First report of Ophiostoma novo-ulmi ssp. novo-ulmi on elms in the Iberian Peninsula

The highly virulent fungal pathogen Ophiostoma novo-ulmi causes leaf wilting and subsequent mortality to all three elm species (Ulmus spp.) native to Europe. Responsible for Dutch elm disease (DED), this fungus is also present in North America, and parts of Asia. Two subspecies have been described (Brasier & Kirk, 2001), the ssp. novo-ulmi and the ssp. americana, which are partially reproductively isolated, behaviourally distinct and exhibit morphological differences (Brasier & Kirk, 2001). They can be distinguished by a number of genetic markers (RAPD's) (Hoegger et al., 1996).

In order to analyse the Spanish DED fungal population, 108 strains were collected from 1992 to 2002 from twigs of DED symptomatic U. minor trees. The strains were isolated and stored as described by Brasier (1981). Determination of growth rates at 20 and 33°C (on Malt extract agar), colony morphology, mating type and subspecies were performed as described by Brasier (1981). Reference strains H172, H363 (O. novo-ulmi ssp. americana), Yu16 and P114 (O. novo-ulmi ssp. novo-ulmi) were used for comparison. Sixteen strains from markedly different isolation origins (ie La Coruña, Murcia, Navarra or Cáceres) were identified as O. novo-ulmi ssp. novo-ulmi, with a mean (±Std Dev) in vitro growth rate of 3.3 ± 1.1 (20°C), and 0.0 ± 0.0 mm/day (30°C). All the colonies were morphologically typical for O. novo-ulmi ssp. novo-ulmi (i.e. powdery and uneven fibrous-striate) and mating type B. The oldest O. novo-ulmi ssp. novo-ulmi strain was collected in Villadiego, Burgos, in 1994. The identification was checked through RAPD markers (Hoegger et al., 1996). Pathogenicity tests were conducted by trunk inoculations on 5-year-old U. minor grown from seed. On average, 60 % (Std Dev.=12) of the leaves wilted 2 months after inoculation, while the control trees remained healthy. The pathogen was consistently re-isolated from the elm twigs with symptoms.

This is the first report of O. novo-ulmi ssp. novo-ulmi in the Iberian Peninsula. Before this finding, all DED fungi from the Iberian Peninsula were identified as ssp. americana. The ssp. americana spread eastwards and southwards from Britain into the Netherlands, France, Spain, and many other west European countries. In contrast, the novo-ulmi ssp. has spread from central Asia westward into and across Europe. Evidence of hybridisation among the two subspecies has been found in Ireland, Netherlands, Scandinavia, Germany and Italy (Brasier et al., 2004), leading to unpredictable new strains, potentially capable of enhancing their virulence. Research is in progress to detect the presence of hybrids in Spain.