Interactions between poplar clones and Melampsora populations and their implications for breeding for durable resistance.

In the 1950s, bacterial canker (Xanthomonas populi (Ride) Ride and Ride) was a major concern in the northern part of Europe in the area used for poplar cultivation. During the 1960s, the major foliar disease on poplar in Europe was Marssonina brunnea (Ell. and Ev.) Magn., following its introduction from North America. In the past, rust caused problems sporadically, only in favourable weather conditions and following infections by Discosporium populeum (Sacc.) Sutton, which resulted in decline. Three Melampsora species are pathogenic on cultivated poplars in Europe: M. larici-populina Kleb., M. allii-populina Kleb. and M. medusae Thuem. The latter is confined to south-western France, Spain and Portugal. To date, no significant damage caused by M. medusae has been observed, even though susceptible cultivars are widely planted (Pinon, 1986). M. alliipopulina was frequent in western France (Pinon, 1991) and its frequency increased in Belgium (M. Steenackers, personal communication) due to the cultivation of ‘Beaupre’ which is relatively susceptible to M. alliipopulina. Since 1997 the frequency of this species has decreased significantly with ‘Beaupre’ becoming severely infected by new pathotypes of M. larici-populina. This latter species is now a major concern all over Europe. Variability within a given Melampsora species is mainly defined by the presence of pathotypes (or physiological races), this second level of variability becoming increasingly important. The first evidence of pathotypes was mentioned for M. laricipopulina by Van Vloten (1949), although the occurrence of different pathotypes in the field did not become obvious until the 1980s. At present, eight virulences are known within M. larici-populina, with potentially 256 pathotypes (Table 12.1). During the past 20 years, many cultivars have lost their complete resistance to M. larici-populina, due to new virulences and development of new pathotypes. Virulences were also described within M. allii-populina (Frey and Pinon, 1997). To date, no major change in clone behaviour has been observed in relation to the pathotypes of M. allii-populina. This is probably due to the fact that no breeder took susceptibility to this species into account and therefore no selection was made for complete resistance. It has been demonstrated that populations of M. larici-populina are not