. Cuadernos de Trabajo de Flora Micológica Ibérica, Vol. 17. Madrid, Spain: Dep. Publicaciones CSIC, No. 1923.

Thuemen F, 1880. Liste des champignons que feu le Dr. Wolffenstein a récoltés pendant un séjour a Malaga en Espagne, déterminés par F. de Thuemen. Revue Mycologique Recueil 2, 150-151.

Vánky K, 1994. European Smut Fungi. Stuttgart, Germany: Gustav Fisher Verlag.

Vánky K, 1999. Taxonomical studies on Ustilaginales. Mycotaxon 73, 135-161.

The British Society for Plant Pathology